psfnight-CAMERA-NIGHT.fits

Summary:: PSF model for the night derived from combining multiple arc lamp calibration exposures.
Naming Convention:: psfnight-CAMERA-NIGHT.fits, where CAMERA is e.g., “b0”, “r5”, etc. and NIGHT is the observation night in YYYYMMDD format.
Regex:: psfnight-[brz][0-9]-[0-9]{8}\.fits
File Type:: FITS, 1 MB

Contents

Number	EXTNAME	Type	Contents
HDU0	XTRACE	IMAGE	Legendre coefficient of the fiber trace X CCD coordinates
HDU1	YTRACE	IMAGE	Legendre coefficient of the fiber trace Y CCD coordinates
HDU2	PSF	BINTABLE	Table with PSF shape parameters

FITS Header Units

HDU0

EXTNAME = XTRACE

Encodes the X coordinate of the fiber traces in the CCD. X is in units of pixels, along the cross-dispersion axis (perpendicular to the fiber traces) and increases with increasing fiber number.

XTRACE is a 2D array of size [nfiber,ncoef]. Each row contains the Legendre polynomial coefficients for a fiber trace. The polynomial applies to a reduced wavelength = (2*wavelength-(WAVEMIN+WAVEMAX))/(WAVEMAX-WAVEMIN) where WAVEMIN and WAVEMAX are header keywords.

Required Header Keywords

Required Header Keywords Table

KEY	Example Value	Type	Comment
NAXIS1	7	int	length of data axis 1
NAXIS2	500	int	length of data axis 2
FIBERMIN	0	int
FIBERMAX	499	int
WAVEMIN	7339.0	float
WAVEMAX	9915.0	float
PSFTYPE	GAUSS-HERMITE	str
PSFVER	3	int

Data: FITS image [float64, 7x500]

HDU1

EXTNAME = YTRACE

Encodes the Y coordinate of the fiber traces in the CCD. Y is in units of pixels, along the dispersion axis and increases with increasing wavelength.

YTRACE is a 2D array of size [nfiber,ncoef]. Each row contains the Legendre polynomial coefficients for a fiber trace. The polynomial applies to a reduced wavelength = (2*wavelength-(WAVEMIN+WAVEMAX))/(WAVEMAX-WAVEMIN) where WAVEMIN and WAVEMAX are header keywords.

Required Header Keywords

Required Header Keywords Table

KEY	Example Value	Type	Comment
NAXIS1	7	int	length of data axis 1
NAXIS2	500	int	length of data axis 2
FIBERMIN	0	int
FIBERMAX	499	int
WAVEMIN	7339.0	float
WAVEMAX	9915.0	float

Data: FITS image [float64, 7x500]

HDU2

EXTNAME = PSF

Binary table with the PSF shape parameters encoded as Legendre polynomials of wavelength, per fiber.

Each row of the table addresses a parameter. The column PARAM provides its name. The column COEFF contains a 2D array of size [nfiber,ncoef] with its associated Legendre polynomial coefficients.

As for XTRACE and YTRACE, each row of the COEFF array provided the Legendre coefficients of a fiber. The polynomial applies to the reduced wavelength = (2*wavelength-(WAVEMIN+WAVEMAX))/(WAVEMAX-WAVEMIN) where WAVEMIN and WAVEMAX are header keywords.

The additional columns LEGDEGX and LEGDEGW are the degrees along the cross-dispersion and dispersion axes used during the fit.

This format can apply to several PSF models. For the one currently used in DESI, based on Gauss-Hermite polynomials, the parameters are the following:

‘GHSIGX’ Gauss-Hermite Gaussian sigma along X
‘GHSIGY’ Gauss-Hermite Gaussian sigma along Y
‘GH-I-J’ with I and J in the range 0 to 6: Gauss-Hermite polynomial coefficient of degree I along X and J along Y
‘TAILAMP’ PSF tail amplitude
‘TAILCORE’ PSF tail core size
‘TAILXSCA’ relative scaling along X
‘TAILYSCA’ relative scaling along Y
‘TAILINDE’ PSF tail asymptotic power law index
‘BUNDLE’ Index of fiber bundle (or fiber block)
‘STATUS’ Not used currently (values = 0)
‘CONT’ Value of continuum at this fiber and wavelength

