Astrodon MonsterMOAG Backfocus Calculations

Published January 16th, 2022

Coming to Focus
It is important to realize that any OAG must be placed within a narrow range of distances from the imaging camera for the guiding camera to come to focus. The distance between the prism and guide camera focal plane (A) must equal the analogous distance between the prism and the focal plane of the imaging camera (B).  If too close, the focal point will be inside the helical focuser on the MMOAG.  If too far, the focal point may be too far above the focuser, and require an eyepiece extender or just be too far away to be stable.


The first steps in determining how to assemble your off-axis guiding system is to collect the back focal distances for the imaging train and the guiding train.

Guide camera back focal distance. How do I find this?
Maximum insertion distance of guide camera body. How do I find this?
Imaging camera back focal distance. How do I find this?
Filter wheel or filter drawer back focal distance (thickness of filter wheel). How do I find this?
Additional back focal distance created by the connection between camera and filter wheel. How do I find this?
MMOAG camera side adapter back focal distance. How do I find this?
Any desired additional imaging train back focal distance. What is this?
Collect and write these numbers down, convert them to millimeters if they are given in imperial units (multiply by 25.4 to convert inches to millimeters). Next enter the values into the appropriate form fields below. Enter 0 in the fields that do not apply to your hardware configuration. For example if you are not using a filter wheel, enter 0 for the filter wheel BFD value.

Please note that this form uses Javascript to perform the calculations and you must have your browser enabled to allow JS. I promise I will be nice to your computer.


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