Up: Intermediate Dispersion Spectrograph
The layout of the IDS is shown in Figure . The main
- Slit and dekker unit. The same slit and dekker unit is shared by the
IDS and FOS-1. The maximum slit length is 44.3 mm (4 arcmin);
the slit width is variable from 40 m (0.216 arcsec) to 2 mm
(10.82 arcsec) in steps of 5 m (0.027 arcsec). The entire slit
assembly can be removed, although operations staff must be made aware
of this requirement well in advance, as it is necessary to remove the IDS
from the telescope to do this.
Dekker masks are interchangeable
in sets; individual sets of 8 apertures and a clear position, can be made
up according to observing needs. Several standard dekkers, incorporating
single slots, pairs of slots, a ``comb'', and a coronographic set-up, are
always available. Detailed information can be found in a folder at the
Console Room of the INT.
- Filter slides. In addition to the above-slit filter slides in
the A&G unit (see Section ), there are 2
below-slit filter slides, each of which has 3 colour or ND filter
positions. One of
these positions is always clear. Five colour filters are currently
available: UG11, BG18, BG24 and BG38, measuring
19 mm 60 mm 2 mm and made from Schott glass, plus a
25 mm x 25 mm solid copper-sulphate crystal filter.
The wavelength dependence of the
transmission of these filters is shown in Appendix .
The copper sulphate
filter cuts out the red part of the spectrum from 600 -- 650 nm upwards.
Its is usually used to cut out the first order FOS-1 spectrum, or the
red leak of the UG11 filter. The neutral density filters
below the slit provide a choice of ND = 0.5, 1.0 and 1.5.
Note that the use of filters below the slit may make it necessary to
refocus the spectrograph.
- Collimator. The focal length of the collimator is 1275 mm which
gives a collimated
beam with a diameter of 85 mm. There is a choice of three different
collimators, with different coatings for maximum reflectivity in the
wavelength range of interest.
The collimators can be
exchanged during the night by the support staff.
The collimator is mounted on a remotely-driven sliding carriage, making
it possible to focus the spectrograph under computer control.
- Cross-disperser. A transmission grism, R60B with 60 lines mm
blazed at 4800 Å, can be inserted into the collimated beam in the
235 mm camera to provide a cross-dispersion option. This is normally
used in conjunction with an R150 echelle grating, which is blazed at
roughly 3 m. The FOS-1 dekker mask is used to limit the slitlength
arcsec in order to avoid order confusion.
The wavelength range and dispersion provided by this grating in each
order, for a grating angle of 62, are summarised in Table
Table: Characteristics of the IDS echelle grating
- Gratings. The gratings have a ruled area of 102 mm 128 mm.
A wide selection is available, and some of the important characteristics of
each grating are summarised in Tables and .
can be made during the night by support staff.
Table: Characteristics of the IDS gratings with the 235 mm camera
Table: Characteristics of the IDS gratings with the 500 mm camera
- Cameras. Two cameras are available with the Intermediate Dispersion
Spectrograph. Both are of the folded short-Schmidt type, described
in detail by Wynne (Monthly Notices of the Royal Astronomical
Society, vol 180, p485, 1977). The focal
lengths of the two cameras are 235 mm and 500 mm.
Along the direction of the slit (i.e. perpendicular to the dispersion
direction), the focal lengths of 235 and 500 mm imply slit to detector
reduction factors of 5.43 and 2.55 respectively, and hence scales
at the detector of 29.4 and 13.8 arcsec/mm.
Along the dispersion direction, the slit to detector reduction factor
is more complicated, since it depends on the grating angle. Tables
give the slit width in arcsec which projects to 45 microns at the
detector, when each grating is used at its central wavelength.
The same tables also list the dispersion provided by each grating and
camera combination. It can be seen that a wide range of dispersions
is available, from 7 to 270 Å/mm.
- Detectors. The IDS can be used with any of three CCD detectors: a
coated EEV, image frame size 12421152 pixels, pixel size
a thinned Tektronix, image frame size 10241024, pixel size
2424m or a thinned GEC, image frame size 385578, pixel
size 2222m. Spectral response curves and other details of the
available chips have been published in Gemini, June 1992, 36, p. 23.
Changing the detector on a particular camera can only be carried out during the
day. It is possible however, to have one CCD set up on one camera and another
CCD on the other, and operate the two systems consecutively during the same
night. Switching from one camera to the other only requires the grating to be
rotated (or exchanged), an operation normally carried out by support staff, and
the control software to be reinitialized.
Up: Intermediate Dispersion Spectrograph
Tue Aug 15 16:42:46 BST 1995