Visual Comet Hunting Print

INTRODUCTION

Visual comet hunting is the systematic search for undiscovered comets by the visual amateur observer using the naked eye, binoculars or telescope. It is a way of exploring the entire sky on a one to one basis, where the searcher checks each moving field of view (FOV) of his or her instrument in the hope of discovering that faint misty patch of light of an unknown comet first beginning to shine. Comet hunting is a discipline, a way of observing like no other as the sky is in charge of what you might find during a session compared to the conventional method of observing. This is what makes it an exciting past time. It will teach you the sky in a way you will never forget. During the search you will be in for at least several treats….

  1. You will be reacquainted with many of the Messier objects on a regular basis.
  2. You will find more objects from the NGC catalogue than you ever thought your telescope could detect.
  3. You will see countless telescopic satellites passing through the FOV, sometimes even 3 in the same field.
  4. You will be treated to the sudden appearance of telescopic meteors which often do not coincide with the major meteor showers.
  5. You will be astounded by the amazing telescopic fields within the milky way that will astound every time you see sweep these areas.
  6. You may even be lucky enough to sweep up a known comet which will give you an unforgettable thrill and prove to yourself that your technique is good. You could sweep up a known comet which has gone into an unexpected outburst in magnitude that had been previously faint which is important and needs to be reported promptly!

COMET SEEKING EQUIPMENT

Although any kind of telescope can be used for comet hunting I would heartily recommend an instrument of a short focal length (or short focal ratio). These telescopes will give you a much larger field of view for a given eyepiece hence increasing the contrast between nebulous objects and the sky background. They will also decrease the time spent searching as you are seeing more sky in one field. Alta - azimuth mounts are more favourable as they let you scan the sky more comfortably and around the pole where equatorial mounts tend to block. Reflectors are the favoured choice of many comet hunters as they provide very contrasty fields. The Small to moderate aperture reflectors tend to be more successful at bagging the bright twilight comets and the larger light buckets have a tendency to find the fainter comets located further from the sun. I would suggest using at least a 6-inch reflector with a FOV of at least ¾ of a degree although 1 degree or more would be desirable. I use an 8-inch F/6.3 SCT and a 16-inch F/4.5 reflector for my own sweeps. However more important than anything else is the dedication of its user. A scope sitting in a room gathering dust is not going to find any comets!

SEARCH AREAS

Since comets brighten as they near the sun the best place to find a bright newcomer is in the region of sky within the suns vicinity. The region of sky within 90 degrees elongation of the sun is the visual hunting ground and is known as the ‘comet haystack region’. Searching beyond this zone is undesirable as comets located further from the sun tend to be very faint and are unlikely to be picked up by the visual sweeper. Also this is the territory of the professional surveys like LINEAR, NEAT, LONEOS, SPACEWATCH, CSS etc. To avoid these prolific NEO surveys concentrate your search to within 90° solar elongation, below the pole and near the horizon, if your location lets you hunt below -30° in declination then you will have an increased chance of success.

Searching the twilight regions as the sky darkens with the naked eye or binoculars can be rewarding as you may pick out a bright comet that was hidden in the solar glare. A telescopic search can begin when the sky is still bright beginning with those twilight areas that are about to drop below the horizon, as long as you can see stars to mag 4.0 with the naked eye. Then as the sky darkens you can work your way into the higher sky regions and into less fertile hunting grounds. I recommend 2 hours of sweeping in the evening sky and 1 to 2 hours again in the morning sky before dawn. The eastern sky before dawn should be your main area of interest. Statistically 4 times as many comets are found in the morning sky compared to the corresponding dusk regions. Also keep in mind that fewer observers are active at these times especially during the week when many find it very difficult to rise from a snug bed on a freezing cold winters morning so by regular sweeping of this region you will be at an increased advantage though many find it difficult due to there daily routines and work load. If you really want to find a new comet then search the morning sky as much if not more than the evening sector.

SEARCH TIMES

You should search for comets only under a dark sky. This means becoming familiar with the moons cycle and phases. If the moon is bright and in the sky then the chances of making a discovery become so small that your observing time is better spent in doing other things. Finding the moon rise/set times can be obtained from a variety of websites or desktop software like Starry Night Pro. In the evening sky the best time to stalk comets is when the moon is just after full and disappearing from view for a short time before it rises again. This time takes place when the moon is around 2 days after full and so the sky is thrust into darkness for an hour or so. With each night in turn the waning moon will be rising later and later leaving a longer dark window for searching. This is a very important and competitive time. A bright new comet could have appeared in the sky and has been hidden from view by lunar glare but as the sky is suddenly plunged into darkness the comet is sitting there waiting to be found by anyone looking so it needs to be swept!

The next prime search time is just before new moon in the morning sky when the waning crescent moon is not bright enough to interfere with dark sky work. This is the prime area for reasons I have previously mentioned.

PATTERNS OF SEARCH

Searching can be done in a horizontal or vertical manner depending on the individuals taste and setup. I use a horizontal sweep with the 8° because it is easier and more comfortable to use however I use the vertical pattern with the 16-inch as the telescope moves more easily in altitude and its less stressful on my back. Research completed by the Japanese has shown that the human eye is better at finding faint objects with the vertical motion.

I typically search a block of sky 30° in height and 20° or more in length and spend between 1 and 3 seconds on each field depending on my mood, sky conditions or moon phase. I spend much longer in areas of the sky with many galaxies like Virgo, Coma Berenices, Leo, Draco, Ursa Major and Canes Venatici. Spend long enough on each field to scan it for suspects but not too long because the Earths rotation and the skies apparent motion will create gaps in your search. It’s also a good policy to over lap fields and repeat your search regions throughout the monthly dark period. Some comets maybe too faint for even professional scopes but can suddenly flare up in brightness as they approach the sun, repeating your search will increase your chances of bagging one of these ‘outburst’ comets!

COMET SUSPECTS

Sooner or later you will come across a fuzzy object in your moving FOV that will get your heart rate pumping out of the blue, could that be a comet? Probably not but it will be a comet suspect until you can identify its true nature. These fuzzy deep sky objects are hidden all over the sky waiting to catch inexperienced observers, and most of your time and for many years you will be stopped by these objects. When I first started I sketched the object in the eyepiece and its location in the sky then checked it according to a good star atlas. Then I recorded the sketch and visual description in my log book. Eventually when you spend years doing this you begin to learn the whole sky of by heart and don’t really need to refer to an atlas anymore…only for objects you are unsure about.

In addition to a good 8X50 finder you should also consider one of the popular 1x type finders currently in the market like a Telrad, Rigel quickfinder etc. These devices project a red dot or bulls eye onto the naked eye view of the sky and once aligned with the OTA will tell you instantly where your scope is pointing so you can tell where you are searching and the location of any suspects you find. When I find a suspect I just have a quick look at that lovely red bulls eye projected onto the night sky and can tell immediately by its appearance and location if I have seen it before and thus saving lots of time with identification. I use the sky atlas 2000 field addition which serves me well. It contains all the suspects that an 8” scope would find under a dark sky. Other sky atlases like Uanometria go even deeper and desk top software like starry night or the internet can be invaluable. But if in doubt….watch the suspect, if it doesn’t move within 24hrs then its not comet as all comets do show motion eventually. It really helps if you become a deep sky observer.
Observe all of the different type of DSOs there are and get familiar with them so you can identify them on sight. As a general rule galaxies have sharp boundaries and are symmetrical, globular clusters are mottled with bright cores while the light of a comet is very soft often difficult to tell where the comet ends and the sky begins. On the whole it is recommended that you do not begin a serious comet search until you have located and described as many of the Messier objects as you can. In addition to becoming ‘a fuzzy’ observer you also need to become a comet observer. Observe all the comets that come within range of your telescope as watching them from night to night and locating them in various weather/sky conditions builds the discipline that is needed for a proper comet search. This time and effort is well spent as it will teach you the subtle difference between comets and deep sky objects. But what if it does not appear in any atlas?............then what?

IS THAT A COMET?

If you search long enough eventually you will find something that is not in a star atlas. Several years ago while sweeping the eastern sky before dawn through Triangulum I found a fuzzy object very close to M33 which did not turn up in my star atlas. I got very excited. I checked a book by Stephen James O’ Meara called ‘The Messier Objects’ and I found out what it was…..a bright HII region within the outer spiral arms of that galaxy. It was a good job I did some further checking!

But what happens if you find an object and you are convinced it is a comet?? First of all make a sketch of the object in relation to nearby field stars in the eyepiece at low and high magnification. Make another series of sketches showing its location through the finder and/or Telrad. Then watch for motion…..sometimes comet motion can be detected within 15mins but if the suspect has shown no motion by then…….wait longer. All comets show motion eventually so you should wait to confirm motion even if that means you must wait another night. David Levy waited 3 nights before confirming a comet find before he reported it. Its good policy indeed, be patient and don’t report it anyway just so you can be first to get credit for it as it could very well end up embarrassing you. There is one observer who reported the Andromeda galaxy as a comet….imagine the aftermath! A comet can show motion quickly if it is close to the earth or sun or its motion maybe detected easily if its location is close to a field star. Very slow motion is an indication that the comet is far away or if its in the east that it is heading towards the Earth. If you do detect motion and can confirm using the internet that it is not a known comet then you need to obtain the following information…..

  1. The date and time in UT
  2. Comets magnitude
  3. The coma diameter
  4. The comas D.C value
  5. A physical description in words including the length and P.A of any tail.
  6. The comets RA and DEC (2000) and its direction of motion + rate of speed or simply 2 positions separated by time.
  7. Name and Address of the observer

Sent your discovery report to the CBAT via email then send a second for back up.
Make sure you know it’s a real unknown comet before reporting. Daniel Green from the CBAT as said that 90% of the reports he receives from unknown observers turn out to be false.

