Choosing a lens
Astrophotography is all about light gathering so usually fast primes are the ones that do better. Coma and chromatic aberrations tend to be lower in primes vs zooms as well.
Modern Pentax cameras will work well with the old Pentax Takumar, K, M, A, F and FA lens series. Though these lenses don't incorporate modern "ED" glass, they are fairly sharp and are widely available in the used equipment market. I have used the M28/2.8, A50/1.4, A135/2.8, M200/4 and A400/5.6. With the astrotracer the M28/2.8 and A50/1.4 are the best matches.
I consider the A50/1.4 as the best astrotracer lens in my lineup as, on APS-C sensors, it produces razor sharp images with no vignetting, very little chromatic aberrations and coma from f/2.0.
The Samyang company manufactures high quality manual focus lenses for the Pentax K mount at a very reasonnable price. I have the 16 mm F2.0 ED which is the lens I use most often with the astrotracer. The 135 mm F2.0 ED is a wonder to work with. Both lenses are very usable wide opened. Rokinon and Bower are other brand names of the Samyang company.
Though I'm not familiar with Irix company products, it too is manufacturing high quality manual focus lenses for the K mount.
Samyang 16 mm F2.0
smc Pentax-A 50 mm F1.4
Rokinon 135 mm F2.0