FA-56214 / Astronomical coordinate conventions / Member archive
Spectrograph grating coordinate: Inverse grating coordinate fails to remove the incident-angle contribution · case 04
The adapter reports an incorrect inferred diffraction sine while other fields remain valid.
Case contract
A stipulated grating adapter supplies spacing g, signed sine of incidence a, signed sine of diffraction b, positive order m, focal length F, and separate calibration line wavelength line. Wavelength is g*(a+b)/m. A Littrow reference has b=a; supplied diffraction cosine c permits a local angular dispersion derivative. Output fields are defined by: wavelength = d['g']*(d['a']+d['b'])/d['m']; littrow_wavelength = 2*d['g']*d['a']/d['m']; angular_dispersion = d['g']*d['c']/d['m']; linear_dispersion = d['g']*d['c']/(d['m']*d['F']); inferred_diffraction_sine = d['m']*d['line']/d['g']-d['a']; next_order_wavelength = d['g']*(d['a']+d['b'])/(d['m']+1)
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 0 | {"angular_dispersion": 20.5, "inferred_diffraction_sine": 0.7439024390243902, "linear_dispersion": 2.05, "littrow_wavelength": 41.0, "next_order_wavelength": 23.0625, "wavelength": 30.75} | {"angular_dispersion": 20.5, "inferred_diffraction_sine": -0.25609756097560976, "linear_dispersion": 2.05, "littrow_wavelength": 41.0, "next_order_wavelength": 23.0625, "wavelength": 30.75} | Failed |
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