gphot

gphot measures the total flux of every star in a frame from a growth curve.

The growth curve (the fraction of a star’s light inside each aperture) is built from the bright, isolated stars and then used to estimate the total flux of all stars. Faint stars get small, low-noise apertures and bright stars get large ones, all corrected to the same total, so the fluxes stay accurate across a wide range of brightness. phcal then calibrates the result; it picks these fluxes by default when the frame has them (--photometry grow).

Usage

munipack gphot [options] file(s)

Run find and aphot first. aphot should use many apertures, from about 1 FWHM out to several FWHM, all inside the sky ring, so the curve can reach its plateau. gphot needs at least 3 bright isolated stars and stops with an error if it finds fewer.

By default the input file is updated in place. Use -t to write into another directory, or give input and output as one quoted argument separated by a space: 'frame.fits result.fits'. The comma form shown by --help does not work.

Options

Growth curve

Option	Argument	Default	Description
--var	0, 1, 2	2	Highest polynomial order for how the curve varies over the field; raised only while significant. Not with --legacy.
--radial	—	off	Let the curve vary with distance from the field centre instead of x,y. Not with --legacy.
--centre	x,y [pix]	NAXIS/2	Field centre for --radial (requires --radial).
--legacy	—	off	Use the growth-curve fit of earlier Munipack versions (before 0.6.2+hg1653).
-th, --threshold	real	0.1	With --legacy only: curve stars must be brighter than this times the sky in the aperture.

Output and general

Option	Argument	Default	Description
-t, --target-directory	directory	—	Write results into this (existing) directory instead of updating the input.
-O	—	off	Turn on advanced output filenames.
--pattern	regex	(.+)\.(.+)	Pattern matched against input filenames (with -O).
--mask	string	—	Output filename built from the pattern’s groups (with -O).
--format	printf format	—	Format for numbered output files, e.g. out_%02d.fits (with -O).
--verbose	—	off	Print the fitted growth curve and how it was used.
-h, --help	—	—	Show the help and exit.

Output

gphot adds two tables to the frame, replacing them if they already exist:

• GROWPHOT, one row per star: position X, Y, SKY, SKYERR, the total flux GCOUNT [ADU] with its error GCOUNTERR, and GROWFLAG (1 for stars used to build the curve, 0 otherwise). Stars without a valid flux have GCOUNT = GCOUNTERR = -1. The header records the curve’s HWHM, the 90%-flux radius RADFLX90, and EE* keys describing the fit.

• GROWCURVE, one row per aperture: radius R, the growth curve GROWCURVE with its error, and the radial profile RADIALPROFILE.

Examples

The full chain from detection to calibration:

munipack find frame.fits
munipack aphot --apertures 1,1.313,1.7241,2.2637,2.9724,3.9028,5.1245,6.7287,8.8349,11.601,15.232,20,23,26.4,30.4,35,40 \
               --ring 45,60 frame.fits
munipack gphot frame.fits
munipack phcal --photometry grow --photsys-ref Johnson -f V \
               --col-ra RAJ2000 --col-dec DEJ2000 --col-mag Vmag --col-magerr e_Vmag \
               -c catalogue.fits frame.fits

The calibrated stars end up in frame_cal.fits.

Wide-field optics, where the PSF changes with distance from the optical axis:

munipack gphot --radial --centre 1024,1024 frame.fits

A constant growth curve, written to a new file, with the fit printed:

munipack gphot --var 0 --verbose 'frame.fits frame_grow.fits'

Notes

• GCOUNT is the flux inside the largest (plateau) apertures, not an extrapolation to infinity. Light in faint, extended PSF wings is missed, but it is the same fraction for every star, so the photometric calibration absorbs it.

• The growth curves come from bright, isolated stars, and isolation is judged from the FWHM written by find. If that FWHM is badly off, for example because stars were split into several detections, check the find result first. On a frame with very few stars, use aphot --psf-file or psfphot instead.

• psfphot is the other way to get total fluxes.