Telescope Autofocus Step-Size Calculator

What this tool does

Your JMI EVRZ or EVRP focuser moves the drawtube in very fine native motor steps — fine enough that a single step changes a star's size by only a small fraction of a pixel, far too little for an autofocus routine to measure. This calculator works out, from your optics and camera, how far the focuser must move between exposures so the star grows measurably, and reports that as a step size in native motor steps you can drop straight into your imaging software.

Quick start

1. Pick your telescope from the preset list (274 scopes) to auto-fill focal length and f-ratio — or type your own.
2. If you use a reducer or Barlow, set its factor — the panel shows the effective focal length and f-ratio.
3. Pick your camera from the preset list (or type a pixel size).
4. Pick your focuser. The JMI EVRZ and EVRP load their measured factory calibration automatically; choose Manual to calibrate your own.
5. Choose your autofocus software and read the recommended step in the table.
6. For best accuracy on small refractors, enter your measured in-focus HFR in the advanced panel.

The core idea

Far from focus, a star's half-flux radius grows linearly with drawtube travel. The slope of that line is fixed by hardware:

slope (HFR px per native step) = d / (2 x N' x p)

where d = µm per native step, N' = effective f-ratio, p = pixel size (µm). A good step size moves the star so the outer points of the sweep bloat to roughly 2.5–3x the in-focus size — the target nearly every autofocus package is built around.

Outputs & formulas

Autofocus software guide

Every program consumes a step size in native focuser steps, so this tool's output unit is correct for all of them. What differs is the curve fit (which changes the ideal sweep width) and each program's own rule of thumb.

Separate from step size: most programs handle backlash by always approaching focus from one direction; temperature by triggering a refocus on a temperature delta; and filter offsets by storing per-filter focuser offsets.

Calibrating your µm/step

The JMI EVRZ (2.54 µm/step) and EVRP (2.63 µm/step) presets already load their measured factory calibration, so most users can skip this. To calibrate any other focuser:

1. Set a dial indicator or calipers against the drawtube (or measure the image-plane focus shift).
2. Command a known number of native motor steps in one direction. 1,000 is a good default — large enough to average out any single-step error.
3. Read the actual travel off your gauge in millimetres or decimal inches — whichever your gauge shows. You never need to convert to microns yourself.
4. Enter the step count and the travel in the calibration helper, choose the matching units, and press Compute. It converts for you and sets µm/step = travel / steps.

Worked example: an EVRZ moved 2.54 mm (the same as 0.1 inch) over 1,000 steps — that is 2,540 µm / 1,000 = 2.54 µm per step.

Glossary

HFR / HFD — Half-Flux Radius / Diameter, a robust star-size measure (HFD = 2 x HFR). CFZ — Critical Focus Zone. NCFZ — seeing-aware New CFZ. V-curve — star size vs. focuser position. Backlash — mechanical slack when a focuser reverses direction. Sampling — pixels per seeing FWHM.

References

Wilmslow Astro & GoldAstro (CFZ / NCFZ), Innovations Foresight, FocusMax / Weber & Brady (V-curve), and the official NINA, Hocus Focus, SGP, APT, MaxIm DL, SharpCap, Voyager, TheSkyX, Ekos, CCDciel and PRISM documentation.