I can't thank you enough for the thorough reply, Jrista. My wife is interested in starting astrophotography, and there was much useful information in that post. Sorry that darker skies aren't available in your area, though I wonder if summertime in the Rockies further west would allow access to high elevations and perhaps less atmospheric interference? The West Coast is blessed with potential access to dark sky zones from Stone Mountain Provincial Park and Wells Grey Provincial Park (both in British Columbia), all the way down to the Warner Mtns and Siskiyous in northernmost California. Go west, young man, go west!
It isn't that dark skies are not available...within about a two hour drive, there are skies that approach some of the darkest on earth. In the north western corner of the state, there are skies that should actually be about the darkest on earth. Darkness isn't the problem...seeing (atmospheric turbulence) is the problem. That affects any state, any region, where the main path of the jetstream passes over. During late winter/spring, the jetstream tends to stretch from the north western region of the country, down through colorado, and back up to the north eastern region of the country. Seeing in that whole band tends to be pretty crappy. There are periods of the year, the heart of summer and early fall, the heart of winter (mainly december) where the jetstream moves off more, and seeing improves. You have to get really high, in regions where the jetstream doesn't frequent, to get "exceptional" seeing...there aren't many places on earth like that. The mountains of Chile are one such region.
I am hoping that July and August will bring fewer clouds, less weather overall, and better seeing conditions.
A couple follow-up questions:
1) What keeps one from binning? Is it a physical problem resulting from the Bayer sensor, or just an image processing issue? It seems to me like there might solutions to a purely software problem. If you have a TIFF, you might be able to post-process that with some sort of binning algorithm. One might also enhance the camera firmware à la Magic Lantern, which wouldn't necessarily destroy the general utility of the camera for other purposes. Even if the Bayer sensor is the issue, some interpolation might be doable from slightly offset images though obtaining the correct offset might be difficult.
Binning is very specifically a hardware thing. It occurs at the point the sensor is read. It literally means to combine the charges of NxN neighboring pixels into a single output charge. So, if you have a 100x100 pixel sensor, and you bin 2x2, then you effectively have a 50x50 pixel sensor. Binning can also only be done (at least properly) with a monochrome sensor...there is no logical way to bin a color sensor, since each neighboring pixel is not the same thing.
So there are two reason you cannot bin a color DSLR camera: It's color, and not monochrome...and, they usually do not contain the hardware to perform binning in the first place. Binning is hardware-only. The software counterpart would basically be downsampling. By downsampling 2x, you reduce the width and height by a factor of two, averaging together 2x2 regions of pixels (with a simple algorithm...bicubic is a bit more complex). Downsampling has the benefit of reducing noise by averaging, thus SNR improves. Binning has the benefit of increasing the actual signal strength, thus SNR improves. The latter is the better approach, at least for astrophotography, as good signal strength is key to lifting dim nebula or galaxy detail above the read noise floor. Any attempt to lift anything above the read noise floor MUST occur before readout occurs...otherwise its moot.
2) If I am understanding you correctly, a 5Dii/5Diii might make a better pairing with the AT8RC. The increased pixel pitch would offset the slightly longer focal length, and assuming that 1200mm is near the maximum that the 7D can support those bodies fall just a tad more comfortably in the useable range. [I think the AT8RC actually has an eyepiece large enough to allow full frame coverage.] Am I reasoning correctly here? Is this combination popular?
You are indeed correct that, at least at the native focal length, the AT8RC and 5DIII/6D are a better combo. Technically speaking, the 6D is a superior astrophotography camera...it's actually the best in Canon's entire lineup. DSLR modders are also offering 6D modification now as well, and recent tests have indicated it's low read noise results in some exceptional results (you usually need really dark skies to get those results, though.)
In addition to barlow lenses, you can also use focal reducers with scopes like the AT8RC. The most popular for that particular scope is the Astro-Physics CCDT67. It is a 0.67x reducer by default, so it makes the scope's focal length 1089mm (assuming you actually space the imaging train out to actually achieve 0.67x...many people opt for ~0.75x reduction, which again gets you around 1219mm). Focal reducers and barlow lenses can be used to change the focal length of the scope, which for a given sensor changes the image scale. You could make the AT8RC work with a 7D or similar sensor, or you could make it work for sensors with much, much larger pixels (such as the 9µm pixel KAF-11000 series sensors, or the KAF-16803 series sensors, both of which have big pixels).
Changing the focal length obviously changes your field of view. At 1000-1200mm, your still relatively "wide field". At 3300mm, your getting into deep field or narrower field territory. Wide fields work better with small pixels, deep fields work better with large (or binned( pixels.
