Here's some notes I made for my own reference :-
Kiron 28mm f/2
Manual Mode - 30sec @ ISO 400 - wide open
~120 exposures - approx 1hr
Interval drive mode - interval 35 sec
Interval Composite "Bright" - Save Process
Using the default
Astrophoto User Mode will help retain the natural colours of the stars and improve the blackness of the background.
Inevitably, light pollution levels in your area may need some compensation applied to these figures.
Doing an internet search for "pentax star trails" will bring up many knowledgeable articles.
Good luck