COMET COMMENTS

Hunting for comets is part sport and part science. I personally believe that it can be taken up in a serious way by the type of person who is very passionate about the night sky. It's a passion for me and it requires a lot of dedication, commitment, patience and it helps to have an addictive personality as well. It gives you a reason to observe on a regular basis (not that you should need one) however you do have to be prepared for knockbacks as it is the worse feeling in the world when a comet is discovered that you missed, being able to take these blows is very important.

You need to be a very motivated individual and be prepared to sacrifice many other things to get the proper sky coverage that is required during each monthly dark period but it is a great journey with the possibility of finding the holy grail of astronomy on any night. But it takes patience, you have to be in for the long run if you want to have a chance of discovery. There is an interesting statistic that says you must search for an average of 400 hours for a 1st find and 200 hours for each find after that. I don’t think that number applies anymore because of the treat of the surveys. I would realistically place it at well over 1000 hours however do not be discouraged.

Mark Whitaker aged 14 began comet hunting as a summer project with his 4-inch reflector, and on his second night he discovered a new comet! George Alcock found 2 within 5 days, one of which was found using binoculars from inside his upstairs window in his pyjamas! I have heard of another observer who found 2 on the same night within 50mins of each other. On the other end of the scale David Levy bagged his first after 917 hours of searching and Don Machholz found his first after 1700 hours and found his second after another 1700 hours!!! – WOW this is what I like about comets and the search for comets, they are unpredictable objects that can appear at any time from anywhere in the sky and I firmly believe that a relative new comer to the field still has a chance of discovering a comet.

Until the surveys can scan ALL of the sky tight around the sun, then there will still be more amateur finds to come in the future. So, if anyone has been thinking about it then do it because now is the time...I have been comet hunting for over 750 hours and I don’t intend to stop! Good luck, stay vigilant and happy hunting!!!

Martin McKenna



Why observe comets?

 

Resident comet hunter and EAAS member, Martin McKenna has been observing comets since 1997 and has made it his passion in the field of astronomy. He has clocked up over 700 hours observing in 5 years and has taken some time out during cloudy nights to give us his experiences and some excellent in-depth information as to how we can search for, observe and describe comets.

For me, comets are the most amazing objects to graze the night sky and of all the objects available to the amateur astronomer to observe on a given night, comets are my favourite. They are special, they are elite! These objects offer something for everyone, from the beginning naked eye observer to the most experienced deep sky observer. They are transient objects of the very best kind. They change there brightness like variable stars – yet they are not stars, they move among the back ground stars like asteroids – yet they are not asteroids, there ghostly tails are like the aurora and can be visible to the naked eye or only with the largest telescopes. Its there unpredictable nature that allures me. At its most basic level, comet observation is fun however, with a little practice, anyone can contribute observations of scientific value. Professional telescope time is so valuable and astronomers do not have time to observe every comet, so this is an area where the amateur astronomer is invaluable. At any given time there are at least a couple of faint comets within the grasp of moderate to large backyard telescopes. If you wait a little longer one or more may become visible to small telescope users. Once every year or two a comet will become bright enough to be seen in binoculars and every 5 years one will present itself as a nice naked eye object. However on average once a decade a comet comes along that is so bright that it shakes the world of astronomy and gets attention even in television and newspapers throughout the world. These great comets are not to be missed. They can and have profoundly changed people’s lives. I am one of those people! The year was 1997 and it was a cold frosty night. The time was in the hours before dawn, I was with a friend driving through the country. Since we where just driving around I asked my friend to pull over so I could look at the stars from this extremely remote dark location. As we stepped out we were hit by the frost, it was so cold but very calm and quiet, we could have heard a pin drop. As our eyes adapted to the dark we saw more stars than we have ever seen in our lives. We glanced to the east and got a severe shock!..there high in the eastern sky was a giant search beam. We got more confused when we seen no pole or source attached to this beam. It sat there among the stars with a glorious star like head. My friend finally remembered reading in the newspaper about the arrival of a great comet called Hale – Bobb… and this was it!!! What a sight it was, the emotions I felt where similar to how the ancients felt when a great comet appeared in there sky. A combination of fear and complete and utter amazement as if I didn’t know whether to fear or admire it. I have never forgotten that ghostly white tail and it has since changed my life. I now observe every comet that comes within range of my equipment. Comets are double edged swords, they may be responsible for mass extinctions and for delivering the ingredients that led to the formation of life on earth so when you observe a comet you could actually be observing your own ancestors!

COMET NOMENCLATURE

In naming comets, there is a prescribed way of labelling so as to catalogue comets in a descriptive way e.g. 1P/1982 U1 (Halley). First we have the letter C or the letter P followed by a forward slash. C stands for a long period comet ( a period of over 200 years) and P stands for a short period comet ( period of less than 200 years)  more rarely the letters D and X where D stands for comets that have disappeared and X  for those whose orbits cannot be computed due to a lack of observations. Next comes the year of discovery then a space followed by a letter. The letter indicates the half month of the discovery. The letters from A to Y are used however the letters I and Z are omitted. Next is the number which indicates the order of discovery during that half month period.  Lastly we have the name of the discoverers. Up to 3 names can be included. These are the first 3 to report the new comet before it is officially announced. An example comet designation is the recent C/2004 Q2 Machholz.

COMET MAGNITUDE VERSUS APERTURE

It is important to note here that comets are generally fainter objects to observe compared to other deep sky objects of similar magnitude. If your telescope can detect a galaxy at mag 14 then you maybe surprised to learn that a mag 14 comet will be out of your scopes reach. This is because comets are diffuse objects with soft edges that can blend in very well with the back ground sky. Also, even a bright comet may succeed in slipping by you. Comet observer David Levy recalls missing a 7th mag comet with his 16” reflector. This was because the comet was a large very spread out object and hence had a very low surface brightness so watch out for these kinds of interlopers. Below is a list showing aperture versus comet magnitudes …..

Aperture (mm) Faintest Magnitude
50 8.0
80 9.5
150 11.0
200 12.0
300 13.5
400 14.5

LOCATION LOCATION

Your location is an important factor and will greatly influence the number of comets you can observe. The ideal location is a very dark rural site away from any light pollution and with a panoramic view down to the horizon. Even very bright comets maybe difficult if they are close to the Sun as they never get high in the sky and lurk within the bright evening or morning twilight skirting the horizon. A transparent sky is important, more important than having good seeing conditions. Stars to magnitude  6.0 should be visible with the naked eye and cometary observations should only be undertaken in a dark, moonless, clean sky. Observers should compliment there telescopic observations with binocular and naked eye work. Not everyone has the perfect location but we have to make do with the best we got, simple techniques like good dark adaption, patience, averted vision go a long way. Some deep sky observers place a light shroud over their head and eyepiece to cancel out stray light. The famous visual observer Stephen James O’ Meara uses a method of deep breathing to oxygenate the brain and therefore enhance visual acuity resulting in a gain of 1 magnitude. Tapping the telescope tube works well too as the human eye is better at detecting moving objects rather than stationary ones.

FINDING THE COMET

Ok, so you plan to set your telescope up when it gets dark but how do you know if there are any comets visible from your location? This is where the internet comes in handy. I advice visiting the following websites………

http://www.ast.cam.ac.uk/~jds/

http://www.aerith.net/comet/weekly/current.html

http://encke.jpl.nasa.gov/

These sites will tell you which comets are visible from your location. After you have chosen your comet of interest eg: (Q2 Machholz) then click on its ‘Ephemerides’.

This is a table which includes information of the comets RA and DEC, its predicted magnitude, current elongation from the Moon and Sun for every night during each month of its apparition. Then it’s a simple matter of plotting the comets position on a good star atlas like ‘Sky Atlas 2000 or Uranometria  (the latter is favoured by comet observers). If you plot its position for several nights (preferably 5) then you will get a good mental picture of its speed and direction of movement during that time. If the comet is at a respectable distance (elongation) from the Sun and is above the horizon then you can begin to track it down. I prefer to point the telescope at the section of the constellation the comets located then complete a little systematic search using a low magnification wide field eyepiece. Others may be more comfortable with the traditional technique of star hopping yet others will be inclined to use a Go–To telescope which is absolutely fine if you are having difficulty or are rushed for time. I personally encourage the manual approach for 2 reasons…

1)When you do find the comet you will get an unmistakable thrill as you have successfully tracked it down yourself and the process in itself is very satisfying and will teach you the sky in a way you will never forget.

2) You never know what you might find along the way (this applies to all types of observing), amateurs have found new comets and novae before while tracking down other objects. Eg: Of recent times Vance Petriew discovered Comet Q2 Petriew while star hopping to the grab nebula (Messier 1 in Taurus). However you are more likely to accidentally find a new galaxy or binary star you have never seen before enriching your observing experience.

If the comet is brighter than magnitude 8.0 it may already be visible in your finder scope but if not scan the eyepiece field for a faint fuzzy patch of light…that is your comet. Its important to remember that the ephemerides maybe inaccurate for a newly discovered comet if there has been a lack of accurate astrometric positions to nail down its orbit and as such your target maybe a little away from its predicted position so it pays to sweep around a little. Even well known periodic comets have been known to drift from there locations due to non – gravitational forces so bare this in mind.

DEGREE OF CONDENSATION

Comet observation is split up into 2 main areas. Those concerned with the ‘Coma’ and those concerned with the tail/s respectively. Now I will concentrate on the coma.

The ‘degree of condensation’ or DC for short is a way of gauging how condensed the coma is. In a more specific manner it provides a visual description of the coma intensity across distance. It runs on a scale from 0 to 9…

0 = Diffuse coma of uniform brightness

1 = Diffuse coma with slight brightening towards centre

2 = Diffuse coma with definite brightening towards centre

3 = Centre of coma much brighter than edges, though still diffuse

4 = Diffuse condensation at centre of coma

5 = Condensation appears as a diffuse spot at centre of coma – described as moderately condensed.

