CGS4 Introduction
    
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Introduction to CGS4
 General Description of the Instrument
CGS4 is a 1-5um multi-purpose 2D grating spectrometer containing a 256x256
InSb array, installed in a cryostat which is cooled by liquid nitrogen
and closed cycle coolers. Four gratings are available, two of which are
installed in the cryostat at any one time. They are a 40 l/mm grating
which provides resolving powers of 300-2000, a 75 l/mm grating which provides
resolving powers of about 600-2000, a 150 l/mm grating which provides resolving
powers of roughly 2000-6000, and a 31 l/mm echelle which provides a resolving
power of about 20,000 (15 km/sec). The 75 l/mm grating is rarely
used thesedays. These resolving powers are achieved with the 150 mm focal
length camera optics and a one-pixel-wide slit, which provide a scale of
1.22 arcsec/pixel for the two moderate resolution gratings and roughly
1.0x1.5 arcsec/pixel for the echelle (1.0 arcsec/pixel in the direction
of the dispersion). A 300 mm focal length camera mirror sometimes is installed
in place of the standard camera; with it pixel scales and wavelength coverages
are halved and resolving powers are doubled (in principle).
CGS4 slit lengths are about 80-90 arcseconds. It is possible to orient
a slit at any angle on the sky. Slit widths of one pixel (~1.23 arcsec
with the standard camera, 0.61 arcsec with the long focal length camera),
two, and four pixels are available. Since the resolution is matched to
one pixel on the array, with the one-pixel wide slit fully- (or over-)
sampled spectra must be obtained by mechanically shifting the array in
the dispersion direction, while holding the grating fixed. The array may
be "shifted" over two pixel widths; so that isolated bad pixels do not
result in gaps in the spectrum.
For optimum noise performance and efficiency in short exposures either
multiple non-destructive readout of the array (NDSTARE) or a single read
mode (STARE) can be used. The array can also be read in synch with the
chopper. Chopping often is employed when observing at medium resolution
at thermal wavelengths (beyond the K window).
The instrument has a calibration unit, containing a black-body source
for flat-fielding and argon, krypton, and xenon arc lamps for wavelength
calibration. Five broad band filters continuously cover the wavelength
band 0.95-5.4um. Longward of 1.3um, CVFs serve as order-blockers for the
echelle. Shortward of 1.3um, narrow band filters (1.083um, 1.233um, 1.257um,
1.282um), allow echelle observations at important wavelengths.
Spectropolarimetry is available with CGS4 at all wavelengths.
Software
The spectrometer is completely under computer control. After running up
the acquisition software, the observer sees our standard ADAM screen management
system (SMS), and proceeds to select menus and menu items and to initiate
actions. Other screens display the CGS4 motor status and information about
exposures, integrations and the sequence of observations.
The spectrometer settings (grating selection, wavelength, slit, etc.)
and data acquisition parameters (exposure times, sampling, chop/stare,
etc.) are held in an ascii file called a "config" which is defined by the
observer in two menus. The observer may then observe by activating the
config and starting an observation. More commonly, the observer defines
and then runs an "exec", which is a sequence of commands that recall one
or more "configs" and obtains one or more sets of observations. For example,
within a single exec the observer may specify a sequence of different measurements
(e.g. a flat-field, arc, and star) and also can instruct the telescope
to nod either to sky or along the slit, turn auto-guiding on or off, etc.
The execs and configs may be written off-line, using a separate program
(UKIRT_PREP) and an exec may be modified in the course of running it.
The telescope control program and the CGS4 data acquisition program
are in communication with one another, and transmit instructions and information
relating to chopping, nodding, and slit orientation. An auto-peakup routine
allows an astronomical object, once it is found, to be centered in a specified
row of the array.
CGS4 data are written to disk in standard Starlink NDF format. Data
tapes containing either FITS files or the NDF files are written after your
run.
A sophisticated data reduction package, CGS4DR, which is under continual
development, runs in parallel with the data-acquisition package and is
capable of plotting reduced spectra or other data as images, line graphs,
surface plots, or histograms. Numerous data reduction options are available.
Normally the data reduction package is set up to automatically receive
and process data as it is obtained according to a defined set of instructions,
so that the observer can concentrate on running the data acquisition software
efficiently. CGS4DR can also be run off-line to re-process data. Further
data reduction procedures (flux-calibration, precise wavelength calibration,
removing sky lines) may be done using CGS4DR or by using other programs
such as FIGARO. CGS4DR is available through Starlink.
Further Information
Assistance with the CGS4 data acquisition and reduction systems, as well
as other technical information are available on request. Contact persons
are:
in Hawaii: Your Support Scientist
The CGS4 Instrument Scientist, Tom Kerr
Previous: Current
Status | Up: CGS4
manual | Next: Optical
Layout
Last Modification Date 1998/02/04 - Last Modification Author: Tom Kerr
Tom Kerr (tkerr)
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