PSF(X,Y) = PSF_CORE(X,Y) + PSF_TAIL(X,Y)

PSF_CORE(X,Y) = [ SUM_ij (GH-i-j)*HERM(i,X/GHSIGX)*HERM(j,Y/GHSIGY)*GAUS(X,GHSIGX)*GAUS(Y,GHSIGY)

PSF_TAIL(X,Y) = TAILAMP*R^2/(TAILCORE^2+R^2)^(1+TAILINDE/2) with R^2=(X/TAILXSCA)^2+(Y/TAILYSCA)^2’)

Required Header Keywords

Required Header Keywords Table

KEY	Example Value	Type	Comment
NAXIS1	8016	int	width of table in bytes
NAXIS2	59	int	number of rows in table
PSFTYPE	GAUSS-HERMITE	str
PSFVER	3	str
MJD	0	int	MJD of arc lamp exposure
PLATEID	0	int	plate ID of arc lamp exposure
CAMERA	r9	str	camera ID
ARCEXP	0	int	ID of arc lamp exposure used to fit PSF
NPIX_X	4114	int	number of columns in input CCD image
NPIX_Y	4128	int	number of rows in input CCD image
HSIZEX	8	int	Half size of PSF in fit, NX=2*HSIZEX+1
HSIZEY	5	int	Half size of PSF in fit, NY=2*HSIZEY+1
FIBERMIN	0	int	first fiber (starting at 0)
FIBERMAX	499	int	last fiber (included)
NPARAMS	57	int	number of PSF parameters
LEGDEG	1	int	degree of Legendre pol.(wave) for parameters
GHDEGX	6	int	degree of Hermite polynomial along CCD columns
GHDEGY	6	int	degree of Hermite polynomial along CCD rows
WAVEMIN	5546.0	float	minimum wavelength (A), used for the Legendre p
WAVEMAX	7835.0	float	maximum wavelength (A), used for the Legendre p
PSFERROR	0.0	float	assumed PSF fractional error in chi2
READNOIS	0.0	float	assumed read out noise in chi2
GAIN	1.0	float	assumed gain in chi2
B00RCHI2	1.265121999136178	float	best fit chi2/ndf for fiber bundle 0
B00NDATA	140488	int	number of pixels in fit for fiber bundle 0
B00NPAR	1746	int	number of parameters in fit for fiber bundle 0
B01RCHI2	1.244445290488158	float	best fit chi2/ndf for fiber bundle 1
B01NDATA	141467	int	number of pixels in fit for fiber bundle 1
B01NPAR	1786	int	number of parameters in fit for fiber bundle 1
B02RCHI2	1.287672518525548	float	best fit chi2/ndf for fiber bundle 2
B02NDATA	142582	int	number of pixels in fit for fiber bundle 2
B02NPAR	1802	int	number of parameters in fit for fiber bundle 2
B03RCHI2	1.297067743767708	float	best fit chi2/ndf for fiber bundle 3
B03NDATA	140419	int	number of pixels in fit for fiber bundle 3
B03NPAR	1793	int	number of parameters in fit for fiber bundle 3
B04RCHI2	1.28304830748024	float	best fit chi2/ndf for fiber bundle 4
B04NDATA	138599	int	number of pixels in fit for fiber bundle 4
B04NPAR	1785	int	number of parameters in fit for fiber bundle 4
B05RCHI2	1.269246986023668	float	best fit chi2/ndf for fiber bundle 5
B05NDATA	136603	int	number of pixels in fit for fiber bundle 5
B05NPAR	1778	int	number of parameters in fit for fiber bundle 5
B06RCHI2	1.299512103689112	float	best fit chi2/ndf for fiber bundle 6
B06NDATA	137571	int	number of pixels in fit for fiber bundle 6
B06NPAR	1807	int	number of parameters in fit for fiber bundle 6
B07RCHI2	1.347344727978646	float	best fit chi2/ndf for fiber bundle 7
B07NDATA	135041	int	number of pixels in fit for fiber bundle 7
B07NPAR	1788	int	number of parameters in fit for fiber bundle 7
B08RCHI2	1.222211322234266	float	best fit chi2/ndf for fiber bundle 8
B08NDATA	130583	int	number of pixels in fit for fiber bundle 8
