CheckPhot

CheckPhot - Photometric consistency check for a DECam field

Space Telescope Science Institute

Synopsis

Runs the complete photometric consistency workflow for one field and produces two concise summary tables: one for intra-filter consistency (are all exposures of the same filter on the same flux scale?) and one for inter-filter consistency (does the same star have the same DN in the r-band image as in the emission-line images?).

Command Line Usage

CheckPhot.py LMC_c42
CheckPhot.py -np 16 LMC_c42
CheckPhot.py -no_mefphot LMC_c42        # skip MefPhot if TabPhot/ already populated
CheckPhot.py -redo LMC_c42              # force reprocessing of all TabPhot files
CheckPhot.py -summary CheckPhot/LMC_c42_phot_check.fits   # reprint summary only
CheckPhot.py -config myconfig.txt LMC_c42   # alternate image-group config

Flags

-np N

Number of parallel MefPhot workers (default 8).

-no_mefphot

Skip the MefPhot step; use existing TabPhot/ files.

-redo

Reprocess already-existing CalcZeroPoint/PhotEval outputs.

-o DIR

Write all outputs to DIR (default: CheckPhot).

-config FILE

Alternate image-group config file (default: DeMCELS_images.txt in current directory, then $KRED/config/).

-summary FITS

Read an existing *_phot_check.fits and reprint the formatted summary without running any photometry.

-ref COL

Additional ZP reference column for CalcZeroPoint (may be repeated).

-h

Print this help.

Description

Exposures are grouped by Image label read from config/DeMCELS_images.txt (e.g. N662 = 400-800 s N662 images, N662_s = 30-60 s N662 images). Files in TabPhot/ are assigned to groups by matching the FILTER header keyword and EXPTIME (within 5 s). Four steps then run in sequence:

  1. MefPhot — forced aperture photometry at SMASH and Gaia positions. Already-processed files are skipped unless -redo is given.

  2. CalcZeroPoint — per-file zero-point fit vs catalog for each Image group. Measures how far each file’s header MAGZERO deviates from the measured ZP.

  3. PhotEval — groups detections of the same star across overlapping frames and computes chi2_nu_c: the reduced chi-squared of the MAGZERO-corrected magnitudes relative to photon noise.

  4. PhotAnal — cross-matches the same star between two Image groups and measures the scatter in (magc_1 - magc_2), which sets the minimum stellar residual in CleanStars output. Pairs are formed automatically within each tier (long vs short exposures).

Output

All outputs are written to CheckPhot/ (or the directory given via -o). All filenames include the field name so multiple fields share the directory without collision.

{field}_phot_check.fits — single FITS file with two binary table extensions:

INTRA (ext 1) — one row per Image group. Zero-point comparison columns are named {stat}_{ref} where {ref} encodes the catalog and reference band (e.g. smash_r, gaia_g). Consistency columns: N_sources, chi2_nu_c_med, chi2_nu_c_emp_med, sigma_c_magzero, sigma_c_gaia_g, sigma_c_smash_r.

INTER (ext 2) — one row per Image-group pair: Image1, Image2, Filter1, Filter2, N_stars, mean_delta_mag, sigma_c_magzero, sigma_c_gaia_g, sigma_c_smash_r, sigma_residual, residual_pct, color_term_b, N_pairs, std_dzp_magzero, std_dzp_gaia_g, std_dzp_smash_r.

sigma_c_magzero and sigma_c_gaia_g / sigma_c_smash_r name the same concept in both extensions — per-star scatter after applying a given ZP reference — so the INTRA and INTER tables can be compared directly.

Intermediate files in CheckPhot/:

  • {field}_zeropoints_{label}.fits — CalcZeroPoint per-file ZP table

  • {field}_phot_eval_{label}.fits — PhotEval per-source chi2 table

  • {field}_filter_compare_{l1}_{l2}.fits — PhotAnal matched-star table

Interpreting the tables

Intra-filter:

chi2_nu_c_emp_med ≈ 1 means frames are mutually consistent at the photon-noise level. std_dzp_{ref} is the MAD scatter of per-file MAGZERO deviations (robust to outliers); > 0.06 mag suggests the empirical ZP correction may help. sigma_c_magzero vs sigma_c_smash_r shows whether MAGZERO or the SMASH calibration gives better intra-group consistency.

Inter-filter:

residual_pct is the minimum percentage of stellar flux that will remain as a residual after CleanStars continuum subtraction. sigma_residual is the scatter after removing the color-term trend; this is the irreducible floor set by stellar color diversity. std_dzp_magzero measures image-to-image stability of the inter-filter flux ratio (the inter-filter analogue of std_dzp in the INTRA table).

History

260701 ksl Written 260702 ksl Split ZP stats by (catalog, ref_col); add -ref flag 260703 ksl Image-group approach from DeMCELS_images.txt; -summary flag; column renames

Functions

check_phot(field[, mef_dir, tabphot_dir, config_file, ...])

Run the full photometric consistency check for field.

steer(argv)

summarize(fits_file)

Read a *_phot_check.fits file and print the comparison summary.

Module Contents

CheckPhot.check_phot(field, mef_dir='DECam_MEF', tabphot_dir='TabPhot', config_file=None, np_proc=8, redo=False, run_mefphot=True, checkdir=None, extra_refs=None)

Run the full photometric consistency check for field.

Parameters

fieldstr

Field name, e.g. 'LMC_c42'.

mef_dirstr

Directory containing {field}/mef/*.fits.fz (default DECam_MEF).

tabphot_dirstr

Directory where MefPhot writes its output (default TabPhot).

config_filestr or None

Path to DeMCELS_images.txt. Searched in current directory then $KRED/config/ if not given.

np_procint

MefPhot parallel workers (default 8).

redobool

Reprocess existing TabPhot files (default False).

run_mefphotbool

If False, skip MefPhot and assume TabPhot/ is already populated.

checkdirstr or None

Directory for all CheckPhot outputs (intermediate and summary). Default: CheckPhot.

extra_refslist of str or None

Additional catalog column names to compare against in CalcZeroPoint.

Returns

intraastropy.table.Table

One row per Image group with intra-filter consistency statistics.

interastropy.table.Table

One row per Image-group pair with inter-filter consistency statistics.

CheckPhot.steer(argv)
CheckPhot.summarize(fits_file)

Read a *_phot_check.fits file and print the comparison summary.

Can be called as:

CheckPhot.py -summary CheckPhot/LMC_c42_phot_check.fits