Hi Simon,
Yes the aperture should be fully open, but it isn't, see my photos through the lens:
http://www.astrofriend.eu/astronomy/...645-300ed.html
Now I have to fix:
The aperture
What's deforme the stars
Tilt problem
Never any boring time when having astronomy as your favorite interest :-) But if I can fix all these problem my Pentax 645 lens will eat all other lenses to breakfast. I don't think it will be too difficult.
/Lars