6 = Condensation appears as a bright diffuse spot at centre of coma

7 = Condensation appears like a star that cannot be focused – described as strongly condensed

8 = Coma virtually invisible

9 = Stellar or disk like in appearance.

If your value is between two values say 3 and 4 then it is recorded as 3/

Do not confuse the central condensation with the degree of condensation. This is where confusion arises to beginning comet observers. A central condensation is a distinct brightening or disk within the coma. A condensed comet need not have a central condensation. One example is A2 LINEAR which went into outburst in the winter. The coma was a featureless patch of light, completely uniform in brightness with absolutely no CC. An inexperienced observer would rate this as DC0 however it actually had a DC of 9 because the comas edge was completely sharp like a planetary disk. Confused?...dont worry, with practice you will get better estimates. The best thing to do is to just try it yourself and compare your results to those of other observers on the above web sites. Eventually it will become second nature.

Below are 3 sketches. Top is an example of a moderately diffuse comet (OG108 LONEOS) middle is a moderately condensed comet (C/2002 C1 Ikeya – Zhang) and bottom is a very condensed comet (V1 NEAT).

Other things to record are….

What shape is the coma? Is it circular, elliptical, parabolic, elongated? Elongation could be an indication of comet break up as happened to C/1999 S4 LINEAR in the summer of 2000.

Is the CC placed at the centre of the coma? A closer inspection may reveal it is not symmetrically placed but rather off centre.

What colour is the coma? Most faint comae are a pastel grey colour while others are an obvious blue or green. Some brighter comets near Perihelion have displayed gold colours.

THE FALSE NUCLEUS

Is there a false nucleus? Not to be confused with the CC. The false nucleus is a star like body deep within the coma or CC. This is known as the false nucleus because the true nucleus is hidden from view by sunlight reflecting off dust particles issuing from the true nucleus which obscures our view. There are no confirmed observations of the true nucleus either visually or with super sensitive CCD devices although there have been claims in the past that have not been confirmed. Some comets have a false nucleus and some don’t. Take note of its position within the coma, its colour and brightness. Magnitude estimates of the false nucleus are valuable and are known as M2 magnitude estimates (you will most likely need a CCD imager for this) where M1 refers to the total magnitude of the coma.

COMA DIAMETER MEASUREMENTS

A simple way to measure the diameter of the coma is by knowing the field of view of each eyepiece. This technique is not very accurate but it is simple and is a good place to start. Eg: using my 32mm SWA eyepiece in conjunction with my 16” F/4.5  reflector I know the FOV is 1 degree or 2 full moon/sun diameters (I have obtained this value at an earlier period) which is 60’. If the coma of a comet takes up ¼ of the FOV then I can estimate that the comet has a coma diameter of 15’.

Another more taxing method is to measure the coma diameter by estimating its length in relation to nearby field stars then using a detailed star atlas to work out its size.

The third method is the ‘drift method’ and it is the one I use for my own estimates. Simply time how long it takes for the coma to drift out of the FOV along the east – west line, if you have a motor drive on then switch it off. Stars will disappear out of the FOV to the west and new stars will drift in from the east. Using the following equation the coma size can be estimated with good accuracy...

D = 15t cos delta where D is the east = west time span

INTERESTING FEATURES TO LOOK OUT FOR

There are other more intricate details hidden within the coma that may not be obvious at a first glance however by being aware and familiar with them in advance you may pick them out especially on an active or bright comet. These include…

Envelopes or hoods which are often seen concentrically placed around the central condensation.

Fans are sectors of material emanating from the central condensation

Jets are radial features projecting from the CC either as straight lines or curved and are most prominent in active or very close/large comets. Hale – Bopp is a classic example however several observers spotted these in the recent Q2 Machholz. (See Sketch)

Spines are bright, sharp, narrow streaks leading from the CC into the tail.

Rays Delicate strips of light coming from the CC usually a soft white/grey or blue colour.

Shadow of The Nucleus Not really a shadow as such but looks that way visually. According to the BAA observing guide to comets it is rare and needs a 16” or larger instrument to be seen easily

Fountains Around 1’ high in the sunward direction. They are diffuse features subtending a few to 100 degrees angle and are more common than jets.

MAGNTIUDE ESTIMATES

This is an area where the amateur observer can contribute important data of scientific value. If you have experience in the field of variable star observation then comet magnitude estimates should present no problem however to a beginner this area of comet photometry can seem daunting and complicated but with a little practise you can obtain data of scientific value in no time. There are various methods employed eg: the in – out method, out – out method, modified out method and the Beyer or extrafocal Extinction method. For the purpose of this article I will choose one…

THE IN – OUT METHOD  ( OR SIDGWICK)

1) Study the coma until you are familiar with its average brightness. This is easy if the entire coma is uniform (has a low degree of condensation or DC) but not so easy if the coma sports a highly developed CC.

2) Using an atlas that shows star magnitudes, find a comparison star at about the same altitude and, if possible, in the same part of the sky as the comet.

3) Throw the star out of focus so that it is the size of the in – focus coma.

4) Compare the stars out of focus brightness with that of the in – focus coma.

5) Repeat steps 2, 3 and 4 with more stars, until you have a star less than half a magnitude brighter than the coma and a second one less than half a magnitude fainter than the coma.

6) Interpolate between the 2 stars to assign the coma its magnitude.

Do not be intimated by this and gave it a go. With constant practice you will become very proficient and will have developed a very useful observing skill that can be used throughout your own observing lifetime.

Things to remember – because of the little understood phenomena of ‘aperture affect’ one should use the smallest optical aid needed to see the comet in order to obtain an accurate magnitude estimate. Example: a 16” telescope should NOT be used to make an estimate of a mag 8.0 comet! Instead a good pair of binoculars would be a more efficient tool and would be more user friendly for this work because of there larger fields and the ability to through stars out of focus quickly with a touch with your finger or thumb. Many enthusiastic comet observers have in there possession a full range of equipment from binoculars to medium and large aperture telescopes. Consider a very bright naked eye comet like Hyakutake, binoculars would give an untrustworthy magnitude estimate, you can see the coma with your naked eye but how can you defocus your eyes to make your own estimate? Surely the suggestion sounds ridiculous? Well there is one way that is not well known among observers, a neat trick that is as yet unnamed. Imagine standing under a clear starry night with a bright comet in front of you. Hold your finger or thumb out in front of you. If you lift your gaze from the background stars onto your thumb you will make the stars become out of focus. By shifting the distance between your eyes and your thumb you can adjust how far out of focus you want the stars to be. With a little concentration you can find bright naked eye comparison stars and interpolate between then as mentioned above to make your own magnitude estimate – simple but it works!

Try to find suitable comparison stars that are placed nearby and at the same altitude as the comet. Choosing one star at the zenith and a second near the horizon is not good practise and should only be used if no other choice is available. More advanced observers apply an equation which corrects for atmospheric extinction when a comet is placed low in the sky close to the horizon which cancels out and corrects the effects of the comparison stars low altitude in thick atmosphere kind, I like to this as a funny kind of adaptive optics with a calculator. As a last pointer avoid using red comparison stars which have a tendency to build up on the retina and appear brighter than they truly are. These chameleon stars produce what is called the ‘purkinjie affect on an observer and as such should be avoided.

Finally NEVER be influenced by other observers magnitude estimates, do not be tempted to edit your own estimate because other observers have results that differ from yours. After all you might have the only correct one and second the magnitudes produced on the ephemerides are the predicted magnitudes. More often than not these are not accurate so your own estimates maybe imperative if few observers have been following your chosen comet. If you stick to lease guide lines then you will be producing high quality magnitude estimates in no time!

A TALE OF 3 TAILS

The tail is the most distinctive feature that comes to mind when you mention the word ‘comet’ to anyone. Comets have 2 distinctive types of tails…

TYPE II  DUST TAILS

 The most prolific type of tail to many visual observers. Type II dust tails are normally the brightest type of tail to observe. They are composed of dust particles that reflect sunlight and can appear curved as each individual dust particle follows its own leisurely orbit around the sun in the wake of its parent nucleus. Dust tails can vary from faint to very bright depending on how active and dusty a comet is and its elongation from the sun and its perihelic distance. They often appear a grey pastel colour like the coma or a bright white or even a lovely yellow/gold caused by the reflection of sunlight. Through binoculars and telescopes they are typically uniform in brightness and texture however very active or ‘great’ comets may exhibit what is known as ‘synchronic bands’. These are bright sections of bands within the dust tail that are caused by episodic outburst of active jets of dust from the rotating nucleus that can look spectacular to anyone who has been privileged enough to witness such a scene. Great comets Hale – Bopp (1997) and West (1976) are classic examples. (Below is a sketch of the dust tail on S4 LINEAR)

TYPE I  ION TAILS

Type 1 ion tails are also known as gas or plasma tails. These types of tails are much more elusive and subtle than there dusty counter parts. They always are straight and point directly away from the sun and exhibit a beautiful electric blue colour.

(The sketch above shows the complex ion tail of Q2 Machholz)

Observing this type of tail requires a very transparent dark sky; they are seldom seen with the naked eye and are best observed visually through binoculars or a telescope. They are very dynamic and can change dramatically in a short period of time often displaying some very intricate details like….

