offline_matched_coadd_ccds_SURVEY

Summary:

The gfa_reduce pipeline performs an “offline” analysis of full-frame DESI Guide-Focus-Alignment (GFA) images which does not influence observing operations in real time but nevertheless may provide useful metrics related to observing conditions and GFA detector behavior. For instance, gfa_reduce provides independent measurements of observing conditions such as seeing and transparency.

Naming Convention:

offline_matched_coadd_ccds_{SURVEY}-thru_{NIGHT}.fits, where {SURVEY} is e.g., ‘main’ and {NIGHT} is the observation night in YYYYMMDD format.

Regex:

offline_matched_coadd_ccds_(main|SV1|SV2|SV3)-thru_[0-9]{8}\.fits

File Type:

FITS, 250 MB

Contents

Number	EXTNAME	Type	Contents
HDU0		IMAGE	Empty HDU
HDU1	CAMERA_SUMMARY	BINTABLE	Summary results of individual guide cameras.
HDU2	EXPOSURE_SUMMARY	BINTABLE	Summary results accumulated per-exposure for all guide cameras.
HDU3	EXPOSURE_SUMMARY_STRICT	BINTABLE	Summary results accumulated per-exposure for all guide cameras with stricter data quality cuts.

FITS Header Units

HDU0

This HDU has no non-standard required keywords.

Empty HDU.

HDU1

EXTNAME = CAMERA_SUMMARY

CAMERA_SUMMARY is a simple concatenation of all per-exposure gfa_reduce _ccds tables. CAMERA_SUMMARY has one row per exposure per guider camera, so the total number of rows is roughly 6 times the number of exposures. There are more than 100 columns in the CAMERA_SUMMARY table, most of which shouldn’t be relevant for the purpose of estimating effective exposure times.

Required Header Keywords

Required Header Keywords Table

KEY	Example Value	Type	Comment
NAXIS1	1749	int	length of dimension 1
NAXIS2	140700	int	length of dimension 2

