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calfos stsdas.hst_calib.fos



calfos -- Calibrate Faint Object Spectrograph (FOS) data.


calfos input output


The calfos task performs the routine calibration of FOS data. The calibration steps carried out by calfos are determined by the values of certain keywords in the science data header. (The science data header file has the extension .d0h). The science data header file may be edited with the chcalpar task to change the values of the keywords (hedit in the images package or chcalpar in the hst_calib.ctools package can also be used), to select the desired calibration steps. These keywords may contain either of two values: OMIT or PERFORM. Some processing steps require the use of reference tables and files. The header keywords for these reference files must be edited to contain the appropriate reference file names (including their directory paths).

The user should be aware of the existence of an external IRAF package, STPOA, available from the ST-ECF Post-Operational Archives group ( which contains improved calibration for specified FOS data. The poa_calfos pipeline in the STPOA package will provide an improved calibration for the following FOS data:

  Detector:  BLUE
  Fgwa_id:   H13, H19, H27, H40, H57, L15, L65    
  Aper_id:   A-1, A-2, A-3, A-4, B-1, B-2, B-3, B-4

Data which corresponds to the preceding list can be processed using the calfos pipeline (stsdas.hst_calib.fos.calfos). The calfos will issue a warning message regarding the availability of an improved calibration using the poa_calfos pipeline, but calfos will continue to process the data.


The calibration steps, listed by keyword, are:

Convert from raw counts to count rates by dividing each data point by the exposure time and correcting for disabled diodes. If DEFDDTBL is true, the disabled diodes are taken from the Unique Data Log (UDL), otherwise, disabled diodes are found in the file DDTHFILE.
Spectra are corrected for the geomagnetically-induced image motion problem (GIMP). The spectrum is shifted in the FOS X-direction (the dispersion direction) to compensate for shifts induced by the Earth's magnetic field. The offsets are based on model calculations of the magnetic field strength at the spacecraft's position. Requires table ccs7 containing detector-specific scale factors for the GIMP-correction. If the on-board GIMP correction was applied during data acquisition, then this post-acquisition correction should be omitted.
Correct the raw count rates for saturation in the detector electronics. Requires table ccg2 containing the paired-pulse correction table.
Subtract the background from sky and object spectra. If no background was taken, a default reference background, BACHFILE, is used. The observed background is smoothed with a median, followed by a mean filter before subtraction. The filter widths are contained in table ccs3. No smoothing is done to the reference file background if used.
If a reference background is used in the BAC_CORR calibration step, it will be scaled according to the geomagnetic position of the spacecraft at the time of observation. The background will be scaled to a mean count rate appropriate for the geomagnetic position. Requires table ccs8 containing predicted background count rates.
Subtract scattered light from object and sky spectra. Can only be performed for detector/grating combinations that contain a region within the recorded spectrum that is not sensitive to dispersed light from which the level of scattered light is measured. Requires table ccs9 containing the beginning and ending diode positions for the region of no sensitivity for each detector/grating combination.
Remove diode-to-diode sensitivity variations and fine structure by multiplying by the flat field response. Requires the flat field response file FL1HFILE. A second flat field file, FL2HFILE, is required for paired-aperture or spectropolarimetry observations.
Subtract the sky from the object spectra. The routine smooths the sky using a median and mean filter, scales the sky by the aperture size, and shifts the sky before the subtraction. The routine requires table ccs3 containing the filter widths, the aperture size table ccs0, the emission line position table ccs2, and the sky shift table ccs5.
Compute a vacuum wavelength scale for each object or sky spectrum. Wavelengths are computed using coefficients stored in table ccs6.
Convert the object spectra to absolute flux units by multiplying by the inverse sensitivity curve. Requires the inverse sensitivity file IV1HFILE. A second inverse sensitivity file, IV2HFILE, is required for paired aperture or spectropolarimetry observations. This method of flux calibration for pre-COSTAR observations is superseded in CALFOS v2.0 by the APR_CORR, AIS_CORR, and TIM_CORR steps (see below).
Correct for changes in aperture throughput due to changes in OTA focus position. Also correct the observed countrates to match that of the AIS reference aperture. Requires OTA focus history table ccsa, relative aperture throughput table ccsb, and aperture throughput vs focus table ccsc.
Convert the object spectra to absolute flux units by multiplying by the average inverse sensitivity (AIS) curve for the reference aperture. Requires the AIS file AISHFILE. This method of flux calibration for pre-COSTAR data supersedes the FLX_CORR step in CALFOS versions 2.0 and later and requires that the APR_CORR step be performed first.
Correct the flux calibrated object spectra for changes in instrument sensitivity over time. Requires the table ccsd containing correction factors as a function of wavelength and time for each detector/grating combination.
Compute the propagated error at each point in the spectrum.
Perform ground software mode dependent reductions for time-resolved, spectropolarimetry, and rapid-readout observations. The spectropolarimetry reductions require the the Wollaston and Waveplate parameter table ccs4, and the retardation reference file, RETHFILE. If the spectropolarimetry data were acquired during the post-COSTAR epoch as indicated by the Science header keyword/value pair, KYDEPLOY=T, and a post-COSTAR calibration is available, the post-COSTAR polarimetry correction reference file, PCPHFILE, is also required for a proper calibration. However, it should be noted that if no post-COSTAR polarimetry calibration file for the PCPHFILE calibration file keyword is provided, calfos will not abort. Instead, warning messages will be issued and the data in the output c3h file will still be calibrated, but the additional post-COSTAR correction will not have been applied.


input [string]
The rootname of the input FOS observation data set.
output [string]
The rootname of the output FOS data set. If no value is specified for this parameter, output will default to the value passed to input.


1. Perform calibration for observation y00vk108r. All data files for this observation are in the directory spec$. Output files are to have the same root name as the input files.

  hr> calfos spec$y00vk108r ""

2. Calibrate the observation y00vk901r and produce output files with a root name of test186 in the subdirectory temp.

  hr> calfos y00vk901r temp/test186


Version 3.2 (May 1999) of calfos incorporates changes to the way that post-COSTAR POLSCAN = 4 data is processed. No post-COSTAR instrumental polarization correction can be made for this data. Consequently, the results contain significant instrumental polarization. Please refer to FOS Instrument Science Report 150 for a discussion of the magnitude and uncertainties of instrumental polarization introduced by COSTAR.


The following references are available from STScI and describe various aspects of the calibration process, simple cookbook, and general usage of STSDAS:

        "HST Data Handbook"
        "FOS Instrument Handbook"
	"FOS Data Products Guide"
        "STSDAS Users Guide"
        "Phase II Proposal Instructions"

The following are technical references meant for internal usage and are not written as "end-user" products. However, these documents can be retrieved if a detailed understanding of the instrument is required.

The algorithms used by CALFOS are described in "SOGS Requirements Document", (SE-06-01).

The document describing the contents and form of the reference data is "Post Observation Data Processing System to Calibration Database System Interface Control Document", (ICD-47).

The document describing the keywords is "Post Observation Data Processing System to Space Telescope Science Data Analysis Software Interface Control Document", (ICD-19).


For assistance using this or any other tasks, please contact or call the help desk at 410-338-1082.


hedit, chcalpar

Type "help fos opt=sys" for a higher-level explanation of the fos package and the process of calibrating a FOS data set.

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