3) The mod I was thinking of was indeed removing the IR and/or UV filters in the stack, but I saw the Astronomik clip-in filters for DSLRs that narrow-pass for hydrogen-alpha, oxygen-iii, and other wavelengths, so one could collect additional stack frames with those to pump up selected color bands, right? It increases the stack size required, but leaves the camera ready for general purpose photography after the filter is removed. [Unfortunately, it looks like Astronomik does not support Canon FF bodies.]
It's generally a bad idea to use narrow band filters with a color sensor of any kind. The color filter array, in either DSLRs or OSC (one-shot color) CCD cameras usually keep the total Q.E. per channel to 33% (R/B) or 40% (G) at most. It usually requires about 20 minute exposures with a high Q.E. CCD camera (~56% or higher) to image any given narrow-band channel. That is with a mono sensor, where you have a full fill factor.
With color sensors, for each narrow band filter, only one set of pixels is going to get any light. So it isn't just that you get around (usually less than) 33% Q.E. with red pixels...you get 33% Q.E. and only 25% fill factor, on top of the significantly lower total light due to the filtration going on. Assuming your telescope is transmitting 90% of the light...that is 0.9 * 0.33 * 0.25, or only 7.425% of the light reaching the scope actually releases an electron in photodiodes. The poor fill factor creates other problems for registration, calibration, and stacking as well.
In contrast, a monochrome sensor with 56% Q.E. is going to gather 0.9 * 0.56 percent of the light and release electrons in all of it's photodiodes, or 50.4% of the light reaching the scope. The 100% fill factor makes registration, calibration, and stacking far more effective. If it takes 20 minutes to properly expose say a single Ha band image deeply enough, then it will take 6.79x longer for the DSLR to expose to the same level (approximately 136 minutes). There are relatively few mounts that can track well enough to do 20-30 minute exposures, and even fewer of those that could track well enough to support 136 minute exposures...ASA's mounts come to mind, as they are direct-drive mounts with an inherent <0.1" periodic error.
There is another issue with exposures that long. Noise. Read noise, ironically, becomes a distant background factor for long exposures like this. Dark current noise becomes a vastly greater problem. For short exposures, the sub-second exposures common in normal photography, dark current is practically a non-problem. CDS takes care of it, and we never really have to think about it. But dark current accumulates over time, and it is temperature dependent. An average KAF sensor, like the KAF-8300M, might have about 0.02e-/px/s dark current noise at 0°C. That means that, at that temperature, for an exposure of 20 minutes, your dark current noise is 24e-. If you are using the 6D at ISO 800, read noise is 5.1e-, or almost 1/5th the amount of dark current noise. In other words, dark current swamps read noise. And that is for a thermally regulated CCD that was designed to have low read noise (and ironically, it isn't even the lowest, Sony's new ICX 694, for example, has the lowest dark current levels ever heard of, at 0.003e-/px/s...after 20 minutes, total dark current accumulation would only be 3.6e-, still under the read noise floor). DSLRs have SIGNIFICANTLY more read noise, say an order of magnitude more (~0.2e-/px/s @ 0°C), and on top of that, they run hotter (these days, my 7D and 5D III run around 27-32°C). Dark current doubles for every 5.8°C, which means that at 30°C, it's ~1.03e-/px/s. After 20 minutes of exposure, dark current would be 1236e-! CDS takes care of some of that, however there is always a residual as the pixels and CDS units cannot count identically....they reside in different regions of the sensor die, and the discrepancies can be quite large. On a warm night, dark current noise can be as high as several hundred e-, again completely swamping read noise (and possibly even topping photon shot noise.)
If you want to do narrow band imaging...you should seriously look into getting a proper thermally regulated CCD camera. You can find some of the entry-level Atik CCDs, some of which use the new ultra low dark current Sony sensors, for around $1500. That is for the camera only...a filter wheel would also be required for color or narrow band imaging, and that is usually a few hundred more. But for astrophotography, if you are considering modding a brand new 7D II, it is the better option by far. (Note that narrow band imaging is a great way to get started with AP in the city under light polluted skies...the narrow bands, which are 3nm to 12nm or so wide, block out not only all the light pollution, but you can also usually image during a full moon if your not within about an arc-hour of it.)
4) Are there comparable resolution monochrome CCDs to the ~20Mp general purpose DSLRs but at lower cost, and can one use color filters on these to efficiently recreate a color image? Would that increase or decrease the stack size needed to create an image relative to the DSLR stack, assuming similar resolution? The CCDs I've seen seem to be lower resolution, or rather expensive once comparable resolution and the requisite cooling unit is factored in. Most of the comparably priced CCDs I've seen are under 4 Mp. Perhaps the cooling unit isn't necessary here, since our mountain nights tend to be chilly.
