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The usual thing with most lenses is they need to be stopped down 2 or 3 stops. So an F2.8 may only be good at F4 for example.
I've used Nikon 50mm F1.4, Nikon 180mm F2.8 ED, Canon FD 85mm,200mm. The Canon's needed to go up to about F5.6, the Nikon ED was good wide open and the Nikon 50mm was good wide open. I needed a Canon FD to EOS adapter (ebay) to use them on an STL11.
Your standard camera lens is not made to the standards of most telescope lenses and also use cheaper glass that gives chromatic aberration.
I got some "cheap" Pentax 67 lenses. The 55mm F4 is supposed to be super sharp, also have a 165 and a 300mm and I think a 105mm. They are quite large for their aperture. The setup is FLI Proline, filter wheel, FLI PDF focuser adapter and the lenses. Then a guide scope ring to hold the longer ones at the end for support.
The 300mm 67 (medium format 6cm x 7cm) F4 EDIF as we know from Marco's work is the king of the crop and very hard to find one. Also expensive at around US$1200 to $1500+.
Pentax 67 lenses have a lot of backfocus. I have a list of various lenses and their backfocus. Pentax aren't the only ones with large backfocus.
Greg.
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