Absolute Photometry

Absolute photometry converts instrumental counts (DN) into physical flux units (erg s⁻¹ cm⁻² Å⁻¹). This is needed to compare emission-line measurements with other observations or theoretical models, and to compute line luminosities.

Two levels of calibration are available:

Magnitude calibration — places instrumental magnitudes on the Gaia or SMASH photometric system. Useful for checking the pipeline MAGZERO against an independent reference and for deriving color terms.
Physical flux calibration — converts DN directly to erg s⁻¹ cm⁻² Å⁻¹ using Gaia XP spectra as spectroscopic standard references. This is the appropriate calibration for the Hα and [SII] emission-line mosaics.

Step 1: Forced photometry on MEF files

All calibration paths start from MefPhot, which measures stellar fluxes at catalog positions in every CCD extension of each MEF file:

MefPhot.py -r 6 -b 8 12 DECam_MEF/LMC_c42/*.fits          # Gaia (default)
MefPhot.py -cat smash -r 6 -b 8 12 DECam_MEF/LMC_c42/*.fits  # SMASH

Key parameters:

-r RADIUS — aperture radius in pixels (default 6)
-b INNER OUTER — background annulus inner and outer radii (default 8, 12)
-np N — parallel processes (default 8)
-cat gaia|smash — reference catalog (default gaia)

Output tables are written to TabPhot/:

TabPhot/<root>.gaia.fits    (Gaia-matched)
TabPhot/<root>.smash.fits   (SMASH-matched, with -cat smash)

Each table carries phot_mag (instrumental magnitude, ZP = 28), the reference catalog magnitudes (G, R for Gaia; R, G for SMASH), and file-level metadata in the extension 1 header (FILTER, EXPTIME, MAGZERO, FIELD, ROOT, DETNAME).

Note

If you have already run MefPhot for the relative photometry checks (Relative Photometry), the same TabPhot/ files are used here. You do not need to re-run MefPhot.

Step 2: Magnitude calibration

ZeroCalc reads one or more TabPhot/ files and fits a zero-point model relating instrumental magnitudes to reference catalog magnitudes.

Simple fit (default):

\[m_{\rm ref} = m_{\rm inst} + c_0\]

Color-corrected fit (-color):

\[m_{\rm ref} = m_{\rm inst} + c_0 + c_1 \times \mathrm{color}\]

where the color predictor is chosen to be independent of the target band:

Target	Catalog	Color
R	Gaia	G − R
G	Gaia	B − R
R	SMASH	G − R
G	SMASH	U − R

The derived zero point is \(28 + c_0\). All fits are weighted by the per-source photometric uncertainty.

ZeroCalc.py -R TabPhot/*.gaia.fits                  # simple, Gaia R
ZeroCalc.py -R -color TabPhot/*.gaia.fits           # color-corrected, Gaia R
ZeroCalc.py -G -color TabPhot/*.gaia.fits           # Gaia G
ZeroCalc.py -R -smash TabPhot/*.smash.fits          # SMASH R
ZeroCalc.py -R -color -smash TabPhot/*.smash.fits   # color-corrected, SMASH R

Output filenames encode the band, catalog, and fit mode:

MagZero.<band>.gaia.txt / MagZero.<band>.gaia.color.txt
MagZero.<band>.smash.txt / MagZero.<band>.smash.color.txt
FigZero/<band>_<root>.<cat>[.color].png — per-file diagnostic plots

Each summary table has one row per input file with columns Filter, Exptime, Root, MagZero (= 28 + c_0), c_0, c_1, rms, and HdrZero (the pipeline MAGZERO).

Comparing the derived MagZero to HdrZero across many files gives a direct statistical measure of how well the NOIRLAB pipeline calibration tracks the true zero point for your dataset:

from astropy.table import Table
import numpy as np

mz = Table.read('MagZero.R.gaia.color.txt', format='ascii.fixed_width_two_line')
delta = mz['MagZero'] - mz['HdrZero']
print(f"Mean offset (derived - pipeline): {np.mean(delta):+.3f}")
print(f"Std  offset                      : {np.std(delta):.3f}")

Step 3: Physical flux calibration

For the Hα (N662) and [SII] (N673) emission-line mosaics, the conversion from DN to physical flux (erg s⁻¹ cm⁻² Å⁻¹) uses Gaia XP spectra as spectroscopic standards.