In terms of resolution, no. In terms of sensor area, yes. There are FF-sized CCDs (36x24mm, i.e. KAF-16803), and there are also 37.8x37.8mm "large format" CCD sensors (i.e. KAF-11002). These sensors are huge as far as astro imaging goes. Regular photographers are actually quite spoiled when it comes to sensor size. Amateur astrophotographers have been using 1/3" and 1/2" sized sensors for a very long time, and those are around or less than 1/2 the area of an APS-C sensor. The KAF-8300 sensor is an APS-C size. The KAF-8300 tends to roll into the middle of the cost range, usually about $4000 or so for a full camera package (camera, filter wheel, filters, and maybe an OAG.)
The larger format cameras, like the 16803 and 11002 are much more costly. They are usually about $8000 at least, and for the higher grade sensors, they can be as much as $45,000. If you want a full frame sensor, and want it to be monochrome, you could do a full mono mod on a 6D. You still wouldn't have binning, and you would have to find a filter wheel that would work with it (there are a few odd products that might.) For best performance, you'll probably want to build a cold box for it as well (peltier-cooled insulating box) with either a radiator or water cooling rig. Overall, the 6D, while it does have a large sensor, is never going to perform the same as a dedicated astro CCD cam that is thermally regulated. If you are serious enough about astrophotography to mod a brand new DSLR, then you should really invest your money into a CCD. Even a smaller APS-C sized KAF-8300 (like the SBIG STF-8300M) would be a superior performer for astro in the long run, especially if you want to do narrow band.
5) If the 7Dii turns out to be a high Mp APS-C camera, say ~36Mp, then the pitch might become so small (around 3 micrometers) that it might not be useable for deep sky photography since the useable focal range might be only about 250mm - 750mm? [Or alternatively would this put enough pixels on each star that one could use firmware to bin, even despite a Bayer sensor?]
It really depends on what you want to image. The focal range from 200mm to 750mm is very wide field. Ultra wide field would be wider than about 180mm down to your 10mm to 14mm primes and zooms. Wide field is a really good place to be for a LOT of stuff. I use my 600mm lens for a reason...it gives me a very nice relatively wide field view of the sky, and has EXCELLENT image quality. If your interested in nebula, 600-800mm is actually sometimes even a little "tight"...it can be tough to frame some of the huge nebula that span hundreds to many thousands of light years at once. For example, even my 5D III cannot fully encompass the North American/Pelican nebula region of Cygnus, and it can't even come close to encompassing the entire molecular cloud within that constellation...I would need to mosaic somewhere around 15x20 panels (300 integrations, each of which would probably need a minimum of 50 subs, so a total of 15,000 individual light frame exposures.)
There are reasons to use pretty much every focal length from 50mm all the way up through 3500mm just for imaging nebula (although the longer the focal length, the more difficult the job gets, as tracking and guiding smoothly at focal lengths over 2000mm can get very difficult...that's where spending the money on high end gear, like high end Astro-Physics, 10Micron, Software Bisque, and ASA mounts, all of which cost somewhere between $10k-$25k or so, becomes REALLY useful.) At long focal lengths, your zeroing in on very small parts of large nebula, say just a small part of pelican or just a small part of orion or just a small part of heart nebula. Your spreading the light out more, you need longer exposures to get deep exposures, mono sensors become increasingly important for their fill factor. At 250mm, you could image the entire region around Orion's belt and sword in one go, gathering data for horse head, flame, running man, and orion nebulas, as well as all the various reflection nebulas scattered about, and even including the greater extent of the molecular cloud around there (which permeates the entire constellation of Orion, and is most visible in Ha band.) At 600mm, you can zero in on just his belt, or just his sword, and get more detail on the flame/horse head region or the running man/orion nebula region.)
The 7D II, even if it had a 30-40mp sensor, could still be used at a good image scale between 0.5" to 1", for imaging large, beautiful nebulous regions of the night sky. It would only be if you wanted to push your focal length and get in real close on much smaller parts of those nebula that you would find the 7D II's pixels to be wanting...too small, not gathering enough light each...then you'll want a KAF-11002 with it's square frame and huge 9 micron pixels, and you may even find that pixels that large are still not quite good enough. But you would have to be pretty advanced, and willing to spend a LOT of money, to even really begin to attempt imaging at that scale.
Start wide. Wide is easy. Wide is forgiving. Wide lets you suck in light from huge regions of the sky that are packed with beautiful detail. And the 7D II would do superbly well, for what it is....a "one shot color" camera. Don't bother using narrow band filtration with it, not worth it. Use DeepSkyStacker and Photoshop to start, and look into PixInsight for more advanced processing once you get the hang of things. Oh, and make sure you get at least a moderatly decent mount (I HIGHLY recommend starting with the Orion Atlas, an EQDIR cable, and EQMOD...GODSEND!), make sure you get a guiding setup of some kind, and use BackyardEOS (it has a focusing module that allows you to control the AF system of your EOS gear, if your using a Canon DSLR with a Canon lens...without BYEOS, I'd have been completely lost, and focusing would have been a much more significant and painful chore).