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Nice setup there Simon. Those Pentax lenses have some of the longest backfocus of any lens so that makes them especially useful for lens imaging.
I used to have a FLI PDF focuser setup on mine to get precise focus. Then I moved onto other things in Astro as you do and sold it.
Now I manually focus. My Pentax 165 and 75 do not seem to shift focus over temperature shifts. Perhaps the 165 a tad and the 75mm almost not at all.
I look for that focus graph to spike, I know what an in-focus image looks like and stop.
Greg.
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