Another option in pancake lenses is the Vivitar 40mm 2.5. After looking at the prices of the Pentax 40mm on eBay, I bought the Vivitar lens. I've been very happy using it on my MZ-5n. The focal length is just different enough to give a different perspective on scenes that I might shoot with a 28mm or 50mm. Plus, by leaving it focussed at the hyperfocal distance, it make a great point-and-shoot lens.

Try landscape mode to make it shoot at the smallest aperture. That should slow the shutter speed down for the blur effect.

Mick, you're better off just stopping down the lens and going for a longer exposure. It's just the physics of shooting stars. Planetary conjunctions can sometimes be shot wide open, but stopping down a couple of stops really sharpens up the photos. With a 135mm, you may be getting up into the area where you need to start guiding the scope, too. The mechanics of your mount may just not be accurate enough unguided.

The 50mm f1.7A is notorious for messed up aperture rings. Mine is the same way. It takes a little more effort to turn the ring, but the aperture works fine. The lens was a lot cheaper than a new one, so I can put up with it. It doesn't effect the optics at all. Besides with the A lenses, there is always Auto mode.