Required Data Table Columns

Name	Type	Units	Description
MEDIAN	float64	adu	Median of raw CCD image.
MEAN	float64	adu	Mean of raw CCD image.
N_NON_FINITE	int64	adu	Number of non-finite pixel values in raw CCD image.
MAX	float64	adu	Max pixel value of raw CCD image.
MIN	float64	adu	Min pixel value of raw CCD image.
SIG_ROBUST	float64	adu	Robust standard deviation of raw CCD image.
SIG	float64	adu	Standard deviation of raw CCD image.
MEDIAN_Q1	float64	adu	Median of quadrant 1 (amp G)
MEAN_Q1	float64	adu	Mean of quadrant 1 (amp G)
N_NON_FINITE_Q1	int64	adu	Number of non-finite pixel values in quadrant 1 (amp G)
MAX_Q1	float64	adu	Maximum raw GFA image pixel value in ADU in quadrant 1 (amp G)
MIN_Q1	float64	adu	Minimum raw GFA image pixel value in ADU in quadrant 1 (amp G)
SIG_ROBUST_Q1	float64	adu	Robust standard deviation of quadrant 1 (amp G)
SIG_Q1	float64	adu	Standard deviation of quadrant 1 (amp G)
MEDIAN_Q2	float64	adu	Median of quadrant 2 (amp H)
MEAN_Q2	float64	adu	Mean of quadrant 2 (amp H)
N_NON_FINITE_Q2	int64	adu	Number of non-finite pixel values in quadrant 2 (amp H)
MAX_Q2	float64	adu	Maximum raw GFA image pixel value in ADU in quadrant 2 (amp H)
MIN_Q2	float64	adu	Minimum raw GFA image pixel value in ADU in quadrant 2 (amp H)
SIG_ROBUST_Q2	float64	adu	Robust standard deviation of quadrant 2 (amp H)
SIG_Q2	float64	adu	Standard deviation of quadrant 2 (amp H)
MEDIAN_Q3	float64	adu	Median of quadrant 3 (amp E)
MEAN_Q3	float64	adu	Mean of quadrant 3 (amp E)
N_NON_FINITE_Q3	int64	adu	Number of non-finite pixel values in quadrant 3 (amp E)
MAX_Q3	float64	adu	Maximum raw GFA image pixel value in ADU in quadrant 3 (amp E)
MIN_Q3	float64	adu	Minimum raw GFA image pixel value in ADU in quadrant 3 (amp E)
SIG_ROBUST_Q3	float64	adu	Robust standard deviation of quadrant 3 (amp E)
SIG_Q3	float64	adu	Standard deviation of quadrant 3 (amp E)
MEDIAN_Q4	float64	adu	Median of quadrant 4 (amp F)
MEAN_Q4	float64	adu	Mean of quadrant 4 (amp F)
N_NON_FINITE_Q4	int64	adu	Number of non-finite pixel values in quadrant 4 (amp F)
MAX_Q4	float64	adu	Maximum raw GFA image pixel value in ADU in quadrant 4 (amp F)
MIN_Q4	float64	adu	Minimum raw GFA image pixel value in ADU in quadrant 4 (amp F)
SIG_ROBUST_Q4	float64	adu	Robust standard deviation of quadrant 4 (amp F)
SIG_Q4	float64	adu	Standard deviation of quadrant 4 (amp F)
CAMERA	char[6]		Camera identifier. Passband and SPECGRPH ([brz][0-9]).
EXTNAME	char[6]		String guide camera name; one of GUIDE0, GUIDE2, GUIDE3, GUIDE5, GUIDE7, GUIDE8
CONTRAST	float64		Pattern-matching contrast for gfa_reduce astrometry; larger CONTRAST indicates a more robust astrometric solution
SKY_MAG_AB	float64	mag arcsec^-2	GFA-measured sky brightness.
SKY_MAG_AB_SUBREGION	float64	mag arcsec^-2	Sky brightness in a subregion of the GFA image.
SKY_MAG_AB_PER_AMP	float32[4]	mag arcsec^-2	Per-amp GFA-measured sky brightness.
PETAL_LOC	binary		Petal location [0-9]
EXPID	int64		DESI Exposure ID number
MJD	float64	d	Modified Julian Date when shutter was opened for this exposure
LST_DEG	float64	deg	Local Sidereal Time in degrees
MOON_ILLUMINATION	float64		Moon illumination fraction.
PROGRAM	char[88]		DESI program type - BRIGHT, DARK, BACKUP, OTHER
SKYRA	float64	deg	Boresight RA from TCS.
SKYDEC	float64	deg	Boresight DEC from TCS.
ZENITH_DIST_DEG	float64	deg	Boresight zenith distance.
DOMSHUTL	char[8]		Dome lower shutter status inherited from TCS.
DOMSHUTU	char[8]		Dome upper shutter status inherited from TCS.
PMCOVER	char[8]		Primary mirror cover status inherited from TCS.
MOONRA	float64	deg	RA of the Moon at the relevant epoch.
MOONDEC	float64	deg	Dec of the Moon at the relevant epoch.
MOON_ZD_DEG	float64	deg	Moon angular distance from zenith.
T_C_FOR_DARK	float64	deg	Temperature used for dark current calculation (degrees Celsius).
T_C_FOR_DARK_IS_GUESS	int64		Flag for whether temperature for dark current was a guess due to lack of metadata.
TIME_S_FOR_DARK	float64	s	Exposure time used for dark current calculation.
