Starguide - A Practical Guide to Astro-Photography

The backfocus of a camera is the distance from the plane of the sensor to the front of the camera. These measurements are available on all of our astrophotography cameras. To calculate the backfocus of your setup, add up the back focus of all the separate components.

Backfocus is also important when you are using an optical flattner or reducer , for example using the TSO 90/600 - ZWO-ASI294 + Riccardi Reducer configuration:

55mm Backfocus  for ASI cameras: The best solution of 55mm back focus length | ZWO ASI (astronomy-imaging-camera.com)

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