Besides its specific active guiding system, the SIS environment is composed of the following main elements:
Changing mask slides is the only operation that the observers (or the TO) may have to do at night in the dome. The mask slides are introduced through a port in the bottom cover of the central octagon (see section 6.11 and 6.12 for practical procedures). Presently, SIS slides can accomodate five mm mask-holders in addition to the open position (figure 4.1). The mask slide is locked into position at each location to ensure that there is no lost motion or flexure.
The tolerances for filters designed to fit in the MOS/SIS filter wheels are quite tight. If you intend to bring your own filters or have them fabricated for your observations, they should have the following specifications (CFHT standard):
A list of filters@ which can be used in SIS is available on the WWW.
The grisms mounted in the MOS/SIS cassettes have a circular cross sections and 65 mm diameters. The maximum practical thickness is 60 mm. If you think that a new grism should be purchased for SIS, first check that it can be designed within the above specifications.
The CFHT grisms available for SIS are listed in table 4.1 along with the wavelength coverages and dispersions. The wavelength range corresponds to an unobstructed spectra; see section 6.11 for an understanding of spectral range limitations. Figure 4.2 shows the grism efficiencies vs. wavelength.
CCD detectors available for use with SIS are listed in table 4.2. A more complete description of CCD's is given in the document ``CCD's at CFHT@''
The system for introducing uniform illumination of wavelength and flat-field sources was designed by Y.P. Georgelin and G. Monnet. The calibration unit fits into one of the ports of the cassegrain bonnette and uses the rear surface of the existing central 45 mirror to introduce light onto the optical axis of the telescope. Either one of two spectral lamps or a flat field halogen lamp can be selected to illuminate a transmissive diffusing screen. The optical scheme uses two commercial 9.87 grooves/mm fresnel lenses as field lenses and one biconvex lens for a relay. Two symmetrically-mounted lamps are used in tandem for each set to ensure better than 5%uniformity in the blue and better than 3%in the red. The calibration system is controlled by the data acquisition and instrument control computer, independently from the MOS/SIS control system.
Once an image of a field has been acquired, the mask preparation is carried out with another HP terminal by starting a LAMA session (login as lama; ask your support astronomer for the current password).
To design a mask, you will display the field image and interactively superimpose the aperture contours on the objects of your choice. The details of the procedure are given in section 6.11; we just note here that you need to select the size of your slitlets (for objects) and round apertures (for centering stars), then use the MOS and PAN icons to move the apertures from one object to the next, and to center them precisely.
The useful area for slitlets is less than the whole area of the CCD. A first limitation comes from the mask holder geometry: on a , 15 m CCD, the slit coordinates should lie in the range 105<X<1898 and 185<Y<1794. Then, unlike for long slit observations, multi-slit spectroscopy implies that each slit has a unique spectral domain. This domain is directly related to the slit position on the aperture mask. For a slit located at Y, and a CCD with Y pixels, (e.g. Y=2048 for LORAL3), the spectral domain is defined as:
Where is the zero deviation wavelength in Åand D is the dispersion in Å/pixel. One important consequence is that if your program requires a given wavelength range, you will be restricted to a fixed area in which to position slits (figure 4.3) and you should compute in advance the limits of this area in Y.
Once you have completed your mask design, select ``do it'' in the SAOIMAGE/MOS menu. This will create the appropriate mask design file which may be recalled on any SAOIMAGE display at later stages, as well as a specially formatted file (the ``YAG file'') for mask drilling with the LAMA machine.
Forward the LAMA mask file name to the LAMA operator (it could be yourself) for drilling.
Aperture masks are cut from 75m thick black anodized aluminium sheets (the ``blanks'') with a YAG LAser MAchine known by the acronym LAMA (DiBiagio et al., 1990, SPIE, 1235, 422). The accuracy of cutting has been measured to be m on the edges of the slits. The blanks are mounted in metallic mask-holders, installed on LAMA and cut according to the prepared YAG file. Then the mask-holders are mounted in multi-position slides and inserted in SIS. The mechanical mountings of the masks ensure a high degree of positional accuracy and stability, both in the LAMA drilling machine and in the mask slide. Several slides are available so that the observer can load the required masks into the slides well in advance of the spectroscopic observations. Two reference aperture masks consisting of a cross and a grid are used to map the coordinate transfer from the output focal plane at the CCD to the LAMA/entrance focal plane. This procedure takes care of scaling, orientation, and optical distortion to allow for an accurate registration of the mask apertures and the sky targets.
A few weeks before your observing run, CFHT will ask you how many blanks you will need for the entire run, in order to have them ready in advance. Try to estimate how many fields you could observe if everything goes well, then multiply by 1.5 for safety. For instance, if you plan to use 10 masks ask for 15 blanks.
When planning your sequence of observations, take into account the recommendations about bonnette rotation (section 6.4).