KINKS = Bends or twists that can be seen to move down wind through the tail by the solar wind over a period of several hours however these are best seen on photographs and CCD Images

STREAMERS =  Delicate thin blue lines of gossamer that emanate from the coma running straight down the gas tail, they can be numerous, very long and be seen superimposed on the dust tail. They are truly a lovely sight and best seen in binoculars. You could almost swear you could see them dancing in real time in the solar wind. (Below is a sketch of streamers in Ikeya – Zhang)

KNOTS = If you are vigilant enough you may catch a glimpse of a dark patch or knot of material move down wind through the tail. Believe me you will know such an event if it appears as it clearly stands out. The only time I have seen this happen was while observing comet Ikeya – Zhang in 2002 with my 8” SCT at 49X , I was very surprised and excited to say the least! (See sketch below)

DISCONNECTION EVENTS = These are best seen with a succession of CCD images but are very rare to see visually unless you are vigilant and an experienced observer with excellent sky conditions. DE’s are caused by a change in direction of the solar wind. Ion tails are like weather vanes in the solar system showing astronomers the secrets of the solar winds clandestine behaviour!

ANTI – TAILS!

These have got to be one of the finest elusive sights in nature. The anti – tail or ‘anti – solar’ tail is a trick of perspective which happens when the earth passes through the plane of the comets orbit and the earth based observer is witnessing sunlight reflecting of sunlit particles that appear to extent in front of the coma pointing towards the sun instead of away from it. The most famous example of this was the spectacular ant –tail on the bright comet Arend – Roland. In 2004 I was lucky enough to see my very first and to date only anti –tail on a comet. I was observing the unexpected spectacular comet F4 BRADFIELD in the morning sky when all of sudden I spotted a lovely long 5’ long spike of ghostly light extending out from the coma in the sunward direction. I was utterly amazed because on the previous morning it was not there! As it turned out I was the only person from Ireland to see it and I learned via the MPML that only a handful of other observers throughout the world seen it also and confirmed it on CCD images. I will not forget that morning! (Below is the anti –tail on F4 BRADFIELD that morning)

WHAT TO WATCH OUT FOR

Watching a comet move among the background stars can be a very pleasing experience. It helps to have one or more field stars near the coma which will help you detect movement more easily compared to a field with a sparse number of stars. The time it takes to detect comet motion can vary widely depending on the comets distance from the sun or earth. Hale – Bopp was a very large active comet but it did not pass anywhere near the earth and so its movement was not as impressive as comets that grazed the earth so to speak like comets Hyakutake and Iras – Araki – Alcock. The latter comet is on record for making the second closest approach from any known comet. Comet Lexell currently holds the record. These 3 comets could be seen moving rapidly even with the naked eye and made astrophotography very difficult as imagers had to guide accurately on the fast moving nuclear region in real time during a time exposure!

Comets in pro grade motion move in the same direction as the earth similar to asteroids at speeds in excess of 40km/sec however (asteroids have slower velocities) many long period comets move in a retrograde motion against the motion of the earth around the sun and can generate speeds of up to 72km/sec…that’s 72 times faster than a bullet fired from a high velocity rifle! Keeping this in mind really brings to life any comet motion you detect. During a winter EAAS observing night a number of us watched comet Q2 Machholz move in near real time within the telescopic FOV when the comet was at perihelion. Another thing to watch out for is comets transiting across field stars. I fondly remember S4 LINEAR moving swiftly across field stars one bright summer’s night. I could actually see the stars dim in real time as the comets wedge shaped dust tail passed in front of them. What a sight!

TAIL LENGTH MEASUREMENTS

Comet tail lengths are defined in degrees and minutes of arc just like the coma diameter measurements. For naked eye and binocular tails you can estimate its length using a good star atlas and by how far the tail extends in relation to naked eye stars. It pays to be careful. Some fan shaped naked eye tails appear to ‘cut off’ at some distance from the coma while others may become faint and diffuse as they blend into the sky background. With a dark transparent sky and proper dark adaption you may be surprised to detect the tail extending much further than previously thought although sky conditions and the observers visual acuity come into play. I fondly remember seeing a blue ion tail 5* long with the naked eye on comet Machholz with a gibbous moon in the sky. It was sky transparency and patience that enabled me to detect it. At the end of the day (or night should I say) it will depend on your own observing location. Basic sky watching techniques can be used for quick estimates on a naked eye comet. If you extend your arm in front you then you can determine your own angular measurements in degrees without any optical aid….

1) Your little finger equates to 1*

2) Your thumb covers approx 2*

3) Three fingers at arms length equals 5* of angular diameter

4) A fist expands 10* of sky

5) Your thumb and little finger will extend to 20*

As mentioned earlier, do not be tempted to alter your estimates if they do not compare favourably with those from other observers. Unbiased observations are important in any area of science. For more accurate tail measurements use the same techniques that apply for the coma diameter like the ‘drift method’ etc. faint comets may possess very faint tails that will require a high magnification/wide field eyepiece to enlarge the image and induce a darker back ground sky. Tapping the telescope tube may reveal a faint extension from the coma or if there is a notable breeze then nature will do this for you. Pay close attention to what the tail tells you about itself. Is it straight or curved? Does it have soft or sharp edges? how many tails can you see? There maybe 2 tails visible at the same time, one of gas and one of dust, determine there angle of separation and note details of both. You may get one tail superimposed on the other, the ion tail is placed in front of the dust tail our vice versa however it will depend on the sun – earth – comet geometry at that period of time Watching a tail grow can be an exhilarating experience, that tiny faint streak will change its morphology and position angle (see next section) as it approaches and rounds the sun constantly changing with our earthly viewpoint. Comet tails are typically at there longest during the post perihelion period and the change can happen fast! When Hyakutake was discovered in 1996 it looked like a featureless patch of light. Within a matter of weeks it sported a spectacular ion tail 110* long with the naked. Can you imagine that? A tail 220 moon diameters long! That’s the excitement of comet observation, they are transient, unpredictable, rogue swords in the sky and no one can tell you in advance just what they will do!

TAIL POSITION ANGLES

The position angle or PA is the terminology used to describe the angle at which the tail or tails leave the coma in relation to the set compass positions within the FOV.

The PA begins with 0* for north increasing through 090* for the east then onto 180* for south, 270* for west and finally to 0* (360*) once again. It is simple and requires no fancy equipment. Use the drift method to obtain the east – west line as accurately as you possible can, with this done and with the scopes inverted image (many scopes have south as up) I can get started very quickly. For the sake of simplicity lets say the east – west line is horizontal and level in the FOV, if the tail points straight down then I know it is pointing to the north at PA 0* if the tail is to the right (new stars enter the FOV from the east) then I know it is pointing to the east at PA 090*. By refining it with trial and error you will get more accurate results. If you sketch the tail in relation to nearby stars and use a detailed star atlas then you can work out the PA from it to. An illuminated reticle eyepiece can prove very useful here. Note that for highly curved dust tails the PA would need to be noted at various points along the curve.

CONCLUSION

I hope this article has been of help to anyone who is interested in taking up comet observation either as a fun past time or for serious observation. I have outlined much of the basic and intermediate type of work an amateur astronomer can do and I hope it will give you a taste of what comet observation has in store. The information I have supplied is just the tip of the iceberg an amateur can do to contribute quality observations to enhance our knowledge on comets as a whole. It has been a joy for me to write it and even more of a joy for me to observe these incredible, beautiful wanderers of the night sky and I look forward to many more!  I will leave you with these words from Leslie Peltier…

"Time has not lessened the age old allure of the comets. In some ways their mystery has only deepened with the years. At each return, a comet brings with it the questions which were asked before, and as it rounds the sun and backs away toward the long, slow night of its aphelion, it leaves behind with us those same questions, still unanswered. To hunt a speck of moving haze may seem a strange pursuit, but even though we fail, the search is still rewarding for in no better way can we come face to face, night after night, with such a wealth of riches as old Croesus never dreamed off."

 

Martin McKenna



Visual comet hunting - A deeper look

 

Resident comet hunter and EAAS member, Martin McKenna has been observing comets since 1997 and has made it his passion in the field of astronomy. He has clocked up over 700 hours observing in 5 years and has taken some time out during cloudy nights to give us his experiences and some excellent in-depth information as to how we can search for, observe and describe comets.

THREATS TO AMATEUR COMET DISCOVERY


Alan Hale

In the early days of comet hunting, amateurs were free to search as much of the sky they wanted with no clear competition from the professionals, they were visual searchers who were limited only by there determination and the field was wide open to them. Times have changed since those days and now amateurs face a real threat from a host of prolific professional search systems that have emerged onto the scene since the late 1990’s…and for good reason! These surveys have been employed to scan the skies for earth threatening asteroids and comets that could some day pose a serious threat to civilisation. There goal is to find 90% of the potentially hazardous objects out there with a diameter of over 1km. As a consequence the number of amateur comet discoveries has declined since that time. A large number of these robotic patrollers of the sky are now in operation with names like LINEAR (Lincoln Institute Of Near Earth Asteroid Research), NEAT (Near Earth Asteroid Tracking), LONEOS (Lowell Observatory Near Earth Object Search), SPACEWATCH, CSS (Catalina Sky Survey), SWAN (Solar Wind Anisotropies) and SOHO (Solar And Heliospheric Observatory). Using examples of the research completed by Shigeki Murakami I hope to show that amateurs can still compete and continue making discoveries despite the threat from these automated surveys.


Thomas Bopp

THE THREAT FROM SWAN

http://www.fmi.fi/research_space/space_7.html

SWAN is one of the instruments aboard the solar observing satellite SOHO to observe Lyman – alpha lines (UV) emitted by hydrogen. The images by SOHO cover most of the sky and are available to the public at its website. Comets are visible in SOHO's images because their main component is H2O. If a comet is brighter than magnitude 10 and has sufficient gaseous emissions then it can be picked up by SWAN, however: is this instrument as efficient as it sounds? A magazine article about the discovery of C/2002 O6 helped to increase the anxieties of comet hunters. It stated that comets of magnitude 11 are now observable by SWAN and that its capability to cover most of the skies will make amateur comet hunters work increasingly difficult. Mr Seiichi writes that he has learned that all the visually discovered comets in the past year were captured by SWAN without exception….he continues: it is not just a passing concern that comets may not be discovered visually any longer. Suddenly fears crossed my mind: are my observing method and observing instruments appropriate? Do I still have a chance if I look for fainter comets? I started thinking about strategies:

- To sweep the skies at higher altitudes in order to find faint comets sacrificing the low altitude skies. This will make it possible to sweep the skies which do not overlap with LINEARS coverage (discussed later).

