After seeing so many great images taken with the Samyang 135mm f2 lens, I finally decided to buy one. My problem was finding a way to adapt a Pentax K-mount lens to a CMOS astro camera. As it happens, no one makes a Pentax K-mount to T-thread adapter! After months of searching, I eventually stumbled on a guide at the Pentax Forums published by Pete_XL. One must obtain a Pentax K to Canon EOS adapter ring, and then a ZWO Canon EOS filter wheel adapter, which has T-threads.

After finding the right Pentax backfocus flange distance of 45.6mm by adding a T-ring spacer, I affixed the whole affair to a Losmandy dovetail bar with a scope ring and ZWO camera ring adapter. Since my Orion Atlas Pro comes with an additional Vixen/Losmandy saddle I can attach at the end of the counterweight bar, I opted to place a guide scope here (see photos). While guiding probably isn’t necessary at 135mm, it couldn’t hurt either, right?

For my QHY 183M/C cameras, I must install 1.25” mounted filters inside the T-Ring spacer with a T-ring to 1.25” adapter ring. There’s no way to stick a filter wheel in this short an image train with QHY cameras. (If one removes the T-ring collar on the QHY183, no threads can be found. D’oh!) With my ZWO ASI1600, I can use a filter wheel since ZWO cameras thread directly into their EFW filter wheel (20mm thick), after removing the T-ring collar. This luckily works out at the correct backfocus distance for Pentax lenses.

I look forward to experimenting with my Pentax lenses over the next several months. Attaching CMOS cameras to these excellent lenses will allow me to capture extremely wide field images along with those all those great hydrogen regions along the Milky Way in a way my DSLR cannot. I hope you enjoy them.

This image is the first of many to come and is just a quick test with one shot color. Too bad the Antares region is so low in the Southwest at the moment. I would love to have gotten more integration time on this amazing area of the sky.

Your comments and feedback are appreciated.