psf_photometry

An easy to use python script for PSF photometry.

Usage

Set your astrometry.net API key in an environment variable and run the script:

$env:ASTROMETRY_API_KEY = "your_key_here"
python psf_photometry.py --fits path\to\image.fits --out results.csv

Optional: increase nonlinear fit robustness with more multi-start seeds:

python psf_photometry.py --fits path\to\image.fits --out results.csv --fit-seed-density 2

The generated results.csv contains one row per PSF-photometry source with these columns:

1. RA (right ascension in degrees from solved WCS)
2. dec (declination in degrees from solved WCS)
3. flux (PSF photometry flux)
4. v_mag (APASS V magnitude if matched, otherwise empty)
5. b-v (APASS B-V color index if matched, otherwise empty)
6. v_mag_cal (calibrated V magnitude from nonlinear regression of flux vs v_mag)
7. fit_model (used calibration model)
8. fit_flux_low, fit_flux_high, fit_m0, fit_slope (4-parameter sigmoid fit values)
9. fit_linear_intercept (only set for linear fallback)
10. fit_seed_density, fit_n_refstars (fit configuration and number of APASS reference stars)

For differential photometry, the script fits a 4-parameter sigmoid (logistic) in the forward direction flux(v_mag) and then computes v_mag(flux) by numerical inversion (bisection). This provides the required inverse mapping for calibration of all detected stars (including those without APASS data). If the nonlinear fit cannot be computed, a linear fallback in log10(flux) is used.

Additionally, a PNG plot v_mag vs flux is written to <out>_vmag_vs_flux.png by default. Plot points are colored by b-v using a red-to-blue palette, with larger b-v values shown in blue. You can set a custom plot path with --plot path\to\plot.png.

For quality control, an additional plot <out>_vmagcal_vs_vmag.png is written. It shows v_mag_cal vs APASS v_mag for stars with known APASS v_mag, includes the reference line v_mag = v_mag_cal (Winkelhalbierende), and colors points by b-v.

Dependencies

• astropy
• astroquery
• photutils
• numpy
• matplotlib

Install with:

python -m pip install astropy astroquery photutils numpy matplotlib