This section deals with the reduction of cross-dispersed grism data. The procedures for doing so are similar to those described for long-slit grism data (§8), so readers should be familar with that material before proceeding.
The types of spectroscopic datasets obtained with the cross-dispersed grisms are constrained by the short length of the slits used, and by the extent of the object being measured. The reduction of most cross-dispersed grism datasets will follow the path:
1) Create BIAS and DARK frames, and linearize object and sky frames.
2) Subtract sky background from object frames, and combine all object frames in the dataset to a single spectral image.
3) Create FLAT frames for each grism, and remove pixel-to-pixel sensitivity variations.
4) Fix known and random bad pixels.
5) Apply a geometrical transformation to align the dispersion direction with image columns, align the spatial direction with image rows, and perform the wavelength calibration.
6) Correct for non-uniform illumination along the slit.
7) Extract one dimensional spectra from the combined spectral images with appropriate background subtraction in the slit direction to remove residual sky features.
8) Flux calibrate the one dimensional spectra.
9) Merge the individual grism orders into a single spectrum.
10) Correct for absorption features in the standard star spectrum, inexact cancellation of terrestrial atmospheric absorption features, and slit losses in the standard star measurement.
The reduction procedures described here continues to use the local MSSSO CASPIR package running in IRAF. Refer to §6 for instructions on how to obtain this package. Users are again advised to familiarize themselves with the general procedures available within IRAF for spectral reduction by reading the documents A User's Guide to CCD Reductions with IRAF and A User's Guide to Reducing Slit Spectra with IRAF. Compressed postscript versions of these documents are available via anonymous ftp to iraf.noao.edu in the files iraf/docs/ccduser2.ps.Z and iraf/docs/spect.ps.Z.