FA-55759 / Astronomical coordinate conventions / Member archive

Celestial sin projection: Inverse oblique SIN conversion subtracts a constant shear instead of direction-dependent displacement · case 04

The adapter reports an incorrect undo obliqueness east while other fields remain valid.

Member previewVariant 4 · 3 implementations · 6 checks per implementation

Case contract

A reduced SIN sky projection supplies direction east e,north n,center z and obliqueness coefficients a,b. Its plane coordinates are e+a*(1-z),n+b*(1-z). Orthographic mode a=b=0 still has front/back hemisphere ambiguity; forward metadata preserves z for inverse branch selection. Output fields are defined by: plane_east = d['e']+d['a']*(1-d['z']); plane_north = d['n']+d['b']*(1-d['z']); hemisphere = 'front' if d['z']>=0 else 'back'; undo_obliqueness_east = d['e']-d['a']*(1-d['z']); undo_obliqueness_north = d['n']-d['b']*(1-d['z']); center_offset = 1-d['z']

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	`{"center_offset": 0.5, "hemisphere": "front", "plane_east": 6.0, "plane_north": 1.5, "undo_obliqueness_east": 6.0, "undo_obliqueness_north": 4.5}`	`{"center_offset": 0.5, "hemisphere": "front", "plane_east": 6.0, "plane_north": 1.5, "undo_obliqueness_east": 4.0, "undo_obliqueness_north": 4.5}`	Failed

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