NIGHT	int64		Night of observation (YYYYMMDD) starting at local noon before observations start
FOCUS	char[39]		Hexapod focus settings inherited from TCS.
EXPTIME	float64	s	Length of time shutter was open
CUBE_INDEX	int64		Integer frame counter within the guide cube; CUBE_INDEX increases with time within each guide cube; 0 for acquisition images; -1 for matched coadds
COADD_INDEX_START	int64		Starting index of the coadd within the guider image cube sequence.
COADD_INDEX_END	int64		Ending index (inclusive) of the coadd within the guider image cube sequence.
COADD_MJDOBS_MIN	float64	d	Minimum MJD of guider frames contributing to the coadd.
COADD_MJDOBS_MAX	float64	d	Maximum MJD of guider frames contributing to the coadd.
RACEN	float64	deg	Central RA of the guider image.
DECCEN	float64	deg	Central Dec of the guider image.
FNAME_RAW	char[76]		Raw file name for guider image cube.
GITREV	char[7]		Git version for gfa_reduce software.
FIBER_FRACFLUX	float64		Fraction of flux in a DESI fiber for a point source.
FIBER_FRACFLUX_ELG	float64		Fraction of flux in a DESI fiber for a fiducial ELG morphology.
FIBER_FRACFLUX_BGS	float64		Fraction of flux in a DESI fiber for a fiducial BGS morphology.
N_SOURCES_FOR_PSF	int64		Number of sources used for PSF model ; key observing conditions quantities rely on the PSF model, so at least 3 sources for PSF-making is desirable
APER_CORR_FAC	float64		Correction factor to go from aperture to total light used for zeropoint determination.
XCENTROID_PSF	float64	pix	Diagnostic giving PSF postage stamp’s x-axis centroid location to potentially flag ill-determined PSF models.
YCENTROID_PSF	float64	pix	Diagnostic giving PSF postage stamp’s y-axis centroid location to potentially flag ill-determined PSF models.
PSF_FWHM_PIX	float64	pix	PSF FWHM in pixels.
PSF_FWHM_ASEC	float64	arcsec	PSF FWHM in arcseconds.
PSF_CENTROID_CBOX	float64	pix	Centering box sidelength used for PSF centroid determination.
PSF_CENTROID_FAILED	int64		Flag indicating whether PSF centroiding failed.
RADPROF_FWHM_ASEC	float64	arcsec	PSF FWHM in arcseconds determined from a fit to a 1-dimensional radial profile of the PSF.
PSF_CENTROIDING_FLAG	int64		Flag indicating potential issues with PSF centroiding.
PSF_ASYMMETRY_RATIO	float32		Dimensionless metric characterizing the level of PSF asymmetry.
PSF_ASYMMETRY_NUMERATOR	float32		Numerator of the PSF_ASYMMETRY_RATIO metric.
PSF_ASYMMETRY_DENOMINATOR	float32		Denominator of the PSF_ASYMMETRY_RATIO metric.
PSF_TOTAL_FLUX	float32	adu	Total PSF flux.
PROFILE_RADIUS_PIX	float32[26]	pix	List of radius values in pixels for the PSF radial profile.
PSF_RADIAL_PROFILE	float32[26]		List of PSF amplitude values for the PSF radial profile.
MOUNTHA_HEADER	float64	deg	MOUNTHA hour angle inherited from TCS.
MOUNTDEC_HEADER	float64	deg	MOUNTDEC declination inherited from TCS.
HA_DEG	float64	deg	Boresight hour angle in degrees calculated independently of MOUNTHA reported by TCS.
HA_DEG_PER_GFA	float64	deg	Hour angle in degrees for the relevant GFA camera.
MOON_SEP_DEG	float64	deg	Boresight angular distance to the Moon.
ZD_DEG_PER_GFA	float64	deg	Zenith distance in degrees calculated for the relevant GFA camera.
HEADER_AIRMASS	float64		AIRMASS inherited from TCS.
AIRMASS	float64		Boresight airmass calculated independently of TCS value.
AIRMASS_PER_GFA	float64		Airmass in degrees for the relevant GFA camera.
ZP_ADU_PER_S	float64	mag	GFA camera zeropoint (r-band mag corresponding to total flux of 1 ADU/second).
N_STARS_FOR_ZP	int64		Number of stars used in computing the zeropoint.
TRANSPARENCY	float64		r-band transparency value for this GFA camera.
KTERM	float32	mag	k-term in zeropoint calculation corresponding to this GFA camera’s zenith distance.
FRACFLUX_NOMINAL_POINTSOURCE	float32		Point source fraction of flux for a circular aperture roughly approximating the size of a DESI fiber.
FRACFLUX_NOMINAL_ELG	float32		Fiducial ELG morphology fraction of flux for a circular aperture roughly approximating the size of a DESI fiber.
