7 Data reduction procedures

7.1 Direct imaging

The basic data processing of direct CCD images includes the following steps:

• Computation of average BIAS image.
• Computation of average, normalized FLAT frames.
• Computation of the master ``fringe'' frame (depends on the CCD you use; this step may not be necessary)
• Subtraction of the master BIAS from the object frames after proper additive correction for each, based on the mean value of the overscan region.
• Division of each object frame by the master FLAT (obtained with the proper filter, of course).
• Subtraction of the master fringe frame after scaling (sometimes division works better).
• Identification and removal of cosmic ray events. If you have several exposures of the same field and if their characteristics are exactly identical (seeing, sky level, maximum count for a given star, etc.), you can try to use a ``combine'' procedure to do that. If not (which is most likely), this procedure would badly alter the data and you are advised to remove ``cosmic'' on each frame. A procedure called COSMICRAYS exists in the IRAF package to do just this. A better one (if its parameters are well adjusted) called SPECTRENET is available from C. Vanderriest.
• Photometric calibration from frames on standard stars.

For detailed data reduction procedures for direct imaging we recommend that you read the ``User's guide to the CCDRED Package'', Francisco Valdez, 1987, (NOAO/IRAF group). N.B. It is assumed that Biases and darks are identical, which is a very good approximation with our present detectors.

8.2 Spectroscopy

7.2.1 Overview

The basic data processing for spectroscopic data includes the following steps, the first ones being similar to data processing of direct images:

• Computation of average BIAS image.
• Computation of average, normalized FLAT frames (with the same grism).
• Subtraction of the master BIAS from the object frames after proper additive correction for each, based on the mean value of the overscan region.
• Flat field correction for each slit.
• Identification and removal of cosmic ray events, with COSMICRAYS or SPECTRENET.
• Sky background subtraction for each slit.
• 1D object spectra extraction.
• Wavelength calibration for each 1D spectra.
• Flux calibration from spectrophotometric standard stars

7.2.2 The MULTIRED package

The package MULTIRED has been written under IRAF by O. Le Fèvre to process MOS/SIS multi-slit data following the above steps. You can get this package via ftp . The file multired.tar is a tar file containing all the necessary files. To unpack and recreate the files, type the command:

tar xvf multired.tar
Proceed with the installation of the package in IRAF, by following the steps described in the file README. MULTIRED is a very efficient package which helps you keep track of slit numbers, works on multiple images per mask, and performs the data processing steps in a painless sequence to produce flat/sky corrected 2D spectra, as well as wavelength and flux calibrated 1D spectra. On line help is available for each MULTIRED task.