Design

The layout of the IDS is shown in Figure . The main components are:

• 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 and 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 to 25 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 . Grating changes 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 and 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 22.522.5m, 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.