FRACFLUX_NOMINAL_BGS	float32		Fiducial BGS morphology fraction of flux for a circular aperture roughly approximating the size of a DESI fiber.
DET_SN_THRESH	float64		Detection signal-to-noise threshold used.
NPIX_BAD_TOTAL	int64		Number of bad pixels in overscan/prescan ; NPIX_BAD_TOTAL of 10 or larger may indicate insufficient GFA “denoising” (bad A/D conversion)
NPIX_BAD_PER_AMP	int64[4]		Number of bad pixels for each amplifier in this GFA camera.
OVERSCAN_MEDIANS_ADU	float32[4]	adu	Overscan median pixel value per amp.
PRESCAN_MEDIANS_ADU	float32[4]	adu	Prescan median pixel value per amp.
FWHM_MAJOR_PIX	float64	pix	FWHM along major axis of PSF.
FWHM_MINOR_PIX	float64	pix	FWHM along minor axis of PSF.
FWHM_PIX	float64	pix	PSF FWHM in pixels.
FWHM_ASEC	float64	arcsec	PSF FWHM in arcseconds.
N_SOURCES	int64		Number of sources used for PSF determination.
N_SOURCES_FOR_SHAPE	int64		Number of sources whose shapes were individually measured.
NAXIS	int64[2]	pix	WCS solution parameter for GFA astrometry registered to Gaia.
CD	float64[4]		WCS solution parameter for GFA astrometry registered to Gaia.
CDELT	float64[2]		WCS solution parameter for GFA astrometry registered to Gaia.
CRPIX	float64[2]	pix	WCS solution parameter for GFA astrometry registered to Gaia.
CRVAL	float64[2]	deg	WCS solution parameter for GFA astrometry registered to Gaia.
CTYPE	char[16]		WCS solution parameter for GFA astrometry registered to Gaia.
LONGPOLE	float64	deg	WCS solution parameter for GFA astrometry registered to Gaia.
LATPOLE	float64	deg	WCS solution parameter for GFA astrometry registered to Gaia.
PV2	float64[2]		WCS solution parameter for GFA astrometry registered to Gaia.
FNAME_MASTER_DARK	char[112]		File name of master dark used for dark current correction.
DO_FIT_DARK_SCALING	binary		Whether or not the master dark scaling was fit based on the GFA image itself.
MASTER_DARK_EXPTIME	float64	s	Integration time of the dark exposures used to make the relevant master dark.
MASTER_DARK_GCCDTEMP	float64	deg	Average CCD temperature of the dark exposures used to make the relevant master dark (Celsius).
DARK_TEMP_SCALING_FACTOR	float64		Factor by which the master dark was scaled based on CCD temperature.
TOTAL_DARK_SCALING_FACTOR	float64		Total factor by which the master dark was scaled accounting for both exposure time and CCD temperature.
DARK_RESCALE_FACTORS_PER_AMP	float64[4]		Per-amp factor by which the master dark was scaled accounting for both exposure time and CCD temperature.
DARK_RESCALE_FACTOR_BESTFIT	float64		Master dark rescaling factor fit from the GFA image data itself.
DARK_RESCALE_FACTOR_ADOPTED	float64		Adopted master dark rescaling from either CCD temperature-based scaling or else from empirical fit to the GFA image iself.
APPLY_DARK_RESCALE_FACTOR	binary		Whether or not the adopted dark rescaling factor was applied.
DARK_RESCALE_NCALLS	int64[4]		Per-amp number of dark current rescaling fit iterations.
DARK_RESCALE_CONVERGED	binary[4]		Per-amp indication of whether the dark current rescaling fit converged.
REQ_MJD_MIN	float64	d	Input MJD lower bound for creating GFA image coadd.
REQ_MJD_MAX	float64	d	Input MJD upper bound for creating GFA image coadd.
N_PMGSTARS_ALL	int64		Number of platemaker stars.
N_PMGSTARS_RETAINED	int64		Number of platemaker stars retained by platemaker.
FIBERFAC	float64		PMGSTARS forced photometry amount of light in 1.52 arcsec diameter aperture normalized to nominal, assuming a point source profile.
FIBERFAC_ELG	float64		PMGSTARS forced photometry amount of light in 1.52 arcsec diameter aperture normalized to nominal, assuming an ELG-like profile (r_half = 0.45 arcsec exponential)
FIBERFAC_BGS	float64		PMGSTARS forced photometry amount of light in 1.52 arcsec diameter aperture normalized to nominal, assuming a BGS-like profile (r_half = 1.5 arcsec de Vaucouleurs)
SPECTRO_EXPID	int64		DESI spectroscopic exposure number.

