Today I thought it might be fun to experinment with my Orion Shorttume 80mm refractor as a long prime lens. The telescope works out to be a 400mm f/5 lens. This isn't a far cry from the Canon EF 70-200mm F4L lens that I use with a Quantaray 2x teleconvertor (400mm, or 640mm with the 10D's 1.6x sensor).
While the 70-200+2x TC works fine, especially in the f/5.6 to f/8 range, the TC costs me 2 stops and requires a little manual tweaking of my exposure compensation.
The greatest hurdle for the telescope would be the manual focusing. I spent some time making sure that my 10D's diopter focus matched that of the sensor plane. After this, images worked out pretty good, unfortunately, the f/5 telescope was too bright for the 10D, maxing out my shutter at 1/4000th of a second at ISO100. However, the Shortube 80 came with a lens cap with a removable center cap that stopped the telescope down to f/11 (35mm opening). This brought the shutter downt o 1/750th or so.
With all that done, I started snapping pictures and am pretty happy with the result, though there is some obvious purple fringing due to the non-APO doublet, especially on any bright flares on the aircraft.
When it stops raining, I'm going to see what happens, and if the telescope can even focus, with the Quantaray 2x.