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assets/2mass/processed/README.md

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+# 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).