HDU2

EXTNAME = EXPOSURE_SUMMARY

This HDU is intended to be the same as EXPOSURE_SUMMARY_STRICT, except that EXPOSURE_SUMMARY employs more permissive quality cuts in the sense that no CONTRAST or N_SOURCES_FOR_PSF cuts are applied. EXPOSURE_SUMMARY still includes minimal quality cuts to remove cases of bad GFA readout such as broken A/D conversion (lack of denoising) and zero-valued quadrants. The idea behind EXPOSURE_SUMMARY is that it avoids cuts that could bias toward retaining cameras with relatively good observing conditions (e.g., N_SOURCES_FOR_PSF is preferentially higher when the transparency is better and the sky brightness is lower). We found that in practice such biases generally don’t matter at any appreciable level, so in general it’s recommended to use EXPOSURE_SUMMARY_STRICT rather than EXPOSURE_SUMMARY. The downside of EXPOSURE_SUMMARY is that in rare cases it may get very wrong values, especially for the transparency when gfa_reduce astrometric pattern matching has catastrophically failed (low CONTRAST parameter).

In some cases the lack of a CONTRAST cut in EXPOSURE_SUMMARY can be valuable. In rare instances (such as very poor observing conditions) a science exposure can be omitted from EXPOSURE_SUMMARY_STRICT due to failed gfa_reduce astrometry. But the PMGSTARS forced photometry is still usable, since the PMGSTARS forced photometry proceeds as usual even if gfa_reduce astrometry has failed. For this reason, as of late April 2021, the spectroscopy pipeline’s tSNR afterburner uses EXPOSURE_SUMMARY rather than EXPOSURE_SUMMARY_STRICT (PR #1245).

Required Header Keywords

Required Header Keywords Table

KEY	Example Value	Type	Comment
NAXIS1	364	int	length of dimension 1
NAXIS2	23290	int	length of dimension 2

