[leon]

Hi There

I use my Pentax IstD for Astro-Photography, mainly wide field images.
You are correct, there are heaps of older style lens that will fit, even the screw on types, but beware......
To use the screw on types you obviously need an adaptor that will take the 42mm thread, they are easy enough to get, but put them on your Ist and you will struggle to get the little buggers off.
The tool you get is not suited to remove the adaptor properly, and i litterly had to put mine to the grinding wheel to modify it, the tool that is.
The problem is that when they made these tools they made the little lugs to long, and each time you insert it into the two slots on your camera body to remove it they go in to far and you get stuck, and you cant rotate the adaptor.
I had a hell of a time with it, and ended up with bits of shaved steel in the camera itself, and on the sensor.
My suggestion, dont worry about the screw mount lenses, they are more grief than there worth.
Now secondly, all other lens other than specific digital friendly ones will give you Chromatic aberration and purple fringing around the bright stars.
This is due to the fact that the sensor is not full frame, (being 35mm) and the colours are not equally focus at the focal plane, where the blue light is the biggest culprit, giving you purple fringing and aberration.
I have heaps of Pentax film lenses but find they don't do the job, and have now recentaly ordered a specific Pentax Lens designed specificaly for the IstD.
I'm lucky in a sense that i can try before i buy from my dealer, so i will eventually find a lens suited for my needs
Having said all that the older style lenses do work well for ordinary photography, but are not much when it comes to Wide Field Astro-Photography.

Clear Skies Leon