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Dennis,
Sorry I should have said - mount is a G11, image scale is 1.4" / pixel (127mm F7.5 ED with Opticstar 145m CCD), exposure was 4 mins (1 full worm roatation) x 20 with Ha filter, flats darks and levels in AA4.
I estimate my RA error has resulted in stars being 4x5 pixels instead of 4x4. I suppoose there must be limit here somewhere as DEC error can be 0 (or the seeing limit) if perfectly aligned but RA will always be bigger as tracking can never be perfect...I think I must reduce my image scale a little.
John
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