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31.8 User Calibrations

Some users may wish to create their own calibration reference files or tables for certain parts of the pipeline calibration. IRAF/STSDAS tools exist to generate certain types of user reference files. In nearly all cases, however, some manual editing of reference file headers may be necessary after producing the new calibration data.

Wavelengths: IRAF/STADAS tasks hst_calib.fos.y_calib.linefind and .hst_calib.fos.y_calib.dispfity are required. linefind is used to find the positions in a WAVE spectrum of candidate comparison lines from a template list of lines. (The FOS comparison line template lists may be found in the Calibration Tools section of the FOS WWW page.) The identified lines are then fit in dispfity with an appropriate polynomial (order 3 for all FOS grating spectra). See the STSDAS help files for detailed information on these tasks. The results of the fit must be entered into an STSDAS table to be used with pipeline calibration. Be careful to insure that sufficient lines are used to characterize the fits throughout the spectrum. Remember that due to the design of the spectrograph an internal to external wavelength system offset exists between internal calibration arc spectra and external astronomical source spectra and that this offset must be included in any new wavelength calibration (see Chapter 32 for further discussion of internal to external offsets, filter-grating wheel offsets, and other instrumental characteristics affecting wavelength accuracies). Also, please refer to FOS ISRs 149 and 156 and references therein.
Flatfields: IRAF/STADAS task stadas.hst_calib.fos.y_calib.flatfield can be used to generate new FOS flatfields. In general, a spectrum of an object with few lines is used. Continuum levels are marked interactively throughout the spectrum and a spline is fit through these points. The original data is then divided by the spline to produce a unity-normalized flatfield, which can be inverted for use as a calfos inverse flatfield. The output of flatfield can be highly subjective in some cases. FOS IDT-developed software exists for more objective generation of FOS flatfields, but its description is beyond the scope of this manual (see FOS ISRs 135 and 157 for recent comprehensive discussions of FOS team flatfield generation techniques.
Sensitivity Functions: Tasks stsdas.hst_calib.fos.y_calib.absseny and absfity are used to generate FOS inverse sensitivity functions (IVS files for use with FLX_CORR only). Unfortunately, all AIS reference files and tables were generated using IDL software written by the FOS team. No STSDAS tasks exist to generate these reference materials. The methods involved are discussed in FOS ISRs 144 and 158.
Background: Refer to FOS ISR 146 and the references contained therein for a discussion of FOS background modeling techniques. The task y_calib.parthity is required for this calibration.

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