2 SIS optical and mechanical layout

The MOS/SIS instrument is attached to the Cassegrain bonnette (acquisition/guiding/rotator unit) at the f/8 focus of the CFH 3.6 m telescope. The image scale at this focus is 139.4 m/". SIS and MOS are located on 2 opposite sides of the large, stiff, octagonal structure with 4 openings, or side ports, attached to this bonnette. The 2 remaining ports are presently unused (figure 2.1).

2.1 Optical layout

The present optics of SIS were designed to work between 0.36 m and 1.00 m and were optimized to provide good images over this wavelength region and over the entire useful field. The SIS optical scheme is shown in Morbey (1992, Applied Optics, 31, 2291). The 46 mm pupil is 160 mm from the last element of the /8 collimator and is made to be coincident with the back side of the grism. The back focal distance is 60 mm from the final lens element of the camera, and the exit pupil is -768 mm from the final /10 focus. The final scale is 173 m/", or 0.0866"/pix for CCD's with 15 m pixels.

With this optical design, all the rays, coming either from the edge or from the centre of the field are constrained to be coincident at the pupil and rays from each object point are parallel as they pass through the grism so that the same region of the grism disperses the light from any point of the field in the same way.

All of the SIS optics were fabricated by Applied Physics Specialties of Toronto. PSK3 and FK54 glasses are used for all optical elements except the grisms.

2.2 Optical performances

2.2.1 Image quality

Spot diagrams for a variety of wavelengths are shown in Crampton et al. (1992, SPIE). The optical quality of the integrated optical system was measured on a grid with 20 m pinholes. The average image degradation from the SIS optics is 21.5 m at the detector focal plane (figure 2.2) and varies only slightly from center to edge.

Considerable attention was paid to minimizing parasitic and scattered light. Focal reducers of this type often suffer from light scattered backwards from the CCD chip into the camera where it is reflected back to the detector in the form of a diffuse spot 5 - 10%above the background in the center of the image. Cures include providing a sufficient distance between the last optical element and the chip, as well as efficient anti-reflection coatings. In this regard, SIS presents a very minimal effect, with a background concentration of 0.5%.

Ghost images of bright stars are also formed by reflection from the last surface of the last camera lens. In SIS, the surface brightness of the central ghost image is about 0.5%of that which is contained within a 1" image on the detector. Observers should not be surprised by strong ``ghosts'' close to very bright stars if, in fact, these ghosts have strengths 0.5%or less than the primary image.

2.2.2 Transmission

Excluding the grism block, the total internal transmittance of the lenses at 3650, 4000, 5000 and 7000Å is estimated to be 70, 86, 95 and 96%respectively. The lens surfaces which are not too steeply curved were coated with a special anti-reflectance coating developed by MATRA which reduces these losses to 0.8%over the entire 3700 - 9000Å range; for the other surfaces, the coating is a single layer of magnesium fluoride with maximum loss 1.5%. The flat which folds the beam from the telescope for feeding SIS is coated to give an average reflectance of 97%over the entire wavelength range of the instrument. The total transmission is then predicted to be around 78%(grism excluded) at 5000Å. Measurements on standard stars have shown the transmission of the SIS optics at this wavelength is actually 76%.

2.3 Mechanics

2.3.1 Mechanical assembly

The SIS train is shown on the right side of Figure 2.1. When mounted on a specially designed storage cart, the whole SIS assembly can be inserted precisely into the ports of the octagon or withdrawn for maintenance.

The main mechanical components in SIS that the observer can control are the following:

Several filter and grism wheels exist. They are easily interchangeable cassettes (Grundmann et al., 1988, in ``ESO conf. on Very Large Telescopes'', M.-H. Ulrich ed., II, 1173) and can be inserted either in SIS or in MOS. The design allows these cassettes to be interchanged quickly and precisely, with minimal danger to their contents; they are encoded to allow the control system to recognize each one. All driving and encoding components are kept inside the body of the spectrograph, external to the cassette so that no attached electronics can be damaged during handling. After each wheel is inserted during the set-up of the instrument, the control system maps the wheel positions for further reference. Both the filter and the grism cassettes are located near the pupil plane. Each filter cassette has 8 positions for 75 mm diameter filters (but adaptors also permit use of square 2-inch filters, for instance). Each grism cassette has 8 positions for 65 mm diameter grisms. The grism mountings allow precise alignment of each individual grism, and a locking pin is used to ensure that the grism cassettes can be located and maintained in position to an accuracy of 20".

2.3.2 Mechanical flexures

Internal SIS flexures, from the entrance focal plane to the CCD focal plane, were measured with MOS/SIS installed at the Cassegrain bonnette. A focal plane mask with 25 m apertures was installed in the mask slide and illuminated with the continuum lamp of the calibration system. The telescope was moved to hour angles between -4h and +4h and declinations between -45 and +65. At each telescope position, an image of the focal plane mask was recorded on the CCD, and the centroids of the apertures measured. One has to note that this measurement also includes flexures of the CCD in its mechanical housing (LICK2 was used for the tests).

The measured flexures are plotted in figure 2.3. The internal flexures of SIS for the telescope tracking a star are therefore expected to be a maximum of 25 m at the detector focal plane for a motion of 4 hr in hour angle or equivalently an average of 0.036 arcsec/hr. The analysis of direct images obtained with SIS active guiding are in good agreement with this figure, which shows that the stiffness of the guide probe system is excellent.

No measurement has been done without active guiding. For this mode of observation, flexures are expected to be similar to those observed with MOS, i.e. 40 m, or 0.06"/hr.

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