At a meeting in 1986, the CFHT users' community identified a low spectral resolution multi-object spectrograph as one of the highest priorities for new instrumentation at CFHT. Although the original intermediate dispersion spectrographs constructed for the CFHT had high throughput and were of excellent optical and mechanical quality, they were designed for single slit observations with image intensifiers or electronographic cameras as detectors. The desire to observe many faint objects simultaneously and also the realization that the image quality at CFHT is routinely better than one arcsecond led to the design of the ``MOS/SIS'' spectrograph, a dual Multi-Object and Subarcsecond Imaging Spectrograph. It is composed, in fact, of two distinct spectrographs sharing a common interface with the telescope after the Cassegrain bonnette: one is optimized for multi-object observations over a large field (MOS), the other (SIS) for high spatial resolution observations incorporating rapid tip/tilt image stabilization similar to that very successfully used in the CFHT/DAO high resolution camera HRCam (McClure et al. 1989). Two movables 45 degree mirrors permit a feed to either MOS or SIS. At the present time, a switch from one spectrograph to the other during the same observing run is not allowed.
The MOS/SIS spectrograph was jointly designed and built by teams from the Dominion Astrophysical Observatory (DAO) in Victoria, the Observatoire de Paris-Meudon (OPM), the Observatoire de Marseille and CFHT. Work began on the designs in May 1988 and resulted in an instrument which saw its first light in July 1992. Since that time, MOS/SIS has quickly become the most popular instrument at CFHT. For instance, during the first semester of 1995, MOS and SIS together, in their different modes of observation (including direct imaging, Fabry-Pérot and integral field spectroscopy) was used for 57%of the nights available for science.
While MOS is primarily designed for multi-aperture spectroscopy over a field (see the ``User's Manual for the CFHT Multi-Object Imaging Spectrograph (MOS)@''), SIS gives a smaller field of view () with a better sampling. Designed to be used with a 15m pixel CCD, SIS still gives a good sampling of almost 3 pixels for aperture widths as small as 0.25". The designed wavelength range is 365 to 1000 nm, and typical efficiencies are approximately 80% for imagery and 60%for spectroscopy.
An important transformation is expected soon. With a change in optics extending the useful wavelength range to 2 m, SIS is about to become ``OSIS''. The project should be completed by the second semester of 1996.
To permit spectra of tens of objects to be obtained simultaneously, an on-line facility for producing precise masks from direct images with a laser drilling machine (LAMA) has been developed. Lama is also discussed in this manual. In addition, SIS has its own precision acquisition and guiding system so that objects can be optimally centered on small apertures. This ensures that as much flux as possible from the target objects enters the apertures, and also allows narrower apertures to be used, thus reducing contamination from the sky or other sources of background light.