FA-56115 / Astronomical coordinate conventions / Member archive

Radio fft channel labels: Shifted FFT channel coordinates put zero between central bins · case 05

The adapter reports an incorrect shifted frequency while other fields remain valid.

Member previewVariant 5 · 3 implementations · 6 checks per implementation

Case contract

An even-length complex radio spectrum has N channels and bandwidth B. Unshifted bin k has signed index k below N/2 and k-N otherwise, including negative Nyquist. After fftshift index j denotes j-N/2. Local oscillator L translates baseband to sky frequency. Output fields are defined by: signed_bin = d['k'] if d['k']<d['N']//2 else d['k']-d['N']; baseband_frequency = (d['k'] if d['k']<d['N']//2 else d['k']-d['N'])*d['B']/d['N']; shifted_frequency = (d['j']-d['N']//2)*d['B']/d['N']; sky_frequency = d['L']+(d['k'] if d['k']<d['N']//2 else d['k']-d['N'])*d['B']/d['N']; shifted_index = (d['k']+d['N']//2)%d['N']; channel_width = d['B']/d['N']

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	`{"baseband_frequency": 4.0, "channel_width": 2.0, "shifted_frequency": 2.0, "shifted_index": 6, "signed_bin": 2, "sky_frequency": 108.0}`	`{"baseband_frequency": 4.0, "channel_width": 2.0, "shifted_frequency": -6.0, "shifted_index": 6, "signed_bin": 2, "sky_frequency": 108.0}`	Failed

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