From Gaia photometry to count rates

The starflux module converts each Gaia star into a detector count rate through the wcc_etc instrument model. The chain, per star:

  1. BP−RP → spectral type. The star’s Gaia BP−RP color is matched to the nearest Pickles dwarf template color (spt_from_bp_rp(), table in wcc_sim/data/bp_rp_to_spt.csv). Stars with missing/NaN colors fall back to G2V.

  2. Normalize in Gaia G. That Pickles spectrum is normalized to the star’s phot_g_mean_mag (Vega system) in the Gaia DR3 G bandpass (SVO filter service GAIA/GAIA3.G, shipped as wcc_sim/data/gaia_dr3_g.dat).

  3. Integrate through the instrument. The normalized spectrum is pushed through the full wcc_etc throughput (telescope + filter + detector QE) to a total point-source rate in e-/s (rate_for_spt()).

Memoization

Computing a synphot integration per star would be prohibitively slow for a full array with tens of thousands of stars. Instead the rate is computed once per (spectral type, sensorfilter) pair at a reference magnitude G = 15 and scaled analytically:

\[\mathrm{rate}(G) = \mathrm{rate}(G{=}15) \times 10^{-0.4\,(G - 15)}\]

This is exact for a fixed spectral shape, and reduces the synphot work from ~50 000 integrations to at most ~45 (one per Pickles dwarf type).

Note

The reference pin used by the test suite: rate_for_spt("G2V", "zwo:r") ≈ 38 484 e-/s at G = 15. If the wcc_etc throughput model changes, test_rate_reference_pin explains how to re-record it.

Sky and dark

sky_and_dark_rates() extracts the per-pixel zodiacal sky background and dark-current rates from the same base simulation; both are added uniformly to the image in the noise stage (Noise and saturation), and both are recorded in field.params and the FITS header (SKYRATE, DARK).

Practical notes

  • Rates assume Pickles dwarf spectra; giants of the same color are slightly off in the band-shape correction, which is acceptable for a context camera simulation.

  • The per-(type, filter) cache lives for the Python session (module-level dict); the first star of each type pays the synphot cost.

  • spt values in the output catalog let you audit exactly which template each star received.