Re: Back Focus Troubles #Absolute_Encoders

Andrew J

On Wed, Jun 2, 2021 at 08:11 AM, ap@... wrote:
the majority of what the external focuser on the SCT is doing is restoring the same focus length (and so same back focus length), not so much changing it.

Thank you for that explanation. Using a focuser to correct or compensate for these changes makes sense in a standard SCT without any correcting elements in the light path. Moving the Primary Mirror or moving camera sensor should have the same net result.

I think this becomes more complicated when using an Edge HD system due to the glass correcting element in the baffle or adding the .07x focal reducer. 146.05mm is a very specific number. I always wondered what the impact would be on these systems when a Crayford focuser is used and the primary mirror is locked at a specific focus position, effectively swapping the variable and fixed portions of the optical chain. It is not something i necessarily need an answer to as I no longer own and Edge HD, but just curious. 


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