I don't have a K-1 but after searching for prime-focus adapters for my crop sensor (APS-C) K-3II I was struck that I did not find any for full-frame Pentax cameras, i.e., the K-1 and K-1 Mark II. Sure, you can use an adapter based on a standard M42 T-ring but you will get serious vignetting. M42 T-ring based prime-focus adapters choke the clear-aperture down to about 38mm. The diagonal on a full-frame sensor and, of course, on 35mm film is 43mm. Prime-focus adapters with integrated bayonets avoid this problem but I did not see any for Pentax, only for Canon and Nikon.

Back in my film days I used an Olympus OM adapter with an integrated bayonet that met the 43mm clear aperture requirement. Before that I used a Lumicon Newtonian 2-inch Newtonian Easy Guider OAG that provided 42mm of clear aperture, which practically speaking proved to be sufficient. I actually had two of the latter, one configured for my OM-1, the other for my Pentax K1000.

Today, a M42 T-ring-based prime-focus adapter is more than sufficient to completely avoid vignetting with my APS-C K-3II. But what do all you K-1 users out there use to avoid vignetting?

Interesting. I have an " Orion 2" Prime Focus Camera Adapter" with a T to K-mount that looks like it should cover the FF sensor. I must try it and see what happens.

With a Pentax-K T-ring you probably need this: Altair-Ex Low Profile T-Thread Camera Adapter (2" barrel) - Altair Astro. Never seen one in just one piece for Pentax. Note that the vignetting doesn't necessarely come from the T-ring or T-adapter. An f/6 telescope will have approximately 29 mm of usable field (w/o a field corrector of course). In such a case, no need to pay extra for a 2" adapter, a much more common 1.25" will do just fine.

I appreciate that some, perhaps most, telescopes aren't fully corrected for a full-frame sensor. But there are some that are, and some can be made so with the addition of a field corrector/reducer. (Flat frames may be and usually are still required to correct gradual illumination drop-off, which is not as serious as what I would classify as actual vignetting.) I should have clarified that I was referring to K-1 users with scopes that actually allow use of the full sensor. Surely there are some here in this group?

Pentax K-mount has a 44 mm throat diameter so it's going to be difficult to find something with 43 clear aperture. This one claims to do better than the T-ring/T-adapter couple: 2" Prime Focus Adapters for Pentax - Telescope Camera Adapters | TelescopeAdapters.com