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 movable 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, theObservatoire de Paris-Meudon (OPM), the Observatoire de Marseille andCFHT. Work began on the designs in May 1988 and resulted in an instrument which saw its first light in July 1992. For several years from that time, MOS/SIS was the most popular instrument at CFHT. With the advent of wide-field imaging and regular AOB observations, it has taken a smaller, but still quite significant role in the observering schedule. MOS/OSIS have accounted for 25 - 30 night per semester over the past few semesters (Sept 2001).
MOS is primarily designed for multi-aperture spectroscopy over a 10´ x 10´ field, just covered with a 2048 x 2048 15 µm pixel CCD. This gives images with a correct spatial sampling of 0.8". This is considered the best compromise between field size and spatial resolution. The designed wavelength range is from 365 to 1000 nm, and typical efficiencies are approximately 80% for imagery and 60% for spectroscopy.
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.