Right Jerry, good advice. The ST-80, and most other 80mm f5 scopes of course have a 400mm fl. But the danged camera sensor can make things longer if it is a cropped sensor like a DX in Nikons. In that case a .8 flattener/focal reducer can help.
I suppose the idea of just using a good 400 mm camera prime lens is maybe the best. I don't know. Same as for the telescope, one would ideally want the ES 80mm APO. All gets down to cost and where on the learning curve you are and how much you want to spend to climb up it in case you decide to bail on it. I bought a 400mm Sigma lens for a song at a star party. Turns out it does not focus worth beans. Focus wheel is so soft you cannot let go of it without turning it. Also the glass just seems poor. Now it sits and clogs up my camera bag. So if you go with a prime, buy a good one.
Wes, Southport NC
PMC-8, ES ED 127, 10" LX200GPS, Astro-Tech 8" Newt, ETX-90
Polemaster, Orion ST-80 and SAG
Electrical Engineer, Retired