Originally posted by kitfoxdrvr Glad you posted. The OP needs to look at your gallery and see how nice that scope is. I would expect that same kind of result from the shorter scopes as well. Nice....
The OP was explicitly looking for shorter fl lenses, because he already has a nice C8 and paraphernalia. So a couple of concrete suggestions:
- 20/2.8 A or FA for extreme wide shots, you could also try the old 16/2.8 A fish-eye
- 24/2.0 FA, is nice if stopped a bit down (f/4- f/56)
- ofcourse the 35/2.0 FA, very sharp
- the 50/1.4 is good in most flavours, though I prefer the A and FA versions, step down to f/2.8 and you'll be happy
- something in the vicinity of 80-100mm is nice. The choices are many (85/2.0 M or the later, much more expensive 85/1.4 A or FA versions, but the 100/2.8 M is also considered nice, which I cannot confirm, as it is the only lens mentioned so far, I do not have). In this range, macro lenses are also a good choice, as they provide a flat field (either the Pentaxes or the Tamron 90/2,5 in one of its many incarnations)
- 135/2.5 K is a very sharp lens
CA and fringing only gets obvious beyond the 135mm limit (mostly). SO here more modern lenses with ED glass is surely of advantage. For istance the K 200/2.5 would be an astrophotographers dream and is plenty sharp - but it also shows noticeable CA, unless you stept down to f/4-5.6. If you can find a Voigtländer 180/4 close-focus (A-version), this is a dream (as would be the A or FA 200/4 Macro by Pentax)
So in the 200mm + department I would go for ED lenses like the 200/2.8 A and later or the 300/4 A and later. The long, fast Pentax lenses all perform admireably, but the Pentaxes (300/2.8 A/FA or 400/2.8 etc.) are all quite heavy and big and put considerable stress on your Celestron's mount.
If you can find the 400/5.6 A, this is a great lens, albeit a bit slow, but you can use it fully open with great results. And the peak of astrophotography with Pentax is the SDHF75 (0500/6.7) ED-Apo telescope. It is fairly small and superbely built and offers really great performance. Add a tc or (if you still can find one) a reducer and you're done. By the way: Lumicon makes a fine 2-inch reducer, which can be screwed into standard 2-inch eyepiece filter threads. It exhibits some distortion, but is otherwise optically quite good and would convert the Pentax 75 into something like a 350mm 4.7 Apo-lens.
Ben