The tolerances for filters designed to fit in the MOS/OSIS visible 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):
GrismsThe grisms mounted in the MOS/OSIS 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 OSIS, first check that it can be designed within the above specifications.
The CFHT grisms available for OSIS are listed on the CFHT Grism Web Page . Efficiencies for the various grisms are plotted below.
FIGURE 9: Visible grism efficiencies
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 a next chapter. 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.
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.
Aperture masks are cut from 75 µm 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 2 µm 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.