Multi-slit masks can be used to multiplex spectroscopic observations at the 4-m with either the RC Spectrograph or Cryogenic Camera. Each mask consists of typically a dozen slits created on thin stainless steel,corresponding to the positions of program objects, say a cluster of galaxies or stars in a cluster. The mask is then used at the focal plane of the telescope to serve as an entrance mask for the spectrograph. The new metal masks offer improved throughput over the old film masks. The slit allows precise determination of the sky spectrum immediately adjacent to the object and thus enables excellent sky subtraction.
The multi-slit masks cover a 5 arcmin (diameter) field, and individual slits are typically 2.5 x 12 arcsec in size, although either the slitlet width or the length can be varied in constructing the masks. (Objects can be located as close as 3 arcsec apart.)
Because the individual slits may be located at differing locations in the dispersion direction, the central wavelength shifts as a function of position of each slit. The maximum shift corresponds to ±225 pixels with the RC Spectrograph with the UVF Camera (demagnification of 4.3) and T2KB ( pixels). Or (as the scale is 0.69 arcsec/pixel, and the radius of the field 156 arcsec), about ±13% of the unvignetted wavelength range. The Cryogenic Camera (demagnification of 8) with the Ford-Loral 800x1200 CCD ( pixels) has a scale of 0.84 arcsec/pixel, and, hence, this shift corresponds to ±180 pixels, or about ±15% of the unvignetted wavelength range.
When multi-slits are used, the normal slit is opened to its maximum and the masks are placed where the normal decker assembly is used, about 12mm above the usual focal plane. For this reason the "slit viewing" optics and TV cannot be used for locating objects. Instead, the "rear slit" viewing TV is used at the start of each exposure to align the telescope on the field. Of course, this TV must be removed from the beam before observations begin and hence exposures are made "blind", although with excellent tracking and guiding this does not prove to be a problem. However, because the decker assembly must be replaced with the mechanism for holding the multi-slit masks, any long-slit observations planned for the same night must be made using a special metal slit which is inserted into the mask holder, and the observations made in the same "blind" mode as with multi-slits. A collection of about twenty metal long-slit masks provides a selection of widths and offsets for "tuning" the central wavelength.