Introduction
Read this documentation before you go
observing:
Why? (1) The reduction software is also used in the observing setup and
for quick-look analysis during observations, and there are
certain files you will need to bring to the telescope in order to use
it there
and (2) there are certain CCD frames you may wish to obtain at the
telescope in order to make optimal use of the reduction software when
you later reduce your data.
Note:
Grayed-out text describes features not yet available.
COSMOS is a set of programs for the acquisition
and reduction of multislit
, longslit,
and IFU spectra. The version described here is tailored for IMACS;
another version will be available for LDSS-3. Parts of the COSMOS
package have several applications, including
- Target acquisition and alignment
- Quick-look analysis of observations
- Data reduction pipeline
As a data pipeline, COSMOS is designed to be amost fully automatic, so
that large amounts of data can be reduced with minimal intervention.
This is made possible by two features of IMACS (1) an excellent
analytical model of the optical system, which allows one to predict
with great accuracy the location in the dewar focal plane of any
location in the telescope/slit mask focal plane, for any wavelength of
dispersed or undispersed light and (2) IMACS' excellent stability,
which means that the predicted positions will not shift significantly
despite changes in temperature or rotation angle, multiple insertions
and removals of the slit masks or disperser elements, and even multiple
mounting and removal of the ccd dewar.
Spectral reduction consists of two kinds of operations (1) making
changes to the intensity values of pixels (e.g. bias correction,
flat-fielding, sky subtraction) and (2) mapping CCD pixels onto the
coordinate system of wavelength and slit position. All of the
hard work, particularly in multi-object spectra, is in the mapping
process, and much of the COSMOS software is related to that. Input to
the COSMOS programs consists of (1) information on the observing setup
(2) a file defining the slit mask used (3) a set of ccd images
comprising direct and/or spectroscopic observations and bias, flat
field and comparison arcs. When used as a data pipleline, output
consists of either 1 or 2-dimensional extracted, wavelength calibrated
spectra.
Whether used for target acquisition, quick looks at the data, or for
data reduction, all COSMOS programs require the following input
data:
- Definition of the observational setup, including slitmask,
camera, disperser element (and tilt, if a grating), and dewar offset.
This information is contained in an observation defintion file.
- The slitmask definition, contained in the mask definition file
for the slit mask used. This file was produced by the program maskgen which was
used to produce the mask, and will have the name maskname.SMF Make sure that you have these files with you at the
telescope. You will need them for the setup of multislit observations,
and for quick-look analysis of your observations.
- The dewar offset, specified by the dewar offset file,
which describes the positional zero-point of a particular instrumental
configuration. Default dewar offset files exist for each of the
principle configurations. However, although IMACS is remarkably stable,
it is not perfect, and there is some slop in the insertion of slitmasks
into their frames. It is therefore useful to create anew, more
accurate dewar offset files for a particular set of observations, using
the program align-mask.
More accurate dewar offset files for the IFU
can be generated with align-ifu, and more accurate dewar
offset files for long slits with align-longslit.
For quick-look or pipeline analysis of spectra, the following
additional input is needed.
- Generation of a slitmask map, from the information contained in
the observation definition file, using the program map-spectra. This map
defines the relation between the dewar coordinate system of ccd
pixel values, and the science coordinate system of wavelength,
and slit position.
- Creation of a mean bias frame (optional).
Spectral reduction proceeds in the following steps.
- Tweaking of the wavelength and slit locations, using a
comparison arc and the program
adjust-map.
- Subtraction of bias, and division by flats, using the programs Sflats and biasflat.
- Subtraction of sky, using the program subsky.
- Extraction of individual 1 or 2 dimensional spectral exposures
using extract, or
extract-2dspec.
- Multiple sets of extracted spectra can be combined, with optional
comic ray removal,
using sumspec.
Generation of the map file, and execution of Steps 1 through 5 above
can be pipelined together for a set of spectral exposures from a single
night using the script
-generators
process-1spec and
process-2spec.
Quick Look Reduction
Steps 1-3 above take typically 1-2 seconds per slit on the current data
taking computers at the Baade Telescope. This is inconveniently
long to do after each exposure for slitmasks containing hundreds of
slits.
To provide quick-look capability, the program
quick-list allows one to
create a truncated mask definition file containing a managable subset
of slits for examination after an exposure. The programs
view-1dspec and
view-2dspec provide
convenient
means of examining these spectra.
Target acquisition and alignment
The program
ifu-image
allows one to reconstruct the focal plane image entering the
object
and sky fiber bundles of the IFU unit for a direct image taken through
the IFU. The program
alignholes
produces data files describing the alignment holes on a
slitmask,
which are used as input to the telescope/instrument alignment program
ialign. mosaic puts
together all 8 chip images into a mosaiced image
. apertures produces a file
with individual chip and mosaic coordinates of all apertures in direct
images, and
spectral-lines
does the same for spectral images for use with the IRAF routine
tvmark which marks their
locations and types in an image display window. Several IRAF routines
allow display or processing of all 8 chips of data with one command.