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| My aim for comet image processing is to get an image of the comet where the nucleus is in 1 position among stars with no trails. I have a newly acquired camera that allows me to take guided shots. Question is should I allow my equatorial mount to track the sky and take many short exposures and forget about guiding? Alternatively, should I try to guide on an adjacent star or even the comet itself and get fewer exposures to achieve the desired result? |
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Comets are my favorite target because they change all the time. When moving really slowly against the stars, taking lots of short exposures works well and reduces the ultimate background noise when stacked. However, when moving more than a few minutes of arc in say 10 minutes, taking longer exposures fixed on the stars is probably a better technique. You then need software (DSS, PixInsight, and/or photoshop) that can fix on the comet and stars separately and then add the two images for a near stationary, fixed in time looking image. Some like to also fix on just the comet and have the stars trail: See my: http://www.astrobin.com/236351/0/ for example. For example see: http://www.harrysastroshed.com/pixinsight/pixinsight%20video%20files/2013%20pix%20vids/cometstack2/cometstack2.mp4 Jan |
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Thanks Jan, I've been intrigued by capturing comets since I've had a telescope. I found out very quickly how difficult it can be. I just took 100 short exposures (30 seconds) of comet Catalina and will use the work plan in the excellent video you suggest. Stacking the comet and stars separately seems promising. I have had no success with DSS as it never manages to keep the comet in 1 spot. Hopefully I do better with Harry's video and PI. Thanks again. |
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Comets are always a tradeoff between exposure length and keeping the comet sharp. If you want sharp stars, i.e. not trailing, then you should guide on a star, but will have to limit your exposures to some short value like 30 seconds or 1 minute, depending on how fast the comet is moving. Much longer than that and you risk smearing the comet data. Comet Catalina, for example, has a very rapid north movement component, something like 300 arcminutes per hour. I've not had 100% success with DSS stacking stars and comet separately...it always seems to have trouble isolating the tail from the stars, and I end up with some artifacts around bright stars, or streak of tail material. Alternatively, if you don't mind trailing stars, then you can try guiding on the comet, using PHD2 and it's comet tracking tool. I've used this with success on Catalina up to 8 minutes exposure. Unfortunately, DSS won't be able to stack these because of the trailed stars. I'm still looking into some other method of manual stacking them. |
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| There is another method giving you a chance for unlimited exposure time for comet - tracking/guiding by comet. Latest PHD version has this option. I am using ASA DDM 60 direct drive mount with built-in comet tracking module I and want to say- its working perfectly! Just enter the delta RA and delta DEC parameters and go.. some of the examples you can find in my gallery. Than processing is simple, as you can align comet picture ( also LRGB combination made by mono camera) with Comet Alignment module of PixInsight ( Pleiades Astrophoto). Using MEDIAN to stack you will have stars removed from comet picture and - if you are lucky - comet removed from stars picture. |
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also - check thru tony cook's images here, in many of the descriptions of his comet images he's given some hints about comet image processing. Tonk |
