I am not thinking of 'digiscoping' where a camera is mounted to focus on the image projected by a telescope eyepiece, I am thinking of a telescope mounted on a camera to project the image directly onto the sensor.
For example, I have a f1000mm refractortelescope of 120mm aperture (f1:8.3 I assume?) I also have a f1000mm Sigma mirror lens of about f1:13 (I am not sure exactly).
There are some pretty big differences between the two! The telescope is physically huge and requires two hands to carry and an equally heavy tripod. However the pictures to my mind seem quite good, that is when I remove the telescope eyepiece and mount the camera with a 'T' to 'K' mount adapter. Of course the mirror lens is miniscule in comparison though it too is near useless without the tripod, images from this lens are rather poor in comparison. For nature work these are both ambush lenses and of course they are both fixed aperture and the mirror lens is for abright sunny day only.
I suppose if I went down to the mall and kidnapped a couple of kids to sell for medical experiments I might get enough to buy that 'Bigma' then I would get better pictures again, but reallywould I and why? What is it that would make a 'real' camera telephoto better than a telescope optically?
(I have not tried to convert a local price for the Bigma but I expect maybe five or perhaps ten times what the telescope cost.)