hpix2upix

Summary:

This file contains a mapping of large nside healpix to adaptively sized uniqpix.

Naming Convention:

hpix2upix-SURVEY-PROGRAM.fits, where SURVEY is main, sv1, sv2, sv3, special, or cmx; and PROGRAM is bright, dark, backup, or other.

Regex:

hpix2upix-(cmx|main|special|sv1|sv2|sv3)-(backup|bright|dark|other).fits

File Type:

FITS, 9 MB

Contents

Number	EXTNAME	Type	Contents
HDU0	HPIX2UPIX	IMAGE	Mapping from healpix to uniqpix
HDU1	HPIX_NTARGETS	IMAGE	Number of unique targets in each uniqpix

FITS Header Units

HDU0

EXTNAME = HPIX2UPIX

Mapping from healpix to uniqpix at the NSIDE given in the header.

Required Header Keywords

Required Header Keywords Table

KEY	Example Value	Type	Comment
NAXIS1	786432	int	Size of array: Number of healpixels at this NSIDE
NSIDE	256	int	The nside used for the healpix calculation.
HPXNSIDE	256	int	Same as NSIDE
HPXNEST	T	bool	Whether the healpix is in nested ordering (T) or ring ordering (F).
SURVEY	main	str	DESI survey name: main, sv1, sv2, sv3, special, or cmx
PROGRAM	dark	str	DESI program name: bright, dark, backup, or other
SPECPROD	matterhorn	str	DESI spectro production name, e.g. matterhorn, nevis

Data: FITS image [int64, 786432]

This array is indexed by the healpixel number, and the value is the corresponding uniqpix number. The healpix number is calculated using the NSIDE header keyword, which is the largest NSIDE used by any uniqpix in this production. If a healpix is not covered by any uniqpix in this production, then the value is -1.

HDU1

EXTNAME = HPIX_NTARGETS

The number of targets per healpix.

Required Header Keywords

Required Header Keywords Table

KEY	Example Value	Type	Comment
NAXIS1	786432	int	Size of array: Number of healpixels at this NSIDE
NSIDE	256	int	The nside used for the healpix calculation.
HPXNSIDE	256	int	Same as NSIDE
HPXNEST	T	bool	Whether the healpix is in nested ordering (T) or ring ordering (F).
SURVEY	main	str	DESI survey name: main, sv1, sv2, sv3, special, or cmx
PROGRAM	dark	str	DESI program name: bright, dark, backup, or other
SPECPROD	matterhorn	str	DESI spectro production name, e.g. matterhorn, nevis

Data: FITS image [int32, 786432]

Even if a healpix is covered by a uniqpix, it may not contain any targets. This array gives the number of targets in each healpix, which can be used to quickly determine if a healpix contains any targets without having to read the corresponding uniqpix.

Notes and Examples

Example usage:

import healpy
import fitsio

with fitsio.FITS('spectra/main/dark/hpix2upix-main-dark.fits') as fx:
    header = fx['HPIX2UPIX'].read_header()
    hpix2upix = fx['HPIX2UPIX'].read()
    hpix_ntargets = fx['HPIX_NTARGETS'].read()

nsidemax = header['NSIDE']
hpix = healpy.ang2pix(nsidemax, ra, dec, lonlat=True, nest=True)
upix = hpix2upix[hpix]
ntargets = hpix_ntargets[hpix]