- To use magnifications twice the aperture in cm, as the minimum effective power would show objects of one or two magnitudes brighter than the limiting magnitude of the aperture.

- To observe from sites with low light – pollution and at a high elevation, even if it means a longer driving time. It is reported that every 3000 feet (914m) the visibility of the object will be increased by 0.5 to 1 magnitude.

The availability of SWAN images to the public is not a recent occurrence. It became well known after the observations of C/1973 E1 (Kohoutek) by rockets, that comets emit Lyman – alpha lines. Swan images are displayed by ecliptic coordinate. SOHO is stationary at the Laglangian point 1.5 million km from the earth towards the sun. The directions to the earth and the sun are excluded from observation from observation of Lyman – alpha emissions, which correspond to areas of approximately 30* X 30* and 30* X 30*. The skies other than these areas are observed three times a week. Observation from the ground is affected by weather, latitudes, and the phase of the moon, but SOHO is an awesome new weapon to capture comets almost at any part of the sky regardless of these factors. Had this new instrument really captured new comets before they were discovered visually? I tried to find if SWAN images had caught all of the new comets which had been discovered since 2001. The following is the summary of the results of his research….

C/2001 Q2 Petriew – It is barely visible in the image of August 5. In the images of August 8 and 9 it is faintly visible, but it seems difficult to determine if it is a comet. It is also seen in the images if August 14 and 16, located in the area interfered with a lot of noise (or Lyman – alpha emissions). It is difficult to identify it as a comet.

153P Ikeya – Zhang – It is confirmed in the January 5 image and in the images from January 8 onward it is clearly seen.

C/2002 E2 Snyder – Murakami – In the image of February 19 it is barely visible. From February 21 on it has become faint but discernible. Around February 28 it is still faint but relatively easy to identify. It is located slightly above the bright C/2000 WM1 LINEAR. In the images of March 9 it has become very difficult temporarily but in March 12 and 14 images it is discernable though faint. Around February 28 it is most noticeable but difficult to determine its true nature.

C/2002 F1 Utsunomiya – At the time of the discovery it was in the area of the vicinity of the sun which lacks data. Even if bright light was emitted near the boundary of this area it would be difficult to determine if it is a comet or solar flare. On April 7 it is clearly seen.

C/2002 O4 Honig – In the images of July 16 and 18 a stain - like spot is seen shifting its positions, but it is difficult to determine it from noise. In the image of July 20 it is not detectable. In the July 25 image it brightens suddenly. It is difficult, however, to identify the comet if you use only the July 16 and 18 images.

C/2002 X5 Kudo – Fujikawa – SWAN images are available only to November 13 as of December 15 and to November 23 as of December 22; therefore, there is no way to use images to find the comet before their visual discovery. The comets magnitude is estimated to be visually about 10. In the images of November 21 and 23 it is extremely faint and impossible to discover only from these two images.

With these results I have comet to the conclusion that, out of 6 comets visually discovered since 2001, only 153P Ikeya – Zhang is possible to discover using SWAN images before its visual discovery! Many comets were certainly visible in the SWAN images before their visual discoveries, but it is not the same as to say they were discoverable as comets. I think this simply indicates the difference in views between observers and searcher. Trying to identify a comet with its position well known is totally different from finding a comet whose existence is completely unknown. It is the same with visual or photographic discoveries, or use of SWAN images. SWAN images contain a lot of noise or Lyman – alpha emissions. As these disappear and appear with time, it is difficult to determine if the seen object is a comet. Visually discoverable comets are not necessarily discoverable from SWAN images…no need to be afraid of SWAN!

THE THREAT FROM LINEAR

http://www.ll.mit.edu/LINEAR/

The sky visible from the northern hemisphere has been exhaustively searched by LINEAR. Australia may be the last sanctuary left for visual comet searchers….is this correct?

Mr Shigki considers the problem….The area of sky outside LINEAR’s coverage is fairly large. In spite of this, there have been hardly any visual discoveries from the northern hemisphere. This may be because comets usually enter LINEAR’s search area at least once before they attain brightness possible for visual discovery. This will enable LINEAR to find comets. Although it is not impossible to discover comets visually from the northern hemisphere, the probability if visual discoveries must be now diminished to a fraction of what it used to be.

COMET DISCOVERIES BY LINEAR AND AMATEURS

Consider the relation between the number of discoveries by LINEAR and solar elongations over a period between the beginning of search by LINEAR and the end of 2002. The total number of discoveries is 99. Most comets were discovered at a solar elongation of more than 80 degrees. Comets discovered at a solar elongation of less than 80 degrees count two in the morning and one in the evening. Among those, two were discovered at a solar elongation of 75 degrees and 70 degrees. It indicates that the areas of sky within a solar elongation of 80 degrees are most likely outside LINEAR’s search coverage.

After LINEAR started to operate at full capacity, four comets were visually discovered at solar elongations of more than 80 degrees; three in the morning and one in the evening. Of those four, three were discovered in the southern sky; C/1998 P1 Williams (Australian), C/1999 H1 Lee (Australian), and C/2000 W1 Utsunomyia – Jones (Japanese and New Zealander). We tend to think the southern sky is beyond the reach of the Japanese observers, but, as explained later, south of -30 degrees in declination are outside LINEAR’s coverage. C/2000 W1 was discovered at a solar elongation of 83 degrees, but the declination -41 degrees at the time was outside LINEAR’s coverage. The last of the four comets discovered at solar elongations of more than 80 degrees is C/2002 O4 Honig. This comet was discovered within LINEAR’s search area. As explained later, when you closely examine LINEAR’s search area over a period of one month, you will notice that a considerable amount of sky has not been searched even though the solar elongations are more than 80 -90 degrees. It is likely caused by bad weather or other conditions. As the area of coverage by LINEAR for one night is limited, a comet can avoid detection by LINEAR depending on its direction of motion, speed, and pattern of brightening. C/2002 Honig was discovered because it literally went through LINEAR’s search nets. Note that most of the discoveries by amateurs were made at solar elongations of less than 80 degrees, outside the coverage by LINEAR. This had been even before LINEAR commenced its operation. As regards the search areas, visual search by amateurs and LINEAR’s coverage do not overlap and there is not much competition between them.

Discoveries by LINEAR and amateurs show that there were twice as many discoveries in the morning than in the evening. I have read such claims in a book that there are more transparent, less light – polluted, and observers have better physical conditions after a night – long sleep. However, this argument is conventional and unconvincing. LINEAR’s telescopes are installed at locations where meteorological conditions are excellent and very little light – pollution exists. And, as they are telescopes, it is irrelevant if the observer has had a good sleep or not. It will be more logical to argue that more new comets appear in the predawn skies than in the evening. This author has no knowledge of convincing theories presented to explain this fact. A conventional thinking goes as follows: An observable part of the evening sky (or constellations) in the west at any particular time is continuously observable for several months prior to that time without any interference from the glow of the sun. On the other hand, an observable part of the predawn eastern sky at any particular time of year is not observable for about two months prior to that time hidden in the glow of the sun. Therefore, at predawn the part of the sky hitherto hidden continuously emerges out of the glow of the sun resulting in more discoveries. However, there is no way to know if this theory is correct. I have once heard that LINEAR has difficulty in detecting diffuse objects and that diffuse comets not detected by LINEAR had been discovered by amateur observers. When a stellar object and a comet of the same magnitude are compared, the stellar object with its light concentrated at a point is easier to detect than a comet with extended, diffuse light. This is the same photographically and visually. But the seemingly convincing argument that LINEAR cannot detect comets with extended light is simply nonsense.

ANALYSIS BY TSUTOMU SEKI

About the issue of whether visual discoveries are possible under the gaze of LINEAR Mr. Tsutomu Seki, the head of OAA Comet section, has attempted an analysis based on the presumption that, of the six comets Mr. Seki has discovered, those at solar elongations of more than 90 degrees with magnitude 18 or brighter would be detectable by LINEAR. He concluded that LINEAR could have detected one comet without any doubt, one without certainty, and could not have discovered the other four. He has also analysed C/1999 A1 Tilbrook (discovered on January 12 1999 UT) and C/1999 N2 Lynn (discovered on July 13 1999 UT). He points out that LINEAR was not able to discover C/1999 A1, in spite of the fact that on December 5, 1998 it was located at +78 degrees in declination with the magnitude being 12 and a solar elongation of 101 degrees. This implies that northern hemisphere comet hunters had a good chance of discovering this comet. He continues that LINEAR could not find C/1999 N2, which was located at -36 degrees in declination in October 1998 at a solar elongation of 120 degrees with magnitude 16, though the solar elongation diminished after that. As I referred to earlier, LINEAR was probably unable to discover it because it could not cover the area south of -30 degrees in declination.
Mr. Seki has also analysed C/2000 W1 Utsunomiya-Jones (discovered on November 19 UT) and P/2001 Q2 Poetries (discovered on August 18 UT). He states that LINEAR could not discover C/2000 W1, although the comet was located at -9 degrees in declination with a solar elongation of 89 degrees at magnitude 11. P/2001 Q2 had maintained small solar elongations up to the discovery and been outside LINEAR's search area.
Mr. Seki's analysis shows that including the comets prior to the start of search by LINEAR, there were many comets which were not discoverable by LINEAR and those which were discoverable but not discovered.

