Re: Back Focus Troubles #Absolute_Encoders

Andrew J

Hi Mike.

This is really good example of the two "types" of back focus, fixed vs. variable. The X back focus distance is fixed will not change when the focuser is repositioned. In this case I could understand the argument for getting the back focus distance as close as possible per the specs of the Field Flattener. However, the Y back focus is variable and will change when the focuser is repositioned. When the back focus is variable, I believe the documented back focus is a "recommended" value to help ensure focus can be achieved within the travel limits of the focuser. In the case where the back focus distance is variable it probably is not worth stressing over every mm of back focus. As long as it is close and still within the travel distance of the focuser then that should be good enough. 

I think you nailed the explanation of the two different types. This has helped me get my head around back focus and understanding the difference between fixed vs. variable back focus. Thank you for your detailed post. 


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