Here are some photos taken with the Pentax 645 300 mm ED lens, all taken at f/4, full opening. All images are not cropped, all the way out to the edge of the sensor and it's a full frame sensor. Taken at Bortle Class 9 environment so no weak objects.
http://www.astrofriend.eu/astronomy/...per-stars.html
I really like this lens. When I look at the oversaturated stars, looks if something is in the way for the light beam, like an aperture. Have to look into the lens and inspect it carefully.
/Lars