Back focus and spacer calculator for astrophotography

Required back focus runs from the corrector’s documented measuring face to the sensor. Camera distance runs from the selected camera mounting face to the sensor. Accessories use the distance they add when fully installed between those faces. Upstream equipment is outside this budget. If you change the corrector’s output adapter, use “Enter my own figures” with the distance for that exact configuration.

Select only the filter in the light path. If your quoted target already includes compensation for that filter, select “No filter” to avoid adding it twice.