Re: Back Focus Troubles #Absolute_Encoders


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. 

I too had that question on my mind for quite some time for the EdgeHDs (I own the C8). 

From my readings, my understanding at this point is that with a crayford-type focuser in the optical chain, the backfocus spacing from the last optical element still needs to be pretty accurate (to < 1 mm) to optimize the field flatness over the entire sensor area / illumination circle. 

Hence, with a crayford on these scopes, the focus point needs to be pre-set with the primary mirror focuser prior to locking the clutches and should then only be fine-tuned by the crayford (auto)focuser to microns precision. The backfocus spacing tolerance for field flatness being an order of magnitude or so greater than that of focus point, field flatness should not, in theory, be impacted (much) by the change in backfocus spacing from the crayford reaching the focus point.

But as I said, this is my understanding only. If someone has a better explanation, I'm very interested in hearing it too.


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