Required Data Table Columns

Name	Type	Units	Description
EXPID	int64		DESI Exposure ID number
CUBE_INDEX	int64		Integer frame counter within the guide cube; CUBE_INDEX increases with time within each guide cube; 0 for acquisition images; -1 for matched coadds
NIGHT	int64		Night of observation (YYYYMMDD) starting at local noon before observations start
EXPTIME	float64	s	Exposure time; usually 5 seconds for guider frames, 15 seconds for acquisition images, and 5 seconds for matched coadds (average rather than sum)
FNAME_RAW	char[76]		Raw file name processed by gfa_reduce
SKYRA	float64	deg	Telescope bore sight RA taken from raw header metadata
SKYDEC	float64	deg	Telescope bore sight Dec taken from raw header metadata
PROGRAM	char[88]		DESI program type - BRIGHT, DARK, BACKUP, OTHER
MOON_ILLUMINATION	float64		Moon illumination fraction (0 to 1)
MOON_ZD_DEG	float64	deg	Moon zenith distance in degrees
MOON_SEP_DEG	float64	deg	Moon angular separation in degrees relative to the observation’s sky location
KTERM	float32		Assumed r band k-term value in magnitudes per airmass; from DESI-5418
FRACFLUX_NOMINAL_POINTSOURCE	float32		Point source nominal fraction of light in 1.52 arcsec diameter fiber-like aperture
FRACFLUX_NOMINAL_ELG	float32		Nominal fraction of light in 1.52 arcsec diameter fiber-like aperture for an ELG-like profile (r_half = 0.45 arcsec exponential)
FRACFLUX_NOMINAL_BGS	float32		Nominal fraction of light in 1.52 arcsec diameter fiber-like aperture for a BGS-like profile (r_half = 1.5 arcsec de Vaucouleurs)
MJD	float64	d	Modified Julian Date when shutter was opened for this exposure
FWHM_ASEC	float64	arcsec	FWHM in arcseconds based on fitting the PSF with a beta = 3.5 Moffat profile
TRANSPARENCY	float64		Transparency in r band; based on comparison against PS1 r magnitudes; zeropoints from DESI-5418; k-term given by KTERM column
SKY_MAG_AB	float64	mag arcsec^-2	r band sky brightness measured from detrended GFA background levels; AB mag per square arcsec; zeropoints from DESI-5418
FIBER_FRACFLUX	float64		Point source fraction of light in 1.52 arcsec diameter aperture ASSUMING THE APERTURE IS PERFECTLY ALIGNED WITH THE PSF CENTROID
FIBER_FRACFLUX_ELG	float64		Same as FIBER_FRACFLUX but for an ELG-like profile (r_half = 0.45 arcsec exponential) rather than a point source
FIBER_FRACFLUX_BGS	float64		Same as FIBER_FRACFLUX but for a BGS-like profile (r_half = 1.5 arcsec de Vaucouleurs) rather than a point source
AIRMASS	float64		Average airmass during this exposure
RADPROF_FWHM_ASEC	float64	arcsec	PSF FWHM in arcsec measured directly from the PSF radial profile
FIBERFAC	float64		PMGSTARS forced photometry amount of light in 1.52 arcsec diameter aperture normalized to nominal, assuming a point source profile
FIBERFAC_ELG	float64		PMGSTARS forced photometry amount of light in 1.52 arcsec diameter aperture normalized to nominal, assuming an ELG-like profile (r_half = 0.45 arcsec exponential)
FIBERFAC_BGS	float64		PMGSTARS forced photometry amount of light in 1.52 arcsec diameter aperture normalized to nominal, assuming a BGS-like profile (r_half = 1.5 arcsec de Vaucouleurs)
MINCONTRAST	float64		Minimum gfa_reduce astrometric pattern matching contrast parameter among retained cameras
MAXCONTRAST	float64		Maximum gfa_reduce astrometric pattern matching contrast parameter among retained cameras

HDU3

EXTNAME = EXPOSURE_SUMMARY_STRICT

This table aggregates information from CAMERA_SUMMARY on a per-exposure basis by taking the median across individual guide cameras, subject to some quality cuts applied to CAMERA_SUMMARY on a per-camera basis to remove potentially bad GFA measurements. These quality cuts are:

• remove rows of CAMERA_SUMMARY with min(MAX_Q1, MAX_Q2, MAX_Q3, MAX_Q4) = 0. Having a maximum raw pixel value of zero in a quadrant indicates a major readout problem.

• remove rows of CAMERA_SUMMARY with NPIX_BAD_TOTAL >= 10, since this is indicative of insufficient denoising (bad A/D conversion).

• remove rows of CAMERA_SUMMARY with N_SOURCES_FOR_PSF < 3.

• remove rows of CAMERA_SUMMARY with CONTRAST < 2 (CONTRAST < 1.85) in the case of acquisition images (matched coadds). The goal is to remove instances where gfa_reduce astrometry pattern matching failed.

