Lars,
I found that all commercial/terrestrial lenses work better on stars when closed a step or two.
Very often (my experience with Canon tele lenses above 135 mm FL) external diaphragm is better than built-in iris.
So, general rule: if you want f/4, use f/3.5 or f/2.8 lens and stop it down to f/4.
400 mm:
https://www.iceinspace.com.au/forum/...d.php?t=138076
200 mm:
https://www.iceinspace.com.au/forum/...ht=Canon+200mm
100 mm:
https://www.iceinspace.com.au/forum/...ad.php?t=58629
50 mm
https://www.iceinspace.com.au/forum/...ad.php?t=39293