Hi Peter,
Pentax has always had a lot of interesting optics. I had a lot of them.
Pentax 500 mm f/4.5
Pentax 135 mm f/2.5
Pentax 50 mm f/1.8
And now the Pentax 645 300 mm ED f/4 medium format.
Nice to hear your 300 mm lens is good for astrophotography.
Since last I have updated my Astroberry systems that control everything. Last update had an update of the auto focuser routine. It looks to do a much better job now (which wasn't bad earlier either).
I have documented what I found for you who find it interesting:
http://www.astrofriend.eu/astronomy/...645-300ed.html
It got a bit slower but I think I can tweak the parameters and get it faster. Just need a clear sky to test it.
/Lars