Figure 2: Sky coverage by LINEAR for dark period 6 (June), 2001

LINEAR’S SEARCH AREA

At the beginning of this article are Mr. Yoshida's remarks that state: "The sky visible in the northern hemisphere has been exhaustively searched by LINEAR." How justifiable is his claim? How many of the comet hunters know the search areas that LINEAR covers? As I explained earlier, outside LINEAR's coverage is the area of sky of a solar elongation of less than 80-90 degrees. I would like to elaborate on the search area covered by LINEAR here.
At LINEAR's website the following facts are provided: Search by LINEAR is conducted on moonless dark nights to detect moving minor planets (comets) by photographing the same fields at several times per night. LINEAR's search areas over a period of one month at 6th dark period, 2001 (June) and the 12th dark period, 2001 (December). The vernal equinox is at the centre ach illustration displayed in equatorial coordinates. The coloured area is LINEAR's search area with the ecliptic drawn in black. The lighter the colour is, the fainter the limiting magnitudes of the search plot. The darkened areas are outside LINEAR's coverage showing that the part of sky south of -30 degrees in declination is excluded from search. I heard from a number of people that the Milky Way was outside the coverage, but as shown in Figures 2 and 3, the Milky Way is within LINEAR's coverage. You can see this by comparing Figures 2 and 3 with star charts. I also heard that finding comets would be difficult in the Milky Way. Some people asked me if I had deliberately searched the Milky Way to find C/2002 E2 Snyder-Murakami thinking that LINEAR could not perform well in the Milky Way. It was not in my mind at all at the time of the discovery. However, I have a tendency to search the Milky Way, if I have a choice, because I can enjoy that part of the sky strewn with so many stars.

Figure 3: Sky coverage by LINEAR for dark period 12 (December), 2001

Regarding the remark that comet discoveries are difficult in the Milky Way, I asked Mr. Akimasa Nakamura, as I did not have much information on it. Mr. Nakamura replied:

"LINEAR's algorithm for detecting objects in motion is different from that of other surveys. Normally, detection of objects in motion is made from a number of frames. In case of LINEAR five frames of the same field are stacked up and 'five stars in a straight line' are picked up as an object in motion. Because of this, new objects are not often missed, even if they overlap with stars. It can also search the summer Milky Way without any problem, while other surveys tend to avoid this part of the sky."
LONEOS and NEAT avoid the Milky Way in their search. The areas searched by these surveys including LINEAR for the latest one-month period are made publicly available at the NEO page of MPC (Minor Planet Centre) (The data will be removed after one month.)

Figure 4: The boundary of search coverage by LINEAR at the end and start of twilight in June 2001

Figure 5: The boundary of search coverage by LINEAR at the end and start of twilight in December 2001

According to this, LINEAR's coverage extends roughly east and west almost symmetrically from the position of opposition. An example of search for a period of approximately one month shows that the search begins with a strip of the sky about 10 to 15 degrees wide (about 25 degrees wide at the northern-most strip) per night parallel to the celestial equator between +80 degrees and -30 degrees in declination. After completing this, it searches a strip of the sky 30 degrees wide at most on both sides of the ecliptic parallel to the ecliptic. Once it has been completed, it resumes search along the celestial equator. No search is conducted for about 5 days before and after full moon.
Figure 4 shows LINEAR's coverage over a period of one month at the 6th dark period (June), which is projected onto the celestial sphere at latitude 36 degrees north at the time of new moon (June 21, Japan Standard Time). The centre of the concentric circles is the zenith and the outermost circle represents the horizon. The area west (right) of the broken line is outside LINEAR's coverage at the end of twilight. The area east (left) of the solid line is outside the coverage at the beginning of twilight.
In other words, the area outside LINEAR's coverage at the end of twilight is the part of sky below 40-60 degrees in altitude (but occasionally near the zenith) west of the meridian. At the beginning of twilight it is the part of sky below 30-65 degrees east of the meridian. The boundary in the illustration is based on the aggregate of all the areas that LINEAR has searched approximately over a period of one month. Please note that the search area on any given day is naturally smaller than this.
LINEAR's search area is often determined by lines drawn along ascension and declination. As a result, the search area on the celestial sphere is bordered by curved lines. In Figure 4, though, the boundary of the search area (Figure 2) is indicated by a series of straight lines joining the representative points on the boundary. I believe this is sufficient for the purpose of this paper.
Figure 5, like Figure 4, shows LINEAR's search area over a period of one month for the 12th dark period (December), based on Figure 3. On a night of new moon, the search area at dawn covers excessively low altitudes and the evening search reaches excessively high altitudes. As the information on the actual search area was not obtainable, I set the date for December 23, JST, with the first-quarter moon so that the broken line and dotted line become symmetrical. At the end of twilight outside LINEAR's search area is the part of sky below 40-80 degrees in altitude west of the meridian and at the beginning of twilight the part of sky below 30-60 degrees east of the meridian.
Between summer solstice in Figure 4 and winter solstice in Figure 5, there is not much difference in the altitude distribution of LINEAR's search area at the beginning of twilight and the end of twilight. This applies to other seasons and there is almost no difference throughout the year. I think that many comet searchers spend one to two hours at a time searching the eastern morning or western evening sky below 40-50 degrees. This part of the sky does not overlap much with LINEAR's search area. Further, in the example of June 2001 a considerable amount of dark areas, which means unsearched, is seen within LINEAR's search area. C/2002 O4 Honig, which I referred to earlier, might have slipped through such areas or it might have fortunately moved through an area not covered by LINEAR. Alternatively, it may be the case that the summer when this comet was discovered was during the rainy season in the state of New Mexico where the telescope of LINEAR was installed and that it may have resulted in reduced operation.

A NEW THREAT

LINEAR is the most effective automatic search system at the moment, but a more effective system is expected to emerge in the future. It has been already announced that one of such new systems will start to operate in 2006 in Hawaii.

It is a system to combine four 1.8m-aperture telescopes and called Pan-STARRS (Panoramic Survey Telescope and Rapid Response System).
(See Figure 6) Each telescope is equipped with CCD cameras containing 1 billion pixels.

Figure 6: An illustration of Pan-STARRS

It is expected to reach the limiting magnitude of 24 with a 30-60 second exposure covering an area of 3 degrees in diameter (7 square degrees). Its search area will amount to 3000 square degrees for one night and in two weeks it covers 10,000 square degrees three times. Because of Hawaii's low latitude, it can cover 70% (28,000 square degrees) of the whole sky.
When I received this information, I felt that this was a death knell to comet hunters. We were given only three years of life. Although what actually will happen is difficult to foresee until the new system starts to operate, I found the following passage in the paper I had downloaded from its home page:
"If we restrict observations to zenith distance of less than about 45 degrees then the total sky available from Hawaii is about 30,000 deg2. The visible sky on any night, say within 4 hours of opposition for concreteness, is then about 10,000 deg2, of which we can observe about 30% in a single night."

The total area of the sky is about 41,000 square degrees and at any given time a half of the sky is visible from any place on the earth. Therefore, if we assume that about 10,000 square degrees is the area of the searchable sky, a considerable amount of low altitude skies will be left unsearched. In this respect there is not much cause for concern compared with LINEAR's impact.
However, the new system's limiting magnitude is 24 for stellar objects, 4 to 5 magnitudes fainter than LINEAR's. Therefore, cometary objects brighter than magnitude 22 are discoverable. This will hurt comet hunters' chance. According to Mr. Seki's analysis, quite a number of comets have not entered in LINEAR's search area. Some comets, like C/2002 O4 Honig, slipped through LINEAR's search area. Therefore, if Pan-STARRS and LINEAR adopt similar search patterns, there will be many comets which stay outside Pan-STARRS's search area or slip through the area of its search. For this reason I never believe that the chances for visual discovery of comets come to naught by the emergence of Pan-STARRS.

It is expected to reach the limiting magnitude of 24 with a 30-60 second exposure covering an area of 3 degrees in diameter (7 square degrees). Its search area will amount to 3000 square degrees for one night and in two weeks it covers 10,000 square degrees three times. Because of Hawaii's low latitude, it can cover 70% (28,000 square degrees) of the whole sky.
When I received this information, I felt that this was a death knell to comet hunters. We were given only three years of life. Although what actually will happen is difficult to foresee until the new system starts to operate, I found the following passage in the paper I had downloaded from its home page:
"If we restrict observations to zenith distance of less than about 45 degrees then the total sky available from Hawaii is about 30,000 deg2. The visible sky on any night, say within 4 hours of opposition for concreteness, is then about 10,000 deg2, of which we can observe about 30% in a single night."

The total area of the sky is about 41,000 square degrees and at any given time a half of the sky is visible from any place on the earth. Therefore, if we assume that about 10,000 square degrees is the area of the searchable sky, a considerable amount of low altitude skies will be left unsearched. In this respect there is not much cause for concern compared with LINEAR's impact.
However, the new system's limiting magnitude is 24 for stellar objects, 4 to 5 magnitudes fainter than LINEAR's. Therefore, cometary objects brighter than magnitude 22 are discoverable. This will hurt comet hunters' chance. According to Mr. Seki's analysis, quite a number of comets have not entered in LINEAR's search area. Some comets, like C/2002 O4 Honig, slipped through LINEAR's search area. Therefore, if Pan-STARRS and LINEAR adopt similar search patterns, there will be many comets which stay outside Pan-STARRS's search area or slip through the area of its search. For this reason I never believe that the chances for visual discovery of comets come to naught by the emergence of Pan-STARRS.

