# 12. Appendices¶

## 12.1. .psf file format description¶

PSF models are FITS binary tables with a single row, containing PSF image components. PSF files derived from MEF images are themselves MEF file, with one PSF_DATA extension per input image extension.

A sample of a PSF binary table extension header is given below:

XTENSION= 'BINTABLE'           / THIS IS A BINARY TABLE (FROM THE LDACTOOLS)
BITPIX  =                    8 /
NAXIS   =                    2 /
NAXIS1  =                25000 / BYTES PER ROW
NAXIS2  =                    1 / NUMBER OF ROWS
PCOUNT  =                    0 / RANDOM PARAMETER COUNT
GCOUNT  =                    1 / GROUP COUNT
TFIELDS =                    1 / FIELDS PER ROWS
EXTNAME = 'PSF_DATA'           / TABLE NAME
ACCEPTED=                 6637 / Number of accepted sources
CHI2    =           1.17986252 / Final reduced Chi2
POLNAXIS=                    2 / Number of context parameters
POLGRP1 =                    1 / Polynom group for this context parameter
POLNAME1= 'X_IMAGE '           / Name of this context parameter
POLZERO1=   2.046996443272E+03 / Offset value for this context parameter
POLSCAL1=   4.074312777519E+03 / Scale value for this context parameter
POLGRP2 =                    1 / Polynom group for this context parameter
POLNAME2= 'Y_IMAGE '           / Name of this context parameter
POLZERO2=   2.047786695004E+03 / Offset value for this context parameter
POLSCAL2=   4.077873875618E+03 / Scale value for this context parameter
POLNGRP =                    1 / Number of context groups
POLDEG1 =                    3 / Polynom degree for this context group
PSF_FWHM=           2.23724842 / PSF FWHM in image pixels
PSF_SAMP=           0.47601029 / Sampling step of the PSF data
PSFNAXIS=                    3 / Dimensionality of the PSF data
PSFAXIS1=                   25 / Number of element along this axis
PSFAXIS2=                   25 / Number of element along this axis
PSFAXIS3=                   10 / Number of element along this axis
TTYPE1  = 'PSF_MASK'           / Tabulated PSF data
TFORM1  = '6250E   '
TDIM1   = '(25, 25, 10)'
END


The file content is largely self-describing. Please note that

• The TDIM1 keyword in the extension header contains the total number of components (conditioned by the PSFVAR_DEGREES configuration parameter) and the dimensions of the tabulated PSF (see the PSF_SIZE configuration parameter).
• Every context parameter (e.g., $$X_j$$) is rescaled before being used as a polynomial variable ($$x_j$$):
(1)$x_j = \frac{X_j - \textrm{POLZERO} j}{\textrm{POLSCAL}j}$
• The PSF FWHM reported by the PSF_FWHM keyword is actually derived from the mode of the half-light diameter distribution of the input source image sample.