It's a long-slit spectrograph which sits at the Cassegrain Focal station of
the 2.5m INT and is equipped with two cameras, called the 235mm and 500mm
Cameras. Their names merely denote their respective focal lengths.
A wide range of gratings are available (16 in all) which can be mounted in
either camera. Possible grating/camera
combinations allow dispersions of between 8.1 and
271 Å/mm. For a TEK chip with 24 micron pixels, this results in a possible
spectral
resolutions ranging from 0.4Å to 13Å FWHM (corresponding to two pixels
FWHM on the detector). The 500mm camera can provide
approximately twice the resolution of the 235mm; and the highest spectral
resolution is only achievable with this
(click here
for
a full list of grating options). However one should note that to achieve the
highest resolution possible with IDS (and, say, a TEK chip) it is
necessary to use a 0.9 arcsec slit-width with the 500mm camera. The seeing at
the INT is often slightly higher than this, which tends to force observers
to widen the slit, and degrade their resolution. Hence unless you really need
a resolution better than ~0.8 Å FWHM, the 235mm Camera is a
much more efficient option. The other advantage of the 500mm Camera is that
it also provides twice the resolution in the spatial direction (see below).
Although both cameras are permanently mounted on IDS, only one CCD is currently
available as an option for either. Hence only one camera can be used during
a particular night - the task of moving a chip cryostat from camera to camera
is not trivial and can only be carried out by the day-time
engineering team.
The CCD which is mounted on IDS approximately 90% of the time is the 1024x1024 pixel TEK3. It's pixels are 24 microns, and a recent information sheet (in postscript form) with
all it's specifications is available here. An html version of this information is available at the
ING detector group home page
You can also get the current
quality control data for the latest information on readout noise and
bias level of this chip in the
Quality Control section of this page.
What is the pixel scale in the spatial direction, and
what is the longest unvignetted slit length that I can use ?
The 500mm camera also provides higher magnification than the 235mm in the
spatial direction as well as in dispersion. The 500mm scale using a TEK
chip with 24 micron pixels is 0.33 arsec/pixel, whereas that of the
235mm is 0.7 arcsec/pixel. With the decker in the clear position, the longest
slit length employable before vignetting occurs is ~2 arcmins
with the 500mm, and ~4 arcmins with the 235mm.
What is the throughput of the instrument - how can I
calculate my exposure times ?
Recent flux standard data have been taken just after the telescope
primary mirror was aluminised in August 1997, and a plot is available above.
In order to calculate the number of detected photons per pixel you will
have to take into account the following factors.
Slit losses: the plot was based on data taken with a wide slit (10").
For a 1" slit in typical 1" seeing,
approximately 60% of the starlight will enter the spectrograph.
Airmass correction: the plot refers to a star observed at zenith.
You should correct for the expected airmass of your object, (see the
ING Observers Guide 1994 for a table of extinction versus wavelength).
As a guide, at 5500 Å, the extinction is 0.1 Mag per unit Airmass (at
3500 Å it is 0.5; at 4000 Å it is 0.3; at 7000 Å it is 0.04).
Grating efficiency: the flux standard data are, by convention, taken with
the lowest resolution grating (R150V).
You will need to scale to the efficiency
of your chosen grating at a particular wavelength. The table of grating options
gives the absolute efficiencies at blaze wavelengths.
e.g. an example calculation of the expected counts (in detected photons)
per pixel in an exposure of 300 seconds of a B=12 star with the R1200B at
around 4000 Å. We assume an Airmass of 1.15
(correspoding to a zenith distance of 30 degrees),
and that we're observing in seeing of 1", with a 1" slit-width.
where 0.6 represents slit-losses; (0.76/0.66) are the relative grating
efficiencies; 0.85 is the dispersion in Å per pixel for the R1200B;
represents the difference in magnitude between target
object and the AB standard mag for IDS (tabulated values on which the
flux standard plot is based are available here). The bottom line accounts
for attenuation due to airmass.
