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Hi Jimmy,
Yes focus and lens aberrations seem to be the problem.
I doubt you could use a Bahtinov mask with a 50mm lens though I've never tried. The technique I described in my earlier post works very well for short fast lenses and yes stop down a little. No faster than f2.8, maybe f4 or 5.6. I think Bojan's comment about taking lots of shots was probably made because he uses Canon cameras which have limited magnification in live view. So he has to take a test exposure and then examine it in preview mode to gain higher magnification.
You haven't said which model Pentax you are using but most have 10x up to 16x magnification in live view. On bright stars, mag -1 to 1, the focusing can be done live without taking test shots and focussing can be achieved very quickly. On fainter stars, mag 1 - mag 3, the stars are invisible when off focus and snap into focus. I prefer the bright star method. Much easier.
cheers
Joe
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