FA-56323 / Astronomical coordinate conventions / Member archive

Wcs header card semantics: CRPIX header defaulting conflates an explicit zero with an absent card · case 03

The adapter reports an incorrect reference pixel while other fields remain valid.

Member previewVariant 3 · 3 implementations · 6 checks per implementation

Case contract

A bounded FITS-like header adapter parses the projection code at CTYPE characters 5:8, supports exact case-sensitive frequency units Hz,kHz,MHz,mHz, defaults absent CRPIX to one but preserves explicit zero, resolves WCSNAME to its suffix, and binds PV parameters to world-axis number. Effective dimensionality is max(NAXIS,WCSAXES). Output fields are defined by: projection = d['ctype'][5:8]; frequency_hz = d['frequency']*{'Hz':1,'kHz':1000,'MHz':1000000,'mHz':0.001}[d['unit']]; reference_pixel = 1 if d['crpix'] is None else d['crpix']; dimension = max(d['naxis'],d['wcsaxes']); selected_suffix = next(k for k,v in d['names'].items() if v==d['name']); projection_parameter = d['pv'][d['world']]

Why this case matters

Catalog, detector, sky-coordinate, and spectroscopy adapters must preserve the association between numeric coordinates and their declared reference conventions.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
astronomical fixture 0	`{"dimension": 3, "frequency_hz": 0.01, "projection": "TAN", "projection_parameter": 7, "reference_pixel": 1, "selected_suffix": "A"}`	`{"dimension": 3, "frequency_hz": 0.01, "projection": "TAN", "projection_parameter": 7, "reference_pixel": 0, "selected_suffix": "A"}`	Failed

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