gfas

Summary:

DESI guide/focus/alignment (GFA) files contain a single binary table covering the entire footprint. They contain objects derived from matches between Gaia and the Legacy Surveys and the associated quantities used by fiber assignment to select sources for guiding and focus.

Naming Convention:

gfas-hp-HP.fits, where HP is the HEALPixel covered at the (nested) HEALPixel nside included in the file header as FILENSID (e.g. 11).

Regex:

gfas-hp-?[0-9]+\.fits

File Type:

FITS, 20 MB - 8.5 GB

Contents

Number	EXTNAME	Type	Contents
HDU0		IMAGE	Empty
HDU1	GFA_TARGETS	BINTABLE	Target table

FITS Header Units

HDU0

This HDU has no non-standard required keywords.

Empty HDU.

HDU1

EXTNAME = GFA_TARGETS

Target selection table

Required Header Keywords

Required Header Keywords Table

KEY	Example Value	Type	Comment
NAXIS1	164	int	width of table in bytes
NAXIS2	142824	int	number of rows in table
DR	9	int	Legacy Surveys Data Release used to find targets
MAGLIM	21.0	float	magnitude limit on GFA targets in Gaia G-band
MINDEC	-90.0	float	minimum declination for GFAs that are not selected from the Legacy Surveys
MINGALB	0.0	float	closest latitude to Galactic Plane for GFAs that are not selected from the Legacy Surveys
NOURAT	F	bool	True if the URAT catalog was not used to supplement missing proper motions
GAIADR	“edr3”	str	Gaia Data Release used to select GFAs
HPXNSIDE	64	int	HEALPix nside for column HPXPIXEL
HPXNEST	T	bool	HEALPix nested (not ring) ordering
FILENSID	2	int	HEALPix nside covered by file
FILENEST	T	bool	HEALPix nested (not ring) ordering
FILEHPX	11	int	HEALPix pixel(s) covered by file

Required Data Table Columns

Name	Type	Units	Description
RELEASE	int32		Legacy Surveys (LS) Release
TARGETID	int64		Unique targeting ID
BRICKID	int32		Brick ID from tractor input
BRICK_OBJID	int32		OBJID (unique to brick, but not to file)
RA	float64	deg	Right ascension
DEC	float64	deg	Declination
RA_IVAR	float32	deg**-2	Right ascension inverse variance
DEC_IVAR	float32	deg**-2	Declination inverse variance
MORPHTYPE	char[4]		Morphological Model type
MASKBITS	int16		Bitmask for coadd/*/*/*maskbits* maps, as on the LS DR9 bitmasks page
FLUX_G	float32	nanomaggy	LS flux from tractor input (g)
FLUX_R	float32	nanomaggy	LS flux from tractor input (r)
FLUX_Z	float32	nanomaggy	LS flux from tractor input (z)
FLUX_IVAR_G	float32	nanomaggy**-2	Inverse Variance of FLUX_G
FLUX_IVAR_R	float32	nanomaggy**-2	Inverse Variance of FLUX_R
FLUX_IVAR_Z	float32	nanomaggy**-2	Inverse Variance of FLUX_Z
REF_ID	int64		Tyc1*1,000,000+Tyc2*10+Tyc3 for Tycho-2; “sourceid” for Gaia DR2
REF_CAT	char[2]		Reference catalog source for star: “T2” for Tycho-2, “G2” for Gaia DR2, “L2” for the SGA, empty otherwise
REF_EPOCH	float32	yr	Reference epoch for Gaia/Tycho astrometry. Typically 2015.5 for Gaia.
PARALLAX	float32	mas	Reference catalog parallax
PARALLAX_IVAR	float32	mas**-2	Inverse variance of parallax
PMRA	float32	mas / yr	Reference catalog proper motion in the RA direction
PMDEC	float32	mas / yr	Reference catalog proper motion in the Dec direction
PMRA_IVAR	float32	yr^2 / mas^2	Inverse variance of PMRA
PMDEC_IVAR	float32	yr^2 / mas^2	Inverse variance of PMDEC
GAIA_PHOT_G_MEAN_MAG	float32	mag	Gaia G band magnitude
GAIA_PHOT_G_MEAN_FLUX_OVER_ERROR	float32		Gaia G band signal-to-noise
GAIA_PHOT_BP_MEAN_MAG	float32	mag	Gaia BP band magnitude
GAIA_PHOT_BP_MEAN_FLUX_OVER_ERROR	float32		Gaia BP band signal-to-noise
GAIA_PHOT_RP_MEAN_MAG	float32	mag	Gaia RP band magnitude
GAIA_PHOT_RP_MEAN_FLUX_OVER_ERROR	float32		Gaia RP band signal-to-noise
GAIA_ASTROMETRIC_EXCESS_NOISE	float32		Gaia astrometric excess noise
URAT_ID	int64		ID in the URAT catalog for sources where URAT supplemented missing Gaia astrometric information
URAT_SEP	float32	arcsec	Separation between URAT and Gaia sources where URAT supplemented missing Gaia astrometric information
GAIA_PHOT_G_N_OBS	int32		Number of observations in Gaia G band
HPXPIXEL	int64		HEALPixel containing target at HPXNSIDE

Notes

Some units in this file do not conform to the FITS standard:

• deg^-2 is incorrectly recorded as 1/deg^2

• nanomaggy^-2 is incorrectly recorded as 1/nanomaggy^2

• mas^-2 is incorrectly recorded as 1/mas^2

Such issues can typically be fixed by parsing the unit through astropy after reading in a Table, e.g.:

import astropy.units as u
from astropy.table import Table
objs = Table.read(filename, 1)
u.Unit(str(objs["RA_IVAR"].unit))

See https://www.legacysurvey.org for more details about columns in the data model.