supplemental skies

Summary:

DESI supplemental sky locations contain a single binary table covering the entire Gaia footprint. They are generated to help find sky locations in regions that are not covered by Legacy Surveys imaging. Supplemental sky locations are produced at random positions across the sky and are subsequently removed if they either lie close to a Gaia source or share an nside = 4096 HEALPixel with an existing pixel-based sky location from the Legacy Surveys.

Naming Convention:

skies-supp-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:

skies-supp-hp-?[0-9]+\.fits

File Type:

FITS, 186 KB - 9.4 MB

Contents

Number	EXTNAME	Type	Contents
HDU0		IMAGE	Empty
HDU1	SKY_TARGETS	BINTABLE	Table of sky locations

FITS Header Units

HDU0

This HDU has no non-standard required keywords.

Empty HDU.

HDU1

EXTNAME = SKY_TARGETS

Table of sky locations

Required Header Keywords

Required Header Keywords Table

KEY	Example Value	Type	Comment
NAXIS1	136	int	width of table in bytes
NAXIS2	2342	int	number of rows in table
SUPP	T	bool	True if sky locations are supplemental (i.e. are not derived from the Legacy Surveys)
GAIADR	2	int	Gaia Data Release used to select supplemental skies
NPERSDEG	18000.0	float	density of sky locations generated per sq. deg.
HPXNSIDE	64	int	HEALPix nside for column HPXPIXEL
HPXNEST	T	bool	HEALPix nested (not ring) ordering
SUBPSEED	12583926	int	random seed used to generate SUBPRIORITY values
RADIUS	2.0	float	separation (arcsec) at which to avoid Gaia sources
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
MASKED	T	bool	True if targets were masked to avoid bright sources
MASKDIR	“masks/”	str	location of directory of masks used to avoid bright sources
CMDLINE	“/global/”	str	command-line call used to generate target file
FILENSID	2	int	HEALPix nside covered by file
FILENEST	T	bool	HEALPix nested (not ring) ordering
FILEHPX	“11”	str	HEALPix pixel(s) covered by file

Required Data Table Columns

Name	Type	Units	Description
RELEASE	int32		Legacy Surveys (LS) Release
BRICKID	int32		Brick ID from tractor input
BRICKNAME	char[8]		Brick name from tractor input
BRICK_OBJID	int32		OBJID (unique to brick, but not to file)
RA	float64	deg	Right ascension
DEC	float64	deg	Declination
BLOBDIST	float32	pix	Maximum distance from a detected Legacy Surveys source
FIBERFLUX_G	float32		g-band object model flux
FIBERFLUX_R	float32		r-band object model flux
FIBERFLUX_Z	float32		z-band object model flux
FIBERFLUX_IVAR_G	float32		Inverse Variance of FIBERFLUX_G
FIBERFLUX_IVAR_R	float32		Inverse Variance of FIBERFLUX_R
FIBERFLUX_IVAR_Z	float32		Inverse Variance of FIBERFLUX_Z
TARGETID	int64		Unique targeting ID
DESI_TARGET	int64		DESI (dark time program) target selection bitmask
BGS_TARGET	int64		BGS (bright time program) target selection bitmask
MWS_TARGET	int64		MWS (bright time program) target selection bitmask
SUBPRIORITY	float64		Random subpriority [0-1] to break assignment ties
OBSCONDITIONS	int64		Flag target to be observed in combinations of dark/bright observing layer
PRIORITY_INIT	int64		Initial priority for target calculated across target selection bitmasks and OBSCONDITIONS
NUMOBS_INIT	int64		Initial number of observations for target calculated across target selection bitmasks and OBSCONDITIONS
HPXPIXEL	int64		HEALPixel containing sky location

Notes

As quantities in the supplemental skies files are derived from Gaia, quantities from the Legacy Surveys that aren’t also in Gaia (i.e. fluxes) are populated with meaningless values.