Measure Backlash

Backlash is the slack in a focuser’s drive. After the focuser changes direction, the motor turns some number of steps before the draw tube follows. MEASURE BACKLASH finds that number on a star field, so you do not have to guess it.

Where it is

Open the focuser settings screen (gear icon) and find the BACKLASH section:

• Value (steps) is the backlash compensation the focuser applies itself. On a NINA rig it is NINA’s own setting. Set it to 0 to turn compensation off.
• Direction sets how autofocus approaches each position. In backs off inward, then settles outward. Out backs off outward, then settles inward; it is slower, because an upward sweep needs a correction at each step.
• MEASURE BACKLASH runs the routine. Once a measurement is stored, the button reads MEASURE AGAIN.

The measurement runs on the ARIS rig backend. A NINA rig does not offer it.

Run it

1. Point the scope at a star field.

2. Tap MEASURE BACKLASH. If the cover is closed, ARIS asks first. The button is off while the focuser moves or autofocus runs, and the run is refused if another routine owns the camera.

3. Watch the progress: the phase, the frame count and the star size (HFR and star count). STOP ends the run.

4. Read the result, then tap APPLY (the button shows the value) or DISMISS.

The run takes about three minutes, 25 to 36 frames.

What the routine does

1. Preparing: it reads the focuser and turns off the focuser’s own backlash compensation, so it measures the real slack.
2. Walking out: it moves the focuser outward in autofocus-sized steps until the stars are clearly out of focus.
3. Reversing: it moves inward in small steps. At first the star size does not change; that flat stretch is the dead band.
4. Returning: it moves outward again the same way, to measure the other direction.
5. Fitting: it finds where each reversal leaves its flat stretch.
6. Restoring: it returns the focuser to where it started.

The focuser’s compensation setting goes back to how it was at the end, whatever happens. Guiding is paused during the run and resumed after it. If Rotate to reference for AF / solve / backlash is on, the run measures through the reference filter (see Filter Focus Offsets).

The result

The result card shows Backlash in steps: the larger of the two directions, because that is what the compensation has to cover. Under it are the two directions (to in and to out, each with its spread) and the value the focuser holds now. Low confidence — try a brighter star field or longer exposure means the result is uncertain.

APPLY writes the value into the focuser’s backlash compensation and turns compensation on. The measurement is stored either way. The BACKLASH section then shows it (“Measured N steps · to in X · to out Y” and the date), with APPLY whenever it differs from the value the focuser holds.

If the run cannot give a result, it says why, for example No usable star, Too noisy to resolve or Not enough focuser travel.

How autofocus uses it

Each autofocus sweep takes up the focuser’s backlash before it measures. With a stored measurement:

• It trusts the focuser’s own compensation only when that value is at least the measured backlash.
• Otherwise it overshoots in software by the measured backlash plus 20%, plus 5 steps.

With no measurement, it uses 50 steps for both the check and the overshoot. Autofocus can also raise its overshoot on its own when a run shows that the slack is larger, and it keeps the value that worked.