Re: Back Focus Troubles #Absolute_Encoders

Andrew J

On Tue, Jun 1, 2021 at 02:01 PM, Dale Ghent wrote:
at it's a good idea to not fixate on a single exact number when it comes to back focus dista
Hi Dale.

I think this is really good advice. In the past I always ordered my Precise Parts adaptors to be exactly the calculated length based on the scope/correcting element's published specs minus the other elements in the optical path. Probably not a good practice as even PP states their parts have +/- 1mm tolerance. Combined with the +/- tolerances of the other components in the optical path you mentioned and it is probably never going to be the calculated distance. I actually sent a note to Precise Parts last night on how to create shims using their configurator for non-standard thread sizes. Agena Astro Products sells Fine Adjustment Spacer Rings for standard thread sizes. Going forward, I will follow your advice and always order my Precise Parts 1mm short and use shims to get to the correct length. 


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