Hi and thanks for the replies. Yes on the "research quality". I am just tuning into comets and am seeing dedicated amateur work documenting things like ion tails and rapid brightenings and tail disruptions and other interesting momentary stuff so it looks like there is a niche for dedicated comet observation. I have a Canon 6D, CGEM-DX mount, backyard EOS as capture software. I have been using my planet telescope the past few months for comets which is a C-14 but I think this telescope is too slow for fine detail. Not sure.
I'm still not sure I'm getting exactly what you are working at but let me take a stab at things. And yes, I know there is a very good chance that you understand what I'm going to say better than I do, but maybe something will be helpful.
I'm assuming that you are planning to use the Canon 6D for your imaging. Wonderful full-frame camera and I am envious!
In a relatively short search I was unable to find a way to define a ROI for the 6D sensor and that means that at the moment I am going to assume you are going to want a telescope with an image circle large enough for a full-frame sensor.
Since you have a CGEM-DX it will be my assumption you wish to use that mount. Since its payload weight rating is 50 pounds and you are going to be doing at least somewhat long exposures I will apply the 50% rule-of-thumb for AP and assume an upper limit for the weight of the payload (OTA, finders, camera, etc.) of 25 pounds. Since the Canon 6D is about 1.7 pounds and you will still need a flattener or corrector as well as a finderscope, I'm going to aim for a bare OTA weight of 22 pounds or less.
Since most comets do not have very long tails I'm going to assume you are going to typically be trying to image rather dim and small targets. This means I'm going to want to try to find a scope with a rather long focal length so that you can get more detail.
The Canon 6D sensor is actually not very noisy so fast optics are not all that necessary but using the C14 at its native focal ratio gives you a focal length of nearly 4 meters. The C14 OTA I think weighs about 46 pounds. I think trying to do the long exposures using the C14 on a CGEM-DX would be extremely frustrating. And in any case I am not at all sure that the C14 image circle is large enough for a full-frame sensor (although if you are very tolerant of vignetting it could certainly be used that way).
A Hyperstar would give you very fast optics and is within your budget. It would give you a focal length of 684mm which would be somewhat more manageable. But for long-exposure AP purposes your mount would still be grossly overloaded so you may still find the whole thing frustrating. The image circle of the C14's Hyperstar is smaller than is your 6D's sensor so vignetting would be a pretty significant problem. But because the Hyperstar's cost is about $600 under your $2000 budget you could still buy a more suitable camera to use but your mount would still be overloaded for AP purposes.
Something like the 8" TPO Ritchey-Chretien (Astronomics sells one for $1400) comes to mind. You get a 16.4 pound instrument with a 1.6 meter focal length so your mount will handle it well. The focal length can give you enough image scale so that you may get some of the details you want. But at that focal length you may be significantly affected by atmospheric turbulence and tracking can also be challenging. But you can probably handle the tracking pretty well if you get a good autoguider system.
I cannot find the image circle of the 8" RC and it would not at all surprise me if you found that your full-frame sensor is bigger than its image circle so vignetting could be a significant issue. But again, since the RC would be $600 under your budget you might be able to get a camera with a smaller sensor for this use. But since you would need an autoguider as well I'm concerned we'd blow your budget.
You can get a 102mm William Optics Gran Turismo for $1997. This gets you a scope with an image circle big enough for a full-frame sensor. You have a decent focal length of 703mm which means your imaging will be comparatively easy (not nearly as sensitive to tracking errors and atmospheric turbulence as a 1.6 meter focal length instrument). You do not have the same potential for capturing detail as if you were using the 8" RC, however, since both your aperture and your focal length are much less.
The GT-102 will have field curvature problems. They do include a matched flattener/reducer. I usually like this but since for cometary imaging purposes I would probably want the full 703mm focal length, I don't think I would want to use the flattener/reducer for most of your cometary work. William Optics does sell a field flattener (FF68II) which should work nicely and preserve the 703mm focal length but it does cost nearly $600?
If not wedded too strongly toward using the Canon 6D as your camera I think this is what I would do:
I would get an 8" Newtonian astrograph for $600 or less. Plenty light for your mount. Good aperture for potential resolution. The focal length is 800mm so you just might get enough detail to satisfy you. That leaves you plenty of money left over to get something like a cooled monochrome ASI1600 or the QHY163M. You can get later add narrow-band filters and such so that you can study the comets in various wavelengths. You will, of course, have to also get a corrector for that scope's aberrations.
Monochrome means better QE, better SNR, and more potential detail which are all pluses.
Even if one were to ignore all those pluses, for scientific purposes I consider monochrome to be far preferable. When using a color camera you are dealing with some form of the Bayer Matrix and you will be using a de-Bayering algorithm to reconstruct some of the detail lost by using a color sensor. This means that for fine detail you cannot be sure that the algorithm properly reconstructed the detail and you may be getting somewhat bad data - and that is a problem if your goal is science rather than making a pretty picture.
So I see quite a few options but for scientific purposes within your budget I prefer the 8" astrograph with the monochrome ASI1600 or the monochrome QHY163.
I'll be quite interested in seeing what you choose.
Edited by OleCuss, 07 December 2017 - 10:28 PM.