I bought this lens in a thrift mall hoping to use it for some lunar and semi-wide field astro shots. It is the camera "mirror lens" version black tube. After much playing around with it, I cannot seem to get good results. Here is the best I could get on the moon.
I have a set of extension tubes of varying lengths and have tried all sorts of back-focus distances but the results all seem to be the same. First photo was taken with the 65mm tube and the 40mm tube. Other than my exposure differences, as you can see, both pics have soft focus and lots of CA.
Any ideas out there on how to correct this, or did I just get one that was poorly made?