MR SEIICHI’S CONCLUSIONS

I wrote this article because I wanted to tell as many comet hunters as possible to pursue their dream of finding new comets and continue their search. This may increase competitors, but it would be great if I could share my dream with other comet hunters. In fact, through my discovery I got to know many people and was able to exchange views with them, which in turn helped me writing this article.
I hope I have convinced the readers that it is premature to abandon your dream now. Starting with the exchanges with Mr. Utsunomiya, I have argued that SWAN is not a threat to comet hunters. The division of the search areas by LINEAR and amateurs has naturally developed without much overlapping. There is still some hope for discovery by amateurs even after Pan-STARRS begins to operate.
I am fully aware that there are other factors which make comet hunting impossible, such as one's work, family, residence, etc. I myself can be transferred and forced into an impossible situation. However, in recent years technological advancement in CCDs and other areas has been remarkable, making spectacular photography of comets possible even at urban sites. With advanced technologies someday you may have a chance of discovery from cities if you continue to pursue your dream.
It is certain that comet hunters can survive for some time in the future and the chances of new discoveries remain strong.

The NEO sky survey and the amateur comet hunters can discover 68% to 76% of the near earth comets (perihelion distance, q<1.0). Most of the undiscovered comets are small ones with faint absolute magnitude. Note that the NEO sky survey misses many near earth comets. Without the amateur comet hunters, the discovery rate of near earth comets will decrease to 60%. The amateur comet hunters compete with the NEO sky survey, and also complement it.

Discovery rate of near earth comets by simulation - visual limit magnitude 10.0

Discovery rate of near earth comets by simulation - visual limit magnitude 12.0

Comet hunters can survive for the time being, though the discovery rate reduced to one third in the northern hemisphere after NEO sky survey. In the southern hemisphere if the search becomes active higher discovery rate is expected than the statistics to date.

When Kaoru Ikeya discovered his 6th comet 153P Ikeya-Zhang, 35 years has been past since his last discovery (Sky & Telescope, July 2002, p.70-73). Shigehisa Fujikawa has spent 24 years to find his 5th comet that named him, C/2002 X5 Kudo-Fujikawa. Both of them continued comet hunting for decades of years without hesitation. Tsutomu Seki emphasizes importance of self-confidence and dedication; "Strong rivals exist at any era and only comet hunters who fight bravely and attack openly are crowned with victory".

Unless complete sky coverage of the NEO sky surveys is realized, amateur comet hunters can survive but they have to swim with the sharks. Comet hunting goes on.

FUTURE OF VISUAL COMET DISCOVERIES

In his latest book David Levy (discoverer of 21 comets) summaries……

Comet hunting has utterly changed in the 37 years since I started comet hunting in 1965. Back then, the threat to visual searches was from photographic surveys, which were succeeding in finding most of the comets. However, these efforts were concentrating on the sky at opposition to the Sun, and not terribly thoroughly at that, leaving many comets available for visual observers. Even after the enormous productivity of the great Palomar photographic surveys of the 1980s and 1990s - surveys specifically designed to discover comets and asteroids - until a few years ago it still seemed possible for comets to be found visually.

Although it is certainly still possible to discover a comet visually, it is far more difficult to do so than it was even a few years ago. Some comets happen to come in at a shallow angle to the Sun that hides them from the big surveys. But the date and time of the last visual comet discovery is approaching. I still believe that there will always be comets for the amateur to seek and find through his or her visual telescope. It's harder than it used to be, and many searchers will give up. In 1967, Robert Burnham Jr., who discovered comets both physically and photographically, advised me that "If you hunt long enough, stay away from the galaxies in Virgo, and never give up, some day you will find a comet." I think this advice still holds, although that "some day" could be decades into the future for most searchers.

As you hunt for comets, remember that it isn't a good idea to have the discovery of a new comet your only goal. In 1997, Leif Robinson, then-editor of Sky and Telescope, came down pretty hard on single-minded searchers. "I've never had any great admiration for comet hunters," he editorialized. "To spend hundreds of hours in failure for each minute of success never seemed like a good deal to me. ... For amateurs there's the allure of getting your name hitched to a star, albeit a hairy one. If you're very lucky, like Thomas Bopp, your name might appear in textbooks for years." If the only reason you spend all this time comet hunting is to find a comet, then Robinson has a point. So did the great Japanese comet hunter Minoru Honda, who discovered 12 comets and 11 novae during his lifetime, in the advice he gave Kaoru Ikeya before the young comet hunter made his first discovery. "If you desperately want to find a new comet, please stop your search because you may never be able to find a new comet. However, if you are content to search the sky without ever experiencing a new comet discovery, please keep searching because someday, you may be able to find a new one." Leslie Peltier, in Starlight Nights, perhaps said it best:

 “In spite of this increasing competition there always will be comets for the amateur to seek and, in some facets of this work, he still has an advantage. In a given time he can cover far more sky than can the camera, he can know within half an hour the true nature of a suspected object and he can search much closer to the sun in regions which would fog a photographic plate.”

COMET HUNTING VIA THE INTERNET

Maybe you are the kind of person who doesn’t like the idea of spending hundreds of hours out in the cold for years with your telescope searching for comets visually the old fashioned way. Perhaps you are the kind of person that likes to stay warm and cosy indoors, or are you the type of person that spends a lot of time on his/her own computer? But would like to discover a comet? Well as it happens you can do that and all you need is a computer and an internet connection! Using the near real time images gathered from the LASCO (large angle spectrometric coronagraph) C2 (red) and C3 (blue) images you can examine each individually or combine a series of images together to check for any sun grazing comets that may be close to the sun. Moat of the comets on these images are very faint tail less spots or star like though the occasional brighter comet can sport a large tail. As with visual comet hunting, persistence and patience are the key. Even if you do not download the latest series of images in a movie format at least make the habit of checking the latest C2 or C3 image everyday. If you are a fan of the www.spaceweather.com web site then while you are there, scroll down and click on the SOHO link and quickly check the latest images showing the comet rich sky in the suns vicinity….with regular checking you never no you luck. On two separate occasions I was lucky enough to have independently ‘discovered’ a comet on the C3 images however I was beaten by two days by the serious SOHO comet seekers. Both these comets were lovely bright objects with long tails and as such where easy to spot but note that the majority do not appear this obvious however with regular practise you can make a discovery if you  get into it in a serious way. Bare in mind that if you do make a comet discovery it will be named after the instrument that finds them (SOHO or SWAN) rather than your own surname which is the prize of the amateur comet discoveries in the night sky using privately owned equipment. Out lined below is an introduction to SOHO comet hunting…..I hope you take it up!

1. Equipment:

There are some things you need to start your search for comets:

- Computer with internet connection and mouse (fast connection preferred)
- Image viewer with features listed below or movie player
- High-resolution monitor, as big as possible (>15" as an absolute minimum)
- Paper and pencil for notes
- and last but not lease: patience!


Features the image viewer must have:

- Getting coordinates in pixels (not only inches or cm)
- image looping or blending (whereas looping should be preferred)

2. Important note:

It is important to keep calm. You mustn't have the expectation to find a comet at your first try. It may take you weeks or months till you finally catch your first discovery. However there are some examples that one found two comets within his first days (Mike Oates). Also, do not post known comets! Only the first one gets credit. Therefore always check the report page before posting something!
Doug Biesecker, formerly responsible for SOHO comets, once summarized: "Please, don't guess. Don't speculate. Don't hope. Watch comets which have been confirmed before trying to report one your self. Examples can be found below.

3. Common terms:

LASCO

Large Angle and Spectrometric Coronagraph Experiment. That's the telescope that exposes the images.

C2

CCD camera that shows the solar corona within XXX solar radii (low angle camera)

C3

CCD camera that shows the solar corona within XXX solar radii (wide angle camera)

FITS

Flexible Image Transport System. Special image file format used to activate the images

Real time

Latest images available on the SOHO homepage

(Kreutz) sungrazer

Comet that moves in a specific direction (from anywhere on the bottom towards the sun). Most of the SOHO comets are of that type

non-Kreutz comet

Comet that has a different motion (might be seen everywhere). This type of comets is quite rare.

FOV

Field of view. The area covered by C2 or C3 CCD

X/
X-comet

Object that might be real but is not confirmed due to violation of the four image limit.

4. First step:

First you have to download some images. They should be consecutive ones with no image missing. Usually C3 images come in twice per hour at xx: 18 UT and xx: 42 UT while C2s are available three times per hour at xx: 06 UT, xx: 30 UT and xx: 54 UT. Sometimes one of those images is not downloadable. This usually happens when the SOHO Team does some calibration work (dark frames or flat fields or polarized images) which is not interesting for us. Common gaps appear around 01:00 UT, 07:00 UT, 13:00 UT and 21:00 UT. It may, however, occur that there are quite big gaps between two images because of delays in receiving data from the spacecraft.
Images are available in different resolutions on the SOHO website:

Images

useful resolutions

C2 .gif

512 x 512 pixel
1024 x 1024 pixel

C3 .gif

512 x 512 pixel
1024 x 1024 pixel

Movies

useful resolutions

*.mpg

512 x 512 pixel
1024 x 1024 pixel

*.avi

512 x 512 pixel
1024 x 1024 pixel

Java

512 x 512 pixel
1024 x 1024 pixel

I personally prefer single images since the movies are too large (~ 35 MByte) to download. I also encourage the use of 1024 x 1024 resolution images as they are more detailed and so faint objects are easier visible. For more advanced comet hunters 512x512 is also acceptable.

5. Second step:

After having received the images, you have to work with your image viewing software. There are two common ways to search through the images:

1. Looping: That's the most popular way. Simply create an animated .gif from your images. It's useful to loop 3 or 4 images as it is possible that there are objects which seem to be comets in two images but vanish in a third and fourth. It needs a bit of practise to handle the looped images and find possible comets within them. If you don't have the possibility to create animations try to open different windows and loop them by hand. It's not perfect but it may do.