Why XP spectra? Most reference catalogs provide broad-band magnitudes (G, R, …) that must be transformed to the narrow bandpass of each emission-line filter using synthetic photometry, which introduces color-term uncertainties. Gaia XP spectra cover 330–1050 nm at low resolution and allow a direct synthetic-photometry prediction for any filter.

Sub-step 3a: Cross-match photometry on tile images

Run PhotCompare on the swarped tile images to produce cross-matched tables linking each detected star to its Gaia source:

PhotCompare.py -forced DECam_SWARP/LMC_c42/T01/*.fits
PhotCompare.py -forced DECam_SWARP/LMC_c42/T02/*.fits

Output is written to TabPhot/xmatch_<name>.fits (one file per tile image) and diagnostic figures to Figs_phot/.

Sub-step 3b: Derive the DN → flux conversion

Run ZeroPoint on the cross-matched tables:

ZeroPoint.py TabPhot/xmatch_*.fits

ZeroPoint retrieves the Gaia XP spectrum for each matched star, integrates it through the filter bandpass to predict the expected count rate, and compares it to the measured DN to determine the conversion factor. Results are accumulated in PhotMaster.txt:

Using SMASH as the Reference Catalog

For Magellanic Cloud fields the SMASH DR2 catalog (Nidever et al. 2017) offers advantages over Gaia for broadband calibration:

Its photometry is in the DECam ugriz system, so the color term \(c_1\) is near zero for r-band images.
It is deeper (r ~ 22) than Gaia (G ~ 20), giving more reference stars per CCD in the crowded LMC/SMC fields.
It includes a stellar probability parameter (prob) for star-galaxy separation.

SMASH is not all-sky and has no coverage for the emission-line filters (N662, N673). ZeroPoint (spectroscopic flux calibration) has no SMASH equivalent and always uses Gaia XP spectra.

PSF Photometry (crowded fields)

In regions where stellar crowding makes aperture photometry unreliable, a PSF-fitting path is available.

Detect sources with StarFind:
```
StarFind.py image.fits
```

Build a PSF model with PsfBuild:

PsfBuild.py image.fits TabPhot/<root>_all_stars.fits

Fit the PSF with PsfPhot:
```
PsfPhot.py image.fits <root>_psf_model.fits TabPhot/<root>_all_stars.fits
```
Output is a photometry table with the same structure as MefPhot output and can be passed directly to ZeroCalc.

MefPhot (forced at fixed catalog positions) is preferred for zero-point determination because the source list is unambiguous. PsfPhot is preferred when completeness or accurate fluxes in crowded regions matter.

Output Files

File	Created by	Contents
`TabPhot/<name>.gaia.fits`	MefPhot	Per-star photometry at Gaia positions
`TabPhot/<name>.smash.fits`	MefPhot -cat smash	Per-star photometry at SMASH positions
`MagZero.<band>.gaia.txt`	ZeroCalc	Per-file ZP fit vs Gaia: `c_0`, `c_1`, `rms`, `HdrZero`
`MagZero.<band>.smash.txt`	ZeroCalc -smash	Per-file ZP fit vs SMASH
`FigZero/`	ZeroCalc	Per-file diagnostic plots
`TabPhot/xmatch_<name>.fits`	PhotCompare	Stars cross-matched between tile images and Gaia
`Figs_phot/`	PhotCompare	Comparison plots (DECam vs catalog magnitudes)
`PhotMaster.txt`	ZeroPoint	Per-tile DN → erg s⁻¹ cm⁻² Å⁻¹ at Hα and [SII]
`GAIA/`	GaiaCat	Cached Gaia source catalogs