STIS Near-IR Fringing. III. A Tutorial on the Use of the IRAF Tasks

STIS CCD spectra at wavelengths longward of about 7000 A (i.e., spectra taken with gratings G750L and G750M) show a phenomenon called “fringing”. This effect is caused by interference of multiple reflections between the two surfaces of the CCD in case the wavelength of the incident light is a small integer multiple of the distance between the two surfaces of the CCD (see Goudfrooij et al. 1998 for a more detailed discussion of fringing). In the case of STIS observations, the best correction for fringing (hereafter “defringing”) is accomplished by the use of “contemporaneous fringe flats”. This process has been described in detail in two earlier STIS Instrument Science Reports (hereafter ISRs): ISR 97-16 (Walsh et al. 1997) deals with defringing spectra of extended objects, and ISR 98-19 (Goudfrooij et al. 1998) deals with defringing spectra of point sources. We have created a four-step procedure to defringe STIS spectra. It is accomplished by the use of four IRAF tasks in the stsdas.hst_calib.stis package: normspflat, prepspec, mkfringeflat, and defringe. While the actual running of these tasks is not difficult, choosing the parameters so as to accomplish the best fringe correction can be non-trivial and should be done with care and thought. This document serves the following purposes: (1) a tutorial to guide the user through the “defringing” process (described in the following sections), and (2) three appendices containing (A) a step-by-step guide on how to retrieve STIS Reference files, (B) Figures illustrating the effect of the defringing process, and (C) the flow charts of the calibration steps performed in the different IRAF tasks.