THELI - Image Header

To perform smooth pipeline operations from data of various instruments and telescopes, we need to specify a set of header keywords for FITS images that enter THELI processing. In particular, the header needs to contain all necessary information for astrometric and photometric calibration of data. To minimise possible confusions we substitute the original image header with one that contains ONLY the minimum set of required information. A header from a raw image that you want to process with THELI typically looks like:

SIMPLE  =                    T
BITPIX  =                   16
NAXIS   =                    2
NAXIS1  =                 2142
NAXIS2  =                 4128
BSCALE  =             1.000000
BZERO   =         32768.000000
CTYPE1  = 'RA---TAN          '
CTYPE2  = 'DEC--TAN          '
CRPIX1  =              -412.00
CRPIX2  =              3888.00
CD1_1   =        -6.610000E-05
CD2_2   =         6.610000E-05
CD2_1   =         0.000000E+00
CD1_2   =         0.000000E+00
CRVAL1  =           170.622467
CRVAL2  =           -21.721230
RADECSYS= 'FK5               '
EQUINOX =              2000.00
FILTER  = 'BB#Rc/162_ESO844  '
OBJECT  = 'DEEP3b            '
AIRMASS =             1.107282
EXPTIME =               299.92
IMAGEID =                    3
GABODSID=                  726
ZP      =            -1.000000
COEFF   =             1.000000
DUMMY1  =                    0
DUMMY2  =                    0
DUMMY3  =                    0
DUMMY4  =                    0
.
.

We note that all THELI processing happens on single-chip images - other FITS formats such as Multi-Extension-FITS files are never used. If your initial data come in some FITS-cube format, you need to split them first. This step typically also takes care of setting up the required image headers. The THELI distribution contains a C-program mefsplit which performs these tasks. For specfic instruments it is typically wrapped by a shell script. See process_split_WFI_eclipse.sh (for WFI@MPG/ESO2.2m) and process_split_MEGAPRIME_eclipse.sh (for MEGAPRIME@CFHT) for examples.

Note

• There are various ways to specify the astrometric system, especially the sky-orientation of the data and the pixel scale. THELI treats ONLY the CD-matrix convention consistently and correctly everywhere!
• The IMAGEID header keywords gives the chip number of the image; for single chip cameras it is always 1.
• The GABODSID gives the observation night as the number of nights that have passed since 31/12/1998. It was introduced to have an easy-to-handle numeric value to distiniguish data from different nights. Conversions from civil dates to GABODSID and vice-versa can be done with the THELI C-programs mjd, nightid and caldate.
• ZP and COEFF are supposed to contain meaningful values for photomtric zeropoint and extinction coefficient after absolute photomtric calibration. The default values ZP=-1.0 and COEFF=1.0 mean that the image has not (yet) been calibrated.
• The header contains at the end 50 dummy keywords which the pipeline can replace with additional keywords in the course of processing. These dummys allow an easy and very fast addition of new keywords (C-program replacekey) without the need to rewrite the complete FITS file (header and data section) for this task.