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Back focus adjustment
Hi all,
I am having difficulty adjusting the back focus on my Esprit 100 with a Starizona Apex 0.65 reducer and ASI 2600mc. The documentation for the reducer says I need 58mm of back focus (which I have exactly using 1mm shim discs) but the stars in the corners are still distorted. I remember once seeing a web site that described whether back focus was too long or too short based on the shape of the distorted stars but I can no longer find that.
Attached is a zoomed image of the top left corner. All corners show similar appearances with elongated stars pointing towards the center of the field. Can anybody suggest whether I should lengthen or shorten back focus?
Thanks, Kevin
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