FA-55719 / Astronomical coordinate conventions / Member archive
Celestial tangent projection: Celestial TAN north coordinate uses east-axis rather than center-direction denominator · case 04
The adapter reports an incorrect chart north while other fields remain valid.
Case contract
A supplied unnormalized direction basis has dot products east e, north n, and center z. The TAN chart accepts z>0 and returns e/z,n/z. Scale s and reference pixels rx,ry map this chart to detector pixels. Behind-plane rays are explicitly returned as None. Output fields are defined by: chart_east = d['e']/d['z'] if d['z']>0 else None; chart_north = d['n']/d['z'] if d['z']>0 else None; pixel_x = d['rx']+d['e']/d['z']/d['s'] if d['z']>0 else None; pixel_y = d['ry']+d['n']/d['z']/d['s'] if d['z']>0 else None; chart_valid = d['z']>0; reciprocal_center = 1/d['z'] if d['z']>0 else None
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 fixtureThis sample comes from the broken implementation of a controlled reproducer.
| Boundary fixture | Actual | Expected | Outcome |
|---|---|---|---|
| astronomical fixture 2 | {"chart_east": null, "chart_north": -1.5, "chart_valid": false, "pixel_x": null, "pixel_y": null, "reciprocal_center": null} | {"chart_east": null, "chart_north": null, "chart_valid": false, "pixel_x": null, "pixel_y": null, "reciprocal_center": null} | Failed |
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