Required Header Keywords

Required Header Keywords Table

KEY	Example Value	Type	Comment
NAXIS1	364	int	length of dimension 1
NAXIS2	23142	int	length of dimension 2

Required Data Table Columns

Name	Type	Units	Description
EXPID	int64		DESI Exposure ID number
CUBE_INDEX	int64		Integer frame counter within the guide cube; CUBE_INDEX increases with time within each guide cube; 0 for acquisition images; -1 for matched coadds
NIGHT	int64		Night of observation (YYYYMMDD) starting at local noon before observations start
EXPTIME	float64	s	Exposure time; usually 5 seconds for guider frames, 15 seconds for acquisition images, and 5 seconds for matched coadds (average rather than sum)
FNAME_RAW	char[76]		Raw file name processed by gfa_reduce
SKYRA	float64	deg	Telescope bore sight RA taken from raw header metadata
SKYDEC	float64	deg	Telescope bore sight Dec taken from raw header metadata
PROGRAM	char[88]		DESI program type - BRIGHT, DARK, BACKUP, OTHER
MOON_ILLUMINATION	float64		Moon illumination fraction (0 to 1)
MOON_ZD_DEG	float64	deg	Moon zenith distance in degrees
MOON_SEP_DEG	float64	deg	Moon angular separation in degrees relative to the observation’s sky location
KTERM	float32		Assumed r band k-term value in magnitudes per airmass; from DESI-5418
FRACFLUX_NOMINAL_POINTSOURCE	float32		Point source nominal fraction of light in 1.52 arcsec diameter fiber-like aperture
FRACFLUX_NOMINAL_ELG	float32		Nominal fraction of light in 1.52 arcsec diameter fiber-like aperture for an ELG-like profile (r_half = 0.45 arcsec exponential)
FRACFLUX_NOMINAL_BGS	float32		Nominal fraction of light in 1.52 arcsec diameter fiber-like aperture for a BGS-like profile (r_half = 1.5 arcsec de Vaucouleurs)
MJD	float64	d	Modified Julian Date when shutter was opened for this exposure
FWHM_ASEC	float64	arcsec	FWHM in arcseconds based on fitting the PSF with a beta = 3.5 Moffat profile
TRANSPARENCY	float64		Transparency in r band; based on comparison against PS1 r magnitudes; zeropoints from DESI-5418; k-term given by KTERM column
SKY_MAG_AB	float64	mag arcsec^-2	r band sky brightness measured from detrended GFA background levels; AB mag per square asec; zeropoints from DESI-5418
FIBER_FRACFLUX	float64		Point source fraction of light in 1.52 arcsec diameter aperture ASSUMING THE APERTURE IS PERFECTLY ALIGNED WITH THE PSF CENTROID
FIBER_FRACFLUX_ELG	float64		Same as FIBER_FRACFLUX but for an ELG-like profile (r_half = 0.45 arcsec exponential) rather than a point source
FIBER_FRACFLUX_BGS	float64		Same as FIBER_FRACFLUX but for a BGS-like profile (r_half = 1.5 arcsec de Vaucouleurs) rather than a point source
AIRMASS	float64		Average airmass during this exposure
RADPROF_FWHM_ASEC	float64	arcsec	PSF FWHM in arcsec measured directly from the PSF radial profile
FIBERFAC	float64		PMGSTARS forced photometry amount of light in 1.52 arcsec diameter aperture normalized to nominal, assuming a point source profile
FIBERFAC_ELG	float64		PMGSTARS forced photometry amount of light in 1.52 arcsec diameter aperture normalized to nominal, assuming an ELG-like profile (r_half = 0.45 arcsec exponential)
FIBERFAC_BGS	float64		PMGSTARS forced photometry amount of light in 1.52 arcsec diameter aperture normalized to nominal, assuming a BGS-like profile (r_half = 1.5 arcsec de Vaucouleurs)
MINCONTRAST	float64		Minimum gfa_reduce astrometric pattern matching contrast parameter among retained cameras
MAXCONTRAST	float64		Maximum gfa_reduce astrometric pattern matching contrast parameter among retained cameras