When the spectra are extracted, the fluxes have to be corrected for the variations of the quantum efficiency of the detector. These variations depend both on the wavelength and on the position of the object on the detector. Since this is a wavelength dependent effect, the flatfielding can only be done after the spectra are extracted (i.e. are wavelength calibrated). In previous releases of $CALNICC_BASE ,the correction factors were derived through interpolation from a monochromatic flatfield image stored as a single FITS file called nicmosFF.fits (in the $CALNICC_BASE/calib directory).
The flat fields can now be specified in two different ways. One is a three-dimensional FITS files, the other one is a list of flatfield files. The file type specified in the grism setup is recognized by the name of the file. Any file name which contains the string "fits" will be interpreted as FITS file. All other filenames will be interpreted as ASCII list.
The three dimensional FITS file has been used in the pre-flight calibration data base and the option remains enabled for backwards compatibility only. It is not recommended to use this feature.
The ASCII input file list is assumed to be located in the $CALNICC_BASE/calib directory. It should contain at least 2 rows with 2 columns each separated by a single space. The first column should contain the file name of a NICMOS flatfield. The second column should contain the effective wavelength of this flatfield. The NICMOS flatfield are assumed to contain the data in the FITS extension 1, as those in the STScI generated nref directory do. The flatfield names are assumed to be relative to the $CALNICC_BASE/calib directory.
$CALNICC_BASE is distributed with one full set of NICMOS flatfields, which are located in $CALNICC_BASE/calib/nref . However, the user should find the most appropriate flatfields for a given data set and generate an appropriate list.