My answer is that a telephoto is optimised for photography whereas a telescope is optimised for viewing directly via the human eye. When designing photographic lenses, one of the big issues is forming an image with a flat plane of focus - especially when it is for a the lage sensor found in an SLR. Lenses don't naturally focus the light coming off centre to the same plane - hence spherical aberration. That, and correcting for chromatic aberration, are the reasons why photographic lenses end up with so many elements and go in for using exotic glasses for some of those elements (and, consequently, are heavy and expesive ...). A telescope, on the other hand, is designed to be viewed directly by the human eye - which has a curved retina. So a flat field of focus is not optimal. Also the human eye does not work like a camera sensor. There is a very good article about this on Cambridge in Colour.
So I decided to set up a test to compare the images I get through my Kowa TSN 823 telescope with the same subject using my Sigma 150-500mm telephoto. I downloaded an ISO 12233 Test Chart from here. The chart comes as a PDF and is intended to be 400x240mm. So I laser printed it on A3 paper, doing my best to setup the printer to achieve that size (and got pretty close), and mounted it on 3mm foam core board. I set the chart up in the garden with the telescope on a tripod positioned so the target just filled the frame (which was at 21m). I then replaced the telecope with the 500mm lens without moving the tripod, and took a couple of pictures with the chart at the centre and edge of the 500mm's field of view.
My reasoning was that if I am out bird watching somewhere and I see something interesting then I have two options (assuming it is not feasible to get closer - which is often the case). I can try digiscoping it or I can use the 500mm and then crop the resulting image. The three images below show the chart through the telescope with the eyepiece magnification set to its lowest power, the chart through the Sigma 150-500mm set to its max focal length, and the latter cropped to just the target. All three are the full original frame images resized to a width of 1024 pixels to achieve an size I could load here. Exposure: It was a bright day with some blue patches - so bright daylight, but not full sunshine. I set Tv mode on the Canon 60D and a shutter speed of 1/1000 (my usual setting for bird photographs), had the leans wide open and the "Auto ISO" setting so that the ISO went as high as necessary to get an exposure. I dialled in +2/3EV exposure compensation to deal with the fact that the target is mainly white paper. The ISO for the digiscope was around 3000 and for the telephoto around 1800.
|Target through digiscope (minimum magnification).|
|Target through Sigma 150-500mm lens at max focal length.|
|Image above cropped to just the target.|
Just for interest, I also took shots through the digiscope at 30x, 40x and 60x eyepice magnifications.These are the full frame that resulted (resized to a width of 1024 pixels).