Telescope backfocus: where every millimeter goes
Build a spacing budget from the correct reference plane, including the camera and filter path.
Start at the specified reference plane
Backfocus here means the required distance from a corrector or reducer reference plane to the imaging sensor. It is not simply how far the focuser moves. Use the drawing for your exact optical configuration. A quoted 55 mm distance is an example, not a universal requirement.
Count each length once
Sum the installed shoulder-to-shoulder length of each adapter, the filter drawer or wheel, and the camera flange-to-sensor distance. Do not add exposed thread length separately unless the manufacturer’s reference convention requires it. A 16.5 mm adapter, 21 mm drawer, and 17.5 mm camera distance sum to 55 mm.
Filter correction changes the target, not the drawer length
For a plane-parallel filter, the approximate extra physical spacing is t × (1 − 1/n). At 2 mm thickness and refractive index 1.5, this is 0.667 mm. Apply this only when the specified target excludes the correction. Keep the filter drawer’s mechanical length in the train either way.
Read the sign, then verify on sky
A positive gap means the entered train is shorter than the target. A negative gap means it is longer. Within your chosen tolerance, no spacing change is indicated by this arithmetic. This does not prove good corner stars: tilt, reducer spacing, mechanical clearance, and the optical design still matter.