# Renderer-facing 2MASS star catalog

`2mass_psc_m31_0p5deg_stars.csv` is made from the corresponding table in
`../raw/` by `python3 scripts/process_2mass_psc.py`.

The processor accepts `--input` and `--output`, so the same documented quality
selection and fit are used for the M44 comparison field.

The CSV is intentionally compatible with the current four-column catalog
loader: `ra_deg,dec_deg,temperature_K,amplitude_sr`.  RA and Dec are ICRS/J2000
degrees.  The loader treats them as longitude and latitude when constructing
the source-sky direction.

## Quality selection

All three J/H/Ks magnitudes must be finite and every character of `rd_flg` must
be `1`, `2`, or `3`.  These are the 2MASS default-magnitude origins that
generally indicate the best detections, photometry, and astrometry.  Thus the
script rejects nondetections/upper limits (`0`), poor aperture photometry (`4`),
inconsistent band deblends (`6`), and missing brightness estimates (`9`).

## Two-parameter fit

For J, H, Ks effective wavelengths 1.235, 1.662, and 2.159 micrometres and
Vega zero points 1594, 1024, and 666.7 Jy, respectively, each magnitude is
converted to flux density using `F_nu = F_nu,0 * 10^(-0.4 m)`.

The script then performs an unweighted least-squares fit in `log(F_nu)` to

`F_nu = amplitude_sr * B_nu(T)`.

`temperature_K` is constrained to 300--100000 K; `amplitude_sr` is the fitted
apparent solid angle in steradians.  The fit treats each band as monochromatic
at its effective wavelength and does not deredden the source.  This is the
same two-parameter thermal-source reduction described qualitatively in the
reference paper, but the paper does not publish its fit implementation; this
script therefore records the numerical choice explicitly.

References: [2MASS `rd_flg` definitions](https://irsa.ipac.caltech.edu/data/2MASS/docs/releases/allsky/doc/sec1_6b.html),
[2MASS zero points](https://www.ipac.caltech.edu/2mass/releases/allsky/faq.html).