Today I anlyzed my flats that I have taken with my Pentax 645 300 mm lens, they are takne at full aperture, f/4. As I expected the vignetting is very low, much lpwer compare to my Canon 300 mm L lens. That was one of the goals. Of course I want it to be a sharp lens too. It looks okay but at one corner the stars are a bit elongated, it must come partly because of a tilt of the camera house. The bayonette adapter doesn't fit very well and cause this.
This how the vignetting looks:
http://www.astrofriend.eu/astronomy/...ignetting.html
/Lars