OSIS data reduction procedures
The basic data processing of direct CCD images includes the following steps:
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).
- Computation of average BIAS or DARK 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 or DARK 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.
The basic data processing for spectroscopic data includes the
following steps, the first ones being similar to data processing of
- Computation of average BIAS image.
- Computation of average, normalized FLAT frames (with the same
- ubtraction of the master BIAS from the object frames after
properadditive correction for each, based on the mean value of the
- Flat field correction for each slit.
- Identification and removal of cosmic ray events, with COSMICRAYS
- Sky background subtraction for each slit.
- 1D object spectra extraction.
- Wavelength calibration for each 1D spectra.
- Flux calibration from spectrophotometric standard stars
The MULTIRED package
package has been written under IRAF by O. Le Fèvre to process
MOS/OSIS multi-slit data following the above steps. The package can
be obtained as a gzipped, tar file:
To unpack and recreate the files, type the command:
gunzip -c multired.tar | tar xvf -
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
dataprocessing 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.
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Copyright © 1997, CFHT Corporation. All rights reserved.