Photometry

Although the scientific goal of the KRED pipeline is to map extended emission-line nebulae, stellar photometry plays a central role at two distinct stages of the reduction.

Relative photometry — ensuring that all images are on the same flux scale — is a prerequisite both for combining overlapping exposures into mosaics and for subtracting the stellar continuum from emission-line images. This is handled automatically by MefPrep using the MAGZERO keyword carried in each MEF header, but tools are provided to verify and, if necessary, improve that calibration.

Absolute photometry — converting instrumental counts into physical flux units (erg s⁻¹ cm⁻² Å⁻¹) — is needed for science-ready emission-line images. This requires comparison to reference stars with known spectra (Gaia XP) and is handled by a separate set of tools.

Where Photometry Enters the Pipeline

Relative photometry enters at two points:

Mosaicking (MefPrep → Swarp). Each DECam exposure of the same field is taken on a different night and possibly under different atmospheric conditions. The MAGZERO keyword encodes the throughput correction for each exposure. MefPrep applies this to rescale every CCD image so that 1 DN = mag 28, placing all images on a common flux scale before SWarp combines them. If MAGZERO is inconsistent across exposures, the mosaic will have visible seams between images taken on different nights.
Continuum subtraction (CleanStars). Emission-line images are cleaned of stellar continuum by constructing a continuum image from broadband (r) and a narrowband off-line (N708) filter, then subtracting it from the emission-line image. This subtraction relies on the stellar fluxes in the two images being on the same flux scale. An error in the relative photometric calibration between the r-band and emission-line images will leave stellar residuals in the continuum-subtracted output.

Absolute photometry enters once the mosaics are ready:

Flux calibration (ZeroPoint). Stars in the swarped tile images are cross-matched with Gaia XP spectra, which provide synthetic photometry at the exact wavelength of each filter. The ratio of predicted to measured counts gives the conversion from DN to physical flux units.

Tool Overview

Module	Used for	Purpose
MefPhot	Both	Forced aperture photometry on MEF images at Gaia or SMASH positions; primary source of `TabPhot/` files used by all calibration tools
CalcZeroPoint	Relative	Quick per-file ZP from sigma-clipped median; `delta_zp` shows how far each header `MAGZERO` is from the measured value
PhotEval	Relative	Multi-frame scatter check; `chi2_nu_c` measures whether overlapping frames are on the same flux scale
ZeroCalc	Relative / Absolute	Regression fit of ZP against Gaia or SMASH broadband magnitudes, with optional color term
PhotCompare	Absolute	Aperture photometry on tile images with catalog cross-match and comparison plots
ZeroPoint	Absolute	Physical flux calibration (DN → erg s⁻¹ cm⁻² Å⁻¹) using Gaia XP spectra
GaiaCat	Both	Retrieve and cache Gaia DR3 source positions and photometry
Smash	Relative	Retrieve SMASH DR2 catalog for Magellanic Cloud fields
PhotAnal	Relative	Inter-filter consistency: compares the same star’s DN across two filters (e.g. r and N662); reports scatter and color term that set the floor on CleanStars continuum subtraction quality
CheckPhot	Relative	One-command wrapper: runs MefPhot (SMASH + Gaia), CalcZeroPoint, PhotEval, and PhotAnal in sequence; groups exposures by Image label from `DeMCELS_images.txt`; writes `CheckPhot/{field}_phot_check.fits` with INTRA and INTER table extensions and prints a formatted comparison summary
StarFind	Absolute	Detect stars for PSF construction
PsfBuild	Absolute	Build PSF models from selected stars
PsfPhot	Absolute	PSF-fitting photometry for crowded fields