Hi John,
I also have heard lots of good things about the 200mm but all pertaining to daylight.
I am a Pentax user and have a number of primes but unfortunately they all seem to suffer some aberration to some extent and also some internal reflections on brighter objects at night, some acceptable some not.
I have a rather nice first quality Sigma 24-60 that is great in daylight but suffer the internal reflections terribly at night.
Have you asked on the Pentax forums to see if anyone is doing astro with the 200? Worth a look there I would say.
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