Choosing a lens

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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.  

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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.

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Though I'm not familiar with Irix company products, it too is manufacturing high quality manual focus lenses for the K mount.

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