Autofocus Step Calculator

Your autofocus routine moves the focuser a fixed number of steps between exposures, then fits a curve to the star sizes it measures. If the steps are too small, every exposure looks the same and the fit is meaningless. Too large, and the routine overshoots the minimum. The right number depends on your focal ratio, your pixel size and how far your focuser actually travels per step, which is why a single default cannot be right for two different rigs.

How far your drawtube moves per motor step depends on the focuser the motor drives, not on the motor. The same stepper bolted to two different focusers gives two different answers. We could guess, but a guessed number here produces a wrong step size that looks perfectly plausible, so we ask you to measure once instead. Integrated focusers with a published figure need no measurement, and are marked accordingly.

No. The focus zone tells you how tightly focus must be held once you have found it, which matters for temperature drift and for how often you should refocus. Your step size comes from how quickly star size grows as you defocus, which is a separate calculation. Changing the focus zone setting will change the warnings and the shaded band on the curve, but never the step size or the number of exposures.

Almost certainly not. Autofocus moves the entire imaging train, so it will happily bring the centre of the field to perfect focus while the corners stay distorted. That is a back focus problem: the distance between your flattener or reducer and the sensor is not what the optics require. No focus routine can detect or correct it. Back focus errors tend to appear symmetrically across the field, while sensor or focuser tilt makes one corner worse than the opposite one. Our back focus guide walks through it.

Two ways. Glass in the light path shifts the focal plane backwards by roughly a third of the filter's thickness, so a 3mm filter means adding about 1mm to your back focus spacing. Separately, different wavelengths come to focus at slightly different points, which is why the calculator lets you choose a filter band. Narrowband exposures also need longer sub-exposures during the sweep, since the star has to be measurable at every point on the curve.

When focus has drifted outside the zone the calculator reports. Temperature is the usual culprit, since the tube shrinks as the night cools and the focal plane moves with it. A fast telescope has a much smaller focus zone than a slow one, so it needs refocusing more often for the same temperature change. Backlash matters here too: if your focuser reverses direction to return to focus, it will lose motion unless the routine compensates.

No. Every calculation on this page runs in your browser. Your focuser measurement is saved locally so you do not have to enter it twice, and nothing is transmitted to us or to anyone else.