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Defringe -- Correct STIS CCD G750L or G750M spectrum for fringing
defringe inspec inflat outspec
- Input (2-D) spectral image to be corrected for fringing
- Input flat field image (created by normspflat or mkfringeflat )
- Output spectral image
- (do_cal = no)
- Perform bias and dark subtraction and cosmic ray rejection prior to fringe correction?
- (darkfile = "")
- Name of (reference) superdark image to be used for dark subtraction. If this value is null (the default), the DARKFILE keyword in the main header of inspec will be used (in that case, make sure it is correctly populated). This parameter is only relevant if do_cal = yes
- (pixelflat = "")
- Name of reference flat field image to perform pixel-to-pixel correction. This parameter is only relevant if do_cal = yes
- (do_defringe = yes)
- Perform fringe correction?
DEFRINGE eliminates the fringes that are present in STIS first-order CCD spectra at wavelengths longer than about 700 nm, by dividing by "fringe flat field images" that have been treated in advance by means of tasks normspflat and/or mkfringeflat .
The input 2-D spectral image inspec [typically a STIS fits file with "_raw" file suffix] is first run through the STIS pipeline calstis in order to perform bias subtraction, cosmic ray rejection, dark subtraction, and division by a pixel-to-pixel flat field. In case of G750M data, wavelength calibration and S-distortion correction is performed as well. An appropriate superdark file name can be provided as parameter darkfile ,as can a pixel-to-pixel flat field. If these reference files are not specified, the ones listed in the image header will be used. The log file of the calstis reduction steps is written to output file spec.calstis.log .
The task allows the user to bypass the whole calstis reduction step described in the previous paragraph through boolean parameter do_cal . In case do_cal is set to "no", the assumption is that inspec has been corrected for pixel-to-pixel sensitivity variations with the proper flat field, and that 2-D rectification is performed as well (in case of a G750M spectrum). The user is notified of this implicit assumption if appropriate.
After calstis pipeline reduction, the reduced spectrum is divided by the input flat field image inflat . Note that in case of G750L spectra, the output spectrum outspec will neither have been wavelength calibrated nor rectified. Task x2d (in the stsdas.hst_calib.stis package) can be used to perform those (final) calibration steps.
INPUT AND OUTPUT IMAGES
The input STIS spectral image inspec is typically a "multi-imset" FITS file, with three extensions (or groups) per imset: science (EXTNAME=SCI), error (ERR), and data quality (DQ). If do_cal is set to "yes", DEFRINGE will read each imset, calibrate it, and produce a multi-group output FITS file. However, DEFRINGE can handle FITS "imset" files with three extensions as well as "old-fashioned" FITS files (with only one extension). This holds for the input spectrum inspec as well as for the input flat field inflat .
The output spectrum outspec will have the same format (i.e., a FITS file with three extensions or a FITS file with single extension) as the input spectrum inspec .
1. Perform fringe correction for STIS spectrum. Use superdark file "mysuperdark.fits" to provide the most up-to-date correction of hot pixels in the input image.
cl> defringe o3st20gdr_raw.fits normff.fits defringed.fits \ >>> darkfile="mysuperdark.fits"
2. Same as 1, but bypass basic image reduction (in case it was already done before). For a G750M spectrum:
cl> defringe o3st20gdr_sx2.fits normff.fits defringed.fits \ >>> do_cal-
3. Same as 2, but for a G750L spectrum:
cl> defringe o3st20gda_crj.fits normff.fits defringed.fits \ >>> do_cal-
4. Example of all steps involved in fringe correction for a G750M spectrum o3st20gdr_raw.fits and its associated contemporaneous fringe flat o3st20gds_raw.fits:
cl> normspflat o3st20gds_raw.fits normff750M.fits \ >>> darkfile="mysuperdark.fits" wavecal="o3st20gdr_wav.fits" cl> prepspec o3st20gdr_raw.fits darkfile="mysuperdark.fits" cl> mkfringeflat normffG750M.fits normff750M.fits normff750M_fin.fits cl> defringe o3st20gdr_sx2.fits normff750M_fin.fits \ >>> outspec="o3st20gdr_final.fits" do_cal-
5. Same as 4, but for a G750L spectrum o3st20gda_raw.fits, wavecal o3st20gda_wav.fits and contemporaneous fringe flat o3st20gdb_raw.fits:
cl> normspflat o3st20gdb_raw.fits normff750M.fits \ >>> darkfile="mysuperdark.fits" wavecal="o3st20gda_wav.fits" cl> prepspec o3st20gda_raw.fits darkfile="mysuperdark.fits" cl> mkfringeflat o3st20gda_crj.fits normff750L.fits normff750L_fin.fits cl> defringe o3st20gda_crj.fits normff750L_fin.fits \ >>> outspec="o3st20gda_defr.fits" do_cal-
Task created by Paul Goudfrooij on April 17, 1998. Task modified by Paul Barrett on October 21, 2004.