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John,
Interesting discussion...
I assume the backfocus is measured from the outer end of the fixed body of the focuser....
I don't know what the backfocus of my TS102 is - that's not true, I have a PST Mod adaptor I made to allow interchangeability with the Baader Herschel wedge. This adaptor positions the etalon at 200mm inside the prime focus...
Just measured:
The backfocus from the rear OTA flange, where the focuser body fits, is 250mm. The focuser body is 80mm (including the flange on the drawtube) this leaves a clearance of (250-80)= 170mm for all the diagonals, eyepieces etc.
The Baader Herschel wedge with a 20mm Plossl sits at +26mm.....
(This infers the wedge is about 145mm light path.)
The most accurate method is to use a solar filter (or stop down the objective) and focus the solar image on a card......
Last edited by Merlin66; 11-09-2017 at 03:47 PM.
Reason: Oooops maths error
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