2. Blending: This alter SOHO comets is not that common. You take three or four images and combine them with the "blend 50%" option. So you get one image where stars can be identified as dots closely together in horizontal direction, where as comets should have another direction. It may helpful to give different colours to the images to get a typical colour series for moving objects and an untypical one for noise. However this method may fail if searching for faint comets as they may vanish when combining.native way of searching for

Note: The more you process your images the more noise will mimic as an apparently real object. Therefore don't process your images any further. You won't extract more information from the .gif images by over-contrasting them.

6. Third step:

If you think you have found a possible comet there's a simple way of checking if it is real and worth to report:

1. Self – criticism: Look at the object. To be considered as a real object it has to full fill the following minimum criteria:

Minimum criteria

1. Object has to be visible in at least four consecutive images.

2. Object must be constant in brightness and shape or change in a predictable way.

3. Object must have a reliable orbit (not too fast)

4. Consistent positions (no jumping or strange acceleration/deceleration)

If the object doesn't full fill all of the points DO NOT submit your observations since the object is most likely not real. There are often single image cosmic ray hits which appear only in one image and are reported as comets by inexperienced observers. Planets and sometimes stars might look like comets but with a little bit of knowledge such fail reports can be avoided. You can see examples of planets, stars, spacecraft debris or cosmic ray hits on the report form page. It has to be noted that from experience all SOHO comets, ordinary Kreutz sun grazers as well as other comets, do change their C3 positions by less then 15 pixels from one image to another (for C2 the change is less than about 20 pixels). Always check if your object fits this "empiric law". An additional thing that newbies always should be aware of is the fact that more than 95% of all SOHO comets do not show up like one might expect a comet to look like, i.e. no tails. Tailed objects are most likely cosmic ray hits.

2. Checking for known objects: If the object is within the minimum criteria you have to look whether it has been already reported or whether it is some other known object passing through the LASCO FOV (e.g. a transient planet, known comet or gaseous nebula within the Milky Way. To check the former two you can use this site.).

Already mentioned but important enough to say it again: Please do not report if an object has already been reported! Only the first one gets credit! You can find the known objects on the comet discoverers' site. To check for these you have to look for recent "Potential comet" posts. Real objects are often confirmed quite quickly by "Confirming comet of" reports or red posts by Mr. Sungrazer. To check whether your object is one of those simply extrapolate the positions to your date and time with respect to your coordinate system. At the beginning this may take some minutes but after some practise you will see it without time consuming calculations.

3. Reporting: You still think it is an unknown comet? Well, might be. Now you have to report your observation. The report must be put in the official report form on the sun grazer homepage.
Your reports must include some data:

- LASCO camera: C2 or C3
- Date
- resolution: 512x512 or 1024x1024 and origin of your coordinates (usually upper left corner or lower left corner)
- Image and coordinates: time of each image and the position of the object
- Your name

Some of the required data can be inserted by choosing the corresponding down scroll options, the rest should be written "by hand". Do not forget to choose your name at the end of the report form. As a new user you are supposed to select "New user" and fill in an additional form after having reported your object. This form is supposed to contain your name and email address. After Mr. Sungrazer receives it you will be added to the list of names and a note about the new user will appear on the report page. Never forget to press the "submit" button after your report appears in the lower window. Further help on reporting can be found the report form page.

4. Waiting for confirmation: That's the hardest time if you are not sure about your report. But you can use it constructively! Just download further images (previous or following) and recheck. If you can't find it anymore just retract your report. It's better to retract the report than letting other people do confirmation work for something that doesn't exist.
If it is a real object you will get confirmation soon - either by one of the other observers or by SOHO Team member KB alias Mr Sungrazer who is responsible for the comets. The confirmation (or rejection) will be posted on the website where you posted your report.

INFORMATION NEEDED TO REPORT A DISCOVERY

We need to know the date, time, and position of the object for at least one image. We also need to know which telescope (e.g. C2, C3) it was observed in and the origin of your coordinate system (e.g. (0, 0) in top-left corner). Use the format of past comet discoverers if you are unsure how to make a report.

The object should appear in at least 4 frames to be considered real.

Click here to use a web based tool to measure positions on real-time movies..

The form for reporting comets is at the bottom of this page.


COMMON MISTAKES

There are a variety of common mistakes which inexperienced observers make. They report:

  • 1) Stars, planets, or other known objects.

In every LASCO image you can observe objects whose trajectories are almost exactly horizontal. The majority of these objects are stars. Every C3 image has approximately 200-300 visible stars and every C2 image has approximately 10-30 stars visible. Stars will always move from left-to-right (due to SOHO's orbit about the Sun) at a speed of a few pixels/hour. Occasionally, planets appear in LASCO images and they, too, have a nearly horizontal motion. However, planets can move left or right through the field-of-view and can be faster or slower than the background stars. A list of expected planetary transits is provided at the 'C3 Transits' link. This link also provides dates of expected asteroid appearances in LASCO images. Overall, if you believe you have discovered a comet and its motion is nearly horizontal, chances are you've actually located a star, planet, or asteroid. Check the 'C3 Transits' link for known objects or use an ephemeris program to identify known stars, planets, asteroids, or comets. Click on the image below to see examples of stars and planets visible in a LASCO image.


Note: The "rings" seen around the planets are not real. They appear because the planets frequently saturate the CCD, causing a bleeding of pixels along a single row.

  • 2) cosmic ray noise

Cosmic rays are very high energy particles which come from a variety of sources (e.g. solar flares, supernovae). They are of interest to lots of astronomers, but mostly just noise for us. We see lots of them in every image and occasionally, 3 or 4 of them hit the CCD in just the right places in consecutive images to fool us into believing there is a real object there. The cosmic rays can show up as points or as streaks. (http://helios.gsfc.nasa.gov/cosmic.html). Click on the image below to see typical examples of cosmic rays visible in LASCO images.


Note: ellipse = cosmic ray, circle = star.

A solar flare will heat and eject particles radially outward along open magnetic field lines (called solar energetic particles or solar cosmic rays). Every so often a large flare, positioned so that it is magnetically connected to SOHO, will produce a flood of cosmic rays that will completely blind our CCDs with noise. These somewhat rare events are called "proton storms" and can render our CCD observations useless for days. For an example of a proton storm, click on the image below for a C2 movie of the "Bastille Day Event".

  • 3) debris streaks

Debris can refer to astronomical or human-made debris. The debris may be dust or spacecraft insulation which has flaked off. Visit here http://lasco-www.nrl.navy.mil/debris.html for more detailed information. The image below provides a typical example of debris.

  • 4) Objects already reported by someone else.

To see what others have reported, please see the recent reports page.

The mistakes listed above, although common among inexperienced observers, can be avoided. To avoid the above mistakes a new observer must be willing to spend time studying the typical speed, location, morphology, and brightness of comets that LASCO observes. In other words, study as many past comets as possible before you begin looking for new ones. Also, there are several resources at your disposal which can make the learning period less frustrating. A chat site for LASCO comet finders has been established at http://groups.yahoo.com/group/sohohunter/. Here one can ask questions and pick up tips from past LASCO discoverers who have years of experience. Also, Sebastian Hoenig, one of our experienced comet finders, has created a tutorial on 'How to Discover SOHO Comets' that is made specifically for new observers. The tutorial can be found at http://www.sungrazer.org. By studying past comets, avoiding the "common mistakes", and using the resources listed above the inexperienced observer will be able to discover LASCO comets much faster than those who search before acquiring the proper practice and knowledge.


Where should I look in the images to find comets?

A large percentage of LASCO comets belong to one of four known groups (Kreutz=85%, Meyer=6%, Marsden=2%, and Kracht=2%). A group's orbit is fixed in space with respect to the solar system but its location in LASCO images will oscillate annually due to SOHO's orbit about the Sun. Therefore, Mike Oates and Rainer Kracht have put together some web pages for the various groups showing the expected trajectory of the LASCO comets in the C2 telescope. Note how the trajectories vary throughout the year. Similar figures were published by Brian Marsden in his 1989 paper (Astronomical Journal, 98 (6), 2306-2321).

Kreutz trajectories

Other groups

It is recommended that beginning observers concentrate on the Kreutz group because they are far more abundant than the other groups. Also, Kreutz comets are typically easier to locate due to an apparently slower speed during peak observing months (geometric effect) and a more pronounced cometary appearance in LASCO C2 images. In contrast, the other groups' comets usually appear as condensed points that blend with background stars and cosmic rays. Overall, be extra sceptical when suspect comets do not appear along the predicted paths of the known comet groups.


Current tracks of SOHO-observed comet groups in LASCO C3 and C2

The area indicated on the below images (click for a larger image) shows where the comets from the four major SOHO-observed group are currently being seen in the LASCO images. This area will change from month-to-month. The images will be updated appropriately.
Note: These images are designed to give an approximate idea of where to look for SOHO comets in any given month. Some group members may fall slightly outside the indicated area(s). Also, Marsden, Meyer and Kracht comets are occasionally seen in C3, but very rarely, hence their exclusion from the below images.

C2 Kreutz-group

 

C3 Kreutz-group

 

C2 Marsden-group

 

 

     

C2 Meyer-group

 

C2 Kracht-group

 

A FINAL WORD

I hope that after reading the above information any sceptical observers will be re – educated regarding the chances of further visual comet discovery for the time being in the northern hemisphere. I hope that anyone out there who has abandon there dream of comet discovery will re consider there decision and take up this fabulous rewarding past time. With my previous two articles (visual comet hunting and observing comets) I hope that the reader will feel better equipped and more confident by the field of comet observation and discovery. Happy hunting!

Martin McKenna

 

Martin McKenna

Contents and sketches are original work by the author

 

 

 

 

 

 

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