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OSIS data reduction procedures
Direct imaging
The basic data processing of direct CCD images includes the following steps:
- 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.
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).
Spectroscopy
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).
- ubtraction of the master BIAS from the object frames after
properadditive 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
The MULTIRED package
The
MULTIRED
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:
multired.tgz
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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