B08NPAR	1737	int	number of parameters in fit for fiber bundle 8
B09RCHI2	1.283488581543704	float	best fit chi2/ndf for fiber bundle 9
B09NDATA	127868	int	number of pixels in fit for fiber bundle 9
B09NPAR	1716	int	number of parameters in fit for fiber bundle 9
B10RCHI2	1.275975791937288	float	best fit chi2/ndf for fiber bundle 10
B10NDATA	131593	int	number of pixels in fit for fiber bundle 10
B10NPAR	1770	int	number of parameters in fit for fiber bundle 10
B11RCHI2	1.250307604266956	float	best fit chi2/ndf for fiber bundle 11
B11NDATA	133944	int	number of pixels in fit for fiber bundle 11
B11NPAR	1778	int	number of parameters in fit for fiber bundle 11
B12RCHI2	1.231315819103986	float	best fit chi2/ndf for fiber bundle 12
B12NDATA	134637	int	number of pixels in fit for fiber bundle 12
B12NPAR	1767	int	number of parameters in fit for fiber bundle 12
B13RCHI2	1.238502289060944	float	best fit chi2/ndf for fiber bundle 13
B13NDATA	134287	int	number of pixels in fit for fiber bundle 13
B13NPAR	1761	int	number of parameters in fit for fiber bundle 13
B14RCHI2	1.296845822866915	float	best fit chi2/ndf for fiber bundle 14
B14NDATA	139568	int	number of pixels in fit for fiber bundle 14
B14NPAR	1818	int	number of parameters in fit for fiber bundle 14
B15RCHI2	1.319475598189398	float	best fit chi2/ndf for fiber bundle 15
B15NDATA	139759	int	number of pixels in fit for fiber bundle 15
B15NPAR	1802	int	number of parameters in fit for fiber bundle 15
B16RCHI2	1.2373008163902	float	best fit chi2/ndf for fiber bundle 16
B16NDATA	139822	int	number of pixels in fit for fiber bundle 16
B16NPAR	1778	int	number of parameters in fit for fiber bundle 16
B17RCHI2	1.262409294037498	float	best fit chi2/ndf for fiber bundle 17
B17NDATA	140633	int	number of pixels in fit for fiber bundle 17
B17NPAR	1770	int	number of parameters in fit for fiber bundle 17
B18RCHI2	1.270007569982172	float	best fit chi2/ndf for fiber bundle 18
B18NDATA	143004	int	number of pixels in fit for fiber bundle 18
B18NPAR	1790	int	number of parameters in fit for fiber bundle 18
B19RCHI2	1.275991847448398	float	best fit chi2/ndf for fiber bundle 19
B19NDATA	143320	int	number of pixels in fit for fiber bundle 19
B19NPAR	1780	int	number of parameters in fit for fiber bundle 19
EXPID [1]	0.0	float
IN_IMAGE [1]	SPECPROD/preproc/20210508/00087672/preproc-z6-00087672.fits	str
IN_PSF [1]	SPECPROD/calibnight/20210508/psfnight-z6-20210508.fits	str
MEANDY [1]	1.53486325871199e-12	float
MEANDX [1]	-0.02150255816129912	float
MINDY [1]	-3.6379788070917e-12	float
MINDX [1]	-0.02509807125352381	float
MAXDY [1]	1.00044417195022e-11	float
MAXDX [1]	-0.01113212858444967	float

Required Data Table Columns

Name	Type	Description
PARAM	char[8]	Parameter name
COEFF	float64[1000]	Legendre coefficients[nfiber,ncoeff]
LEGDEGX	int32	Legendre polynomial degree in the X (Fiber) direction
LEGDEGW	int32	Legendre polynomail degree in the Y (Wavelength) direction

The dimensionality of COEFF is [nfiber,ncoeff] where nfiber is the number of fibers (always 500 for standard production runs) and ncoeff is the maximum value of any row of LEGDEGX or LEGDEGY.