Re: Back Focus Troubles #Absolute_Encoders


ap@CaptivePhotons.com
 

Andrew:


> In this case the last correcting element does not move with the focuser and distance between the last correcting element and the sensor will change as the focuser is repositioned. In examples where the last correcting element does not move, why is dialing in a specific back focus distance using spacers so critical if the distance is going to change depending on the position of the focuser?

In a terrestrial lens focus distance (indeed even focal length) changes with distance to the subject, due to angles of the corresponding light rays changing with distance.  In Astro work, everything is effectively at the same distance from us (parallel rays, effectively infinity) and the primary reason we actually need a focuser (beyond initial setup) is to correct for temperature change, which is primarily a dimensional changes in the focal path.  While more complex optics make it more complicated (and in theory temperature could affect the glass not just dimensions of the OTA and train), 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.

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