I recently got a CEM25P mount to do some astrophotography. The equipment I already have is a Nikon full frame camera, Micro 4/3 camera, 180mm prime lens, and Sigma 60-600mm lens, which BTW is at least "pretty good" for regular photography. It seems most of the refractor telescopes under $1000 are around 400mm FL and F6, so it overlaps too much with a lens I already have. I want to get a reflector telescope with focal length much more than that for the following things:
Get a very zoomed in picture of the moon.
View and photograph terrestrial objects.
Maybe do some astrophotography if a reducer can somehow work to make the focal length "manageable on this class of mount" like 900mm or so.
There's a few things I am not really clear on. One of them is the notion of "image circle". I get that sometimes the image circle will be smaller than the sensor size of my camera, so then I may as well use the M43 camera, or buy one of those custom astrophotography cameras. Let's say hypothetically there's a 6" reflector with 1500mm focal length. But if I use my M43 camera on it, which has 3.75" pixels, that gives approx 200 x 3.75um / 1500mm = 0.5 arcsec/pixel. I read the "seeing limit" is somewhere around 1-2 arcsec, so would this setup be pointless? Should I just use the full frame camera with 6 um pixels in DX crop mode?
The other question is regarding focal length reducers. My understanding is that they literally take the image circle and shrink it down in size while making it brighter. Let's say originally I was using the FF sensor and it had a little vignetting. I use a focal reducer, and now the image circle got a lot smaller, so I have to use the M43 camera with smaller pixels. But I end up taking (almost) the same picture, and the increased aperture (F stop?) gets cancelled out by the smaller pixels. So how was this useful?
I guess for photographic purposes it has to have large image circle, flat field, low coma, etc?