Hi Simon,
Interesting, maybe you have some photo with lot of stars at full resolution that I can analyze ? My camera is a full frame with 20MPix.
Aperture problem:
I also took a look at the aperture how it behave when rotate the aperture ring. From fully open at f/4 to close down to f/5.6. It doens't always open fully, it have happen before, the aperture ring could move a little bit when it's fully open.
Deformed stars:
Maybe I have found what cause the stars to be that distorted, look at this photo I took just after I bought the Pentax lens:
http://www.astrofriend.eu/astronomy/...645-300ed.html
All star are much better. Could it be my focuser that set some side force of the lens barrel ? I have to have a high tension of the timing belt to not get any backlash.
Tilt problem:
I thought maybe there are something in my lens that are out of adjustment and that cause the tilt ? I did the same inspection of my photos with the TS130 APO refractor. It look almost to be the same tilt, it must be the camera sensor that is out of adjustment. But I think I'm looking after very small tilt error here.
Have a look at the bottom of this page, the last photo:
http://www.astrofriend.eu/astronomy/...645-300ed.html
/Lars