EAAS Meeting Report of Monday 9th January 2006

"Looking Deeper into Space"

The first meeting of the new year was on the 9th January. Paul Evans started the meeting with a short introduction about upcoming events including the NASA "New Horizons mission to Pluto and the Edgeworth-Kuiper Belt" which is due for launch this month. Paul also congratulated EAAS member Martin McKenna on his excellent achievement of being named IFAS Astronomer of the Year for 2005 and then gave a short introduction about the main speaker, Mark Stronge.

Mark’s talk was titled ‘Looking Deeper into Space’ and dealt with Deep Sky Astrophotography. He began the talk with issues and concepts surrounding visual observing. He discussed telescope apertures outlining that bigger is always better for visual observing and explained how to work out your visual limiting magnitude - (7.5 + 5 x log Telescope Aperture (in cm)). He also outlined that the photographic limiting magnitude is only limited by the length of exposure and sky conditions. He then explained the differences in focal ratios with fast ratios being from f/2 to f/5, medium from f/5 to f/8 and slow being f/8 and above. Telescopes with fast ratios are very good for observing deep sky objects whereas scopes with slow ratios are excellent for planetary and lunar observing. Mark then discussed the factors that can limit your visual limiting magnitude including cloud and light pollution. He demonstrated the effect of light pollution by showing the same star field and started off with no light pollution and gradually added sky glow and the number of stars in the field became fewer and fewer.

The next section of the talk dealt with the different camera choices for astrophotography. This ranged from webcams including the Phillips Toucam Pro II and the possibility of modifying webcams to make them practical from deep sky use to CCD cameras including the Meade LPI (Lunar and Planetary Imager) and DSI (Deep Sky Imager) to digital cameras. Mark gave the Nixon CoolPix 5400 particular praise stating that it is capable of 300 second exposures (8 minutes) for astrophotography and is a very good value for money digital camera. He then discussed the concept of image scale. Measuring the image scale is important as it helps you to match your telescope and camera. Image scale is the amount of sky that each pixel of your camera sees. He stated that under excellent seeing conditions the limiting photographic image scale is somewhere around 2 arc seconds/pixel. This is due to the moisture content and winds in the upper atmosphere. He then outlined the need to match the right combination of camera and telescope to achieve the best possible results. He recommended a website www.wodaski.com which has a online CCD calculator and a free downloadable program which indicates which is the best camera to use for different telescopes. If the image scale is too low your pictures will lack sharpness and any benefit of resolution will be lost to the limiting seeing conditions.

The next section was how to quick focus for wide field shots. Mark gave a demonstration of how to set the focus and take an exposure. He also showed how that digital SLR cameras are difficult to focus and the use of bright stars/planets and the moon can be a necessity. Mark then outlined the various considerations for wide field shots. These included composing your shot, the issue of stray light, making sure your tripod is steady and is fully tightened and to watch out for orange trees - unwanted land objects. Orange trees are caused by light pollution and appear in alot of long exposure photographs. He suggested that wide field constellation shots are the best place to start for anyone looking to get into deep sky astrophotography. Mark then showed different images that have been taken including the Milky Way, a Perseid meteor, the ISS, the asteroid Vesta, an Iridium flare, constellations, star trails and aurorae.

Mark then described a device called a scotch mount or barn door tracker. It is a device that can be made from relatively simple materials including a couple of pieces of wood and piano wire. Its purpose is that the user can take 2-3 minute exposures without suffering star trails. Mark then discussed taking tracking wide field shots. The considerations for tracking photographs included all the above in the previous paragraph with a few additional things as well. These were polar alignment, a level mount and dew. Polar alignment is essential for tracking and Mark outlined different methods of how to achieve it including drift alignment and gave a demonstration of how to polar align on his Meade LX200 10” SCT. He also showed how to use the polar scope on the Vixen GP mount of his Orion ED80 Refractor. Mark then outlined two concepts - periodic error and backlash and demonstrated how to correct for both these on his Meade LX200 and Orion ED80 mount. He also showed two computer programs Guidedog and K3CCDTools which are also used to correct periodic error.

Mark then discussed the issue of collimation. This involves aligning the optics of certain types of telescopes. Newtownian, SCTs and some refractors suffer from collimation issues. Mark then gave a demonstration of how to collimate his Meade LX200 SCT. The advantages of a properly aligned telescope are better resolution, easier focusing, and a higher limiting magnitude resulting in sharper and brighter images of the objects you are viewing. He then moved onto the issue of dampening. He outlined that a concrete surface is usually best, followed by a grass surfaces which can depend on the type of soil but one thing that can't be recommended for setting a telescope on is tarmac. Tarmac can cause the tripod to bounce at the slightless movement and vibrations can last for quite a few seconds causing focusing and imaging to be a major problem. He recommended the use of vibration isolation pads which reduce vibrations by 70-80%. He has used vibration isolation pads on tarmac and they have reduced vibrations to only a fraction of a second.

The next section dealt with fine focusing. The key concept is the critical focus zone. This is measured by the following formula:

(focal ratio)2 x 2.2 = critical focus zone in microns (one thousandth of a millimetre is one micron)

This is a very small distance and obviously different types of telescopes will have different sizes of zones depending on the focal ratio of the telescope. As a rule telescopes with fast f/ratios (f/2 to f/5) have a smaller zone than those with slow f/ratios f/8 and above so fast focal ratios are difficult to achieve focus. However, fast f/ratio telescopes can give you much shorter exposure times than with slow f/ratio telescopes. Mark outlined different techniques of fine focusing including using a focus mask and gave a demonstration of how to fine focus using his LX200. Mark then showed another set of images that involved tracking. These included M13, the Milky Way, the Cigar Galaxy, the Dumbbell Nebula, the Pleiades, M51 - the Whirlpool Galaxy, M57 - the Ring Nebula and M42 - the Great Orion Nebula.

Mark then dealt with the issue of guiding and the various techniques involved. These included using a piggyback scope - Mark has an Orion ED 80 Refractor piggybacked onto his Meade LX200. Other techniques included hardware auto-guiders and self-guiding cameras and guiding software like Guidedog and K3CCDTools. Mark then gave a demonstration of Guidedog and K3CCDTools. Mark then looked at the issue of light pollution and how to counter it. He outlined the use of narrowband filters, also the technique of stacking images using software like Registax or K3CCDTools. He also outlined the use of flat fields including techniques such as dark frames and flat field frames which can reduce noise from CCD camera and dust from the optics.

The final part of the talk was on processing images. Mark demonstrated using Paint Shop Pro and the multitude of techniques that could be applied to images to improve them. He did this using two of his own images - the Great Orion Nebula and the Pleiades. He stressed the importance of keeping the original image and not overwriting it so in future if you have greater knowledge of processing techniques, you could further enhance your image.

At the end of the meeting there were a number of questions which Mark was able to answer in detail and helped many in some of the issues they were experiencing while imaging.

Mark has setup a page of Astrophotography Hints and Tips on the EAAS website.


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