Filter Focus Offsets

Filters do not all focus at the same point. A filter focus offset is the number of focuser steps between one filter’s best focus and the reference filter’s. With offsets measured, a filter change moves the focuser by the difference instead of running autofocus.

Offsets live in the FILTER OFFSETS card on the Filter Wheel screen.

The reference filter

Every offset is measured from the reference filter, and the reference is the filter that holds a measured 0. Its row shows REFERENCE and an OFFSET of 0 (Ref).

If no filter holds a 0 yet, each row offers Set as reference. ARIS then confirms it, for example: “L is now the reference filter — every other offset is measured relative to it.”

Each row also shows the filter’s SLOT, its AF EXP and when its offset was last measured: Never, a number of days ago, or AF and the days when it came from autofocus runs.

Measure offsets

MEASURE OFFSETS opens the OFFSET WIZARD. It needs a filter wheel, a focuser and a camera on a star field.

Pick the REFERENCE FILTER. A dark slot cannot be the reference.
Set PASSES, the number of autofocus runs per filter (default 3).
Tick the FILTERS TO MEASURE. The reference is always measured; dark slots start unticked. The wizard shows its estimated time.
Tap Start measurement. For each filter, the rig turns the wheel, confirms the wheel arrived, and runs autofocus once per pass. Abort stops the run.
Review MEASURED OFFSETS. Each filter’s offset is its mean best focus minus the reference’s. Turn Keep off for any you do not want, then tap Apply.

A filter whose passes spread by more than 8 steps is flagged and starts with Keep off: “σ exceeds threshold — recommend re-measure”. After an abort you can still apply the filters that finished, as long as the reference finished too.

Only one wizard runs at a time. It is refused while a sequence runs or while the camera is busy. Its progress is kept in the rig’s memory, so a backend restart loses a run in progress.

Seed from autofocus runs

SEED FROM AF RUNS works offsets out from autofocus runs the rig has already done. Two successful runs through different filters, taken close together, give the offset between those two filters. One filter of the pair must already have a measured offset, such as the reference. Seeding moves nothing, and nothing is written until you apply. The review card says: “Estimated from pairs of autofocus runs — review before applying. Writing an offset stops the rig from autofocusing on that filter change.” With nothing to pair yet, it shows NOTHING TO SEED YET and the reason.

How offsets are used

Turn on Use Filter Offsets (“Auto-adjust focuser position when switching filters in sequences.”). The card notes that this “Requires accurate backlash compensation in Focuser Settings.” See Measure Backlash.

When the filter changes, in a sequence or when you move the wheel by hand, the focuser moves by the new filter’s offset minus the old filter’s:

If the new filter has no measured offset, the focuser does not move.
If the old filter has no measured offset, it counts as 0.
A measured 0 on both filters means they focus together, so nothing moves.
A manual wheel move does not move the focuser while autofocus runs.

To autofocus when an offset is missing, turn on On filter change with If offset undefined under AUTOFOCUS TRIGGERS in the focuser settings. It fires when either filter of the change has no measured offset.

Focus through the reference filter

Rotate to reference for AF / solve / backlash needs Use Filter Offsets on and a reference filter. With it on, autofocus, plate solves, the backlash measurement and a guide calibration started with Recalibrate use the reference filter, and the offsets carry the focus result to the filter in use.

Most runs then turn the wheel back to the filter you were on. The autofocus at the start of a sequence is the exception: it stays on the reference, and the sequence’s first filter step carries the focus on with its offset.