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Info: Faq for applicants and observers
- What is the IDS?
- What are the resolutions available ?
- Can both cameras be used together?
- Which CCD
is normally in use, and what are it's specifications ?
- What is the pixel scale in the spatial direction, and what is the longest unvignetted slit length that I can use ?
- What is the throughput of the instrument - how can I calculate my exposure times?
- If the IDS is not suitable for me, what other possible options are there available at ING ?
- Information on the slit-viewing orientation
on the TV, and correspondance with chip geometry
Back to IDS Homepage
What is the IDS ?
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.
What are the resolutions available ?
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).
Can both cameras be used together ?
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.
Which CCD
is normally in use, and what are it's specifications ?
IDS is offered with a choice of Tek5
or EEV10
detectors.
Click here for characteristics
of the 4kx2k thinned EEV42-80 chip for IDS.
Click here for charcateristics
of the 1kx1k thinned Tek chip for IDS.
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. Table
gives the pixel scale in the spatial direction for both CCD options.
With the dekker in the clear position, the longest slit length
employable before vignetting occurs is ~2 arcmins with the 500mm, and ~4
arcmins with the 235mm.
Table: Pixel scale in the spatial direction
(arcsec/pixel)
|
Detector
|
235 mm Camera
|
500 mm Camera
|
|
Tek5
|
0.7
|
0.33
|
|
EEV10a
|
0.4
|
0.19
|
What is
the throughput of the instrument - how can I calculate my exposure times
?
A number of flux standards were observed in 1997 with IDS to
measure the throughput of the complete
telescope+spectrograph+detector system. The figure below shows the
results, which were consistent for each observed target. The Y-axis is
the apparent AB magnitude of star observed at zenith which gives one
detected photon per second per Angstrom. The lowest resolution
grating (R150V) was used with a wide slit (10 arcsec), the conditions
were photometric. The collimator used was Ag-Red. By comparison,
data form the old EEV chip is shown from 1994. The large
improvements are due to higher QE of the TEKs, cleaning of the
reflecting surfaces within IDS in 1996 and aluminising of the primary
mirror which took place the week before the TEK data were gathered.
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 the IDS
is not suitable for me, what other possible options are there available
at ING ?
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.
Information on the slit-viewing orientation
on the TV, and correspondance with chip geometry
Some observers may want to work out the direction that the slit was pointing,
and which end of the slit corresponds to which direction spatially on the
particular CCDS. Here you have some help:
This page is undergoing continual change and enhancements, if there
is anything you wish to see changed or added then please mail me.
