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I have not used a full frame DSLR/Mirrorless for deep sky work only APS sized but in mono CCD I have used 35mm full frame sized sensors.
Basically you need a corrected circle of light about 44mm for full frame, 54mm for the 16803 chipped cameras which are bigger than full frame.
I had a Tak FS152 with a 2.7 inch focuser. With the full frame CCD I found I got bad coma in the outer corners. When I tried a small flattener I got bad vignetting.
I replaced the focuser with a 4 inch and the troubles disappeared.
My TEC180 scope has a 3.5 inch focuser and that has no problem.
My TEC110 scope has about a 2.7 inch focuser and field flattener (I'd have to measure it maybe its 3 inches) and no problem with a 16803 chip.
So depends on the scope but for sure you will need a field flattener. I think you can forget about a reducer that will work unless you go Tak and larger focuser. And even then it will be problematic and unlikely to work (maybe the APM Riccardi one will work?? For an ED100 probably a Tak 4 inch flattener would work. They seem to be fairly generic in that they work on lots of scopes (I use a 4 inch Tak flattener on an AP140, a TEC180 and it works beautifully).
The first thing you would do is open up the existing aperture and that is usually the DSLR adapter. Astrophysics make a cheap wider open DSLR adapter which would not cost a lot to try out.
Full frame is always better than APS simply by physics. A larger surface collection area of the sensor equals a number of stops better low light performance depending on the sensor.
On the subject of cameras - the Sony A7S is the low light king but the difference only kicks in above ISO6400, at or below that an A7r/A7 matches it so does a 5D3, 6D. Above that and it says good bye to everybody else!
But then what is the advantage of ISO12800 in a deep sky shot? Answer - nothing.
ISO is simply amplified gain of the existing signal. I think sometimes it is done before conversion to digital and sometimes after. So ISO is often optimised for a certain point. For example the Nikon D800 ISO is optimised for ISO1600.
DSO shots and a DSLR are best done around ISO800. If you look at mono CCDs you'll see the gain used on the signal is minimal - like just above 1.
But APS versus full frame - I say go for the real estate. You can always crop a vignetted image but you can't make your image bigger from an APS.
Look at the comparison tool at DPreview to see the noise differences between the various models. Then factor in amp glow which they don't talk about. Then factor in software support. A modified 6D would be my choice at present. Perhaps a modified Sony A7s would be nice as well. A modified Sony A7s could be sensational. It also has well depth of 157,000 electrons which is considerably more than even the best mono CCDs currently in use. QE of 58% is also very high, again on par with most of the best mono CCDs and better than many.12 megapixels also means smaller file sizes. It also has a very large dynamic range - greater than most DSLRs. An SBIG STL11 which has stood the test of time and is the most popular astro camera probably of all time and its 11mp.
Greg.
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