If you are interested in higher resolution than can be provided by the IDS
but are satisfied that a 2-metre telescope is sufficient for your photon
collecting requirements
then you should check out the MUSICOS echelle spectrograph which is currently
available at the INT as a common-user instrument. An intermediate dispersion
spectrograph, ISIS
with a double-arm facility (allowing simultaneous red and blue
light observations) is available on the 4.2m WHT and provides similar
spectral resolution to the IDS and is more efficient, ignoring telescope
mirror diameter. The UES echelle spectrograph on the WHT, provides
a further high resolution option. All the available spectroscopic facilities
currently available at the ING are listed here.
The moving optical parts of the IDS (e.g. grating angle, Hartman shutters, slit jaws etc.) are controlled by the observer from a SPARCstation
running the Instrument Control System (ICS). This is a Unix based facility
which, happily, has replaced the old Perkin-Elmer based Adam control language.
The DAS is defined as the collection of hardware and software used by
observers to grab the images from the CCD detectors, and which dumps this
electronic information to a usable file on a SPARCstation. The DAS also
maintains a record of each observation taken (i.e the night log), and attaches
information to the science images taken (i.e. FITS headers).
The INT Acquisition and Guiding Box is the unit mounted at Cassegrain focus
of the INT between the telescope and the instrument. The IDS sits below
this box, and starlight passes through the A&B box before entering the slit
of the spectrograph. The A&G box provides the calibration sources
(i.e. the arc and tungsten lamps), a selection
of above slit colour and neutral density filters, as well as housing the TV
cameras for target acquisition and the Cassegrain Auto-Guider.
The "User Manual for the Sparc based INT IDS control" contains all you need
to know about moving and changing mechanisms within in the IDS. The
"User Manual for the Sparc based Data-Acquisition system" describes in
detail how to acquire your science images. And the
"User Manual for the Sparc based INT A&G box control"
describes how to invoke the functions of the A&G box. All three of these
functions that make up the IDS observing system can be controlled from
one xterm window on a SPARCstation. Note that all this information is
contained in one folder (``The Black Book'') which is available in the
INT control room for observer reference - this folder also contains helpful
information on IRAF, temporary software workarounds, and how to
write your data to tape.
It is helpful to be familiar with the overall structure of the IDS system.
While reading all of the above listed manuals will not be a particularly
absorbing experience, having an idea on how the IDS itself, the DAS and the
A&G box interact will allow you to understand the origins of your data
better, and trace any faults that you notice. The manuals listed have excellent
reference guides (I can say this, as they weren't written by me!) which will
only really be of use when you are actually at the telescope, but knowing in
which manual to find that elusive command will greatly improve your on-sky
efficiency. Also note that the simplest way for observers to check their
data in real time is with IRAF, and it helps if you are familiar with the
basic workings of this package (see next question).
The science exposures are created as FITS files on the IDS system
SPARCstation which archives them. They can then be transferred to the INT
data-reduction SPARC for analysis using IRAF.
We have the latest version of IRAF
running, and the STARLINK software collection will be also available to
users by the end of 1997. Current upgrades to SOLARIS clustering on the
mountain prevents STARLINK software being immediately available to users.
Check out instructions on how to use IRAF at the INT to
either quickly check your data or reduce it using IRAF.
For observers who are not experienced users of IRAF, and would not be
reducing their data in real time, there are a few very simple procedures
available for them to display their images, and check signal levels.
To do this, you should be familiarize yourself with the general IRAF
environment, an image display tool such as ximtool or SAOimage, and the
IRAF commands 'display', and 'imexam'. An introduction can to these commands
will always be given by your support astronomer at the telescope, but you
can reduce your dead-time between exposures by experimenting before you come
out.
At the minute the IRAF script for quick, automatic pipeline reduction is
not available for the IDS. This is being re-written and will be made
available to Observers in the future.
The new Cassegrain Auto-guider has been commissioned in November 1997, and
is the default system from now on. Draft documentation will be linked from
here soon.
This page last updated: 19 Nov 1997
