Calibrate the 12mm lens on-sky: effective focal length, SQM zero point, f-number

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Tipo di issue
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Chiarezza
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Stato di attività
Tranquilla
Stack tecnologico
python
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backend

Direzione di ricerca

Start with python/PiFinder/optics.py and the 12mm entry in PiFinder.optics.LENSES, then review tests/test_optics.py and the fitting comparison in tests/test_optics_solving.py. An on-sky sweep through the 12 mm lens and comparison with the reference meter and 16 mm data are required to determine effective focal length, SQM offset, and f-number. Done means those three values are updated, effective_focal_length_measured=True, and the corresponding test expectation is updated.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

help wanted

The 12 mm entry in PiFinder.optics.LENSES carries three values that are assumptions rather than measurements. One on-sky sweep settles all three.

1. Effective focal length (currently the nominal 12.0)

Flagged in the data so it cannot be mistaken for calibrated:

"12mm": Lens(
    nominal_focal_length_mm=12.0,
    effective_focal_length_mm=12.0,
    effective_focal_length_measured=False,
    ...
)

The shipped 16 mm measures 15.61 mm — 2.5% short of its label, confirmed independently on two sensors. Assuming the 12 mm is exactly 12.0 is optimistic. Everything angular derives from this value: the FOV gate, the SQM radiometric field width, and the chart's frustum shading.

Method: solve frames taken through the 12 mm and compare tetra3's fitted FOV against the derived one, the same comparison tests/test_optics_solving.py makes for the hq train. The ±15% gate is wide enough to solve through a 2.5% error, so this measures cleanly without a chicken-and-egg problem.

2. SQM zero-point offset (unmeasured)

radiometric_zero_point is calibrated per sensor against a reference meter, on the shipped lens. A different lens changes throughput, so the constant does not transfer. Currently nothing corrects for this — a 12 mm user gets the 16 mm's zero point.

3. f-number (currently the conservative 2.0)

Front elements measure 9.95 mm (16 mm lens) and 9.85 mm (12 mm), but the front element is not the entrance pupil — on M12 lenses it is deliberately oversized to accept off-axis rays. The advertised f/2 on the 16 mm implies an 8.0 mm pupil behind a 9.95 mm element, a 1.24× oversize.

Applying that same oversize to the 12 mm rules out the fast end: f/1.2 would need a pupil larger than the front element, and f/1.4 would need less oversize than the 16 mm, which is backwards for a wider lens. Plausible range is f/1.6–f/2.0; 2.0 is recorded because it never over-claims. Not load-bearing in any current code path.

Settle it photometrically rather than by measuring glass: the 16 mm runs +0.62 mag deeper than an f/2 12 mm but only +0.14 mag deeper than an f/1.6 one, against ~0.08–0.11 mag sweep repeatability — a 4–6σ separation. That is the same measurement as (2), so both close together.

Why one sweep does all three

A referenced sweep through the 12 mm gives the fitted FOV (→ effective focal length), the photometric zero point vs the reference meter (→ SQM offset), and the depth comparison against a 16 mm sweep (→ f-number).

Update python/PiFinder/optics.py with the results and flip effective_focal_length_measured=True. tests/test_optics.py asserts that flag, so the test will need updating alongside — deliberately, so the values cannot be changed without someone noticing they are now claiming a measurement.

Found while implementing #609 / ADR 0027.

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