MOS/SIS

  Observations were obtained with the MultiObject Spectrograph (MOS) and the Subarcsecond Imaging Spectrograph (SIS) [\protect\astronciteLeFèvre et al.1994] at the CanadaFranceHawaii 3.6m Telescope (CFHT) on Mauna Kea, a dormant volcano on the Big Island of Hawaii. MOS and SIS are two distinct spectrographs sharing a common interface behind the telescope bonnette. MOS is optimized for multi-object observations over a large (1010) field with a spatial sampling of 0/pixel for 15 m pixels. SIS is designed for high spatial resolution observations over a smaller field of view (33) with a better pixel sampling of 0/pixel for 15 m pixels. The MOS/SIS interface is mounted at the F/8 Cassegrain focus.

One of the characteristics of SIS is a fast moving mirror providing tip/tilt image stabilization. The point-spread-function (PSF) is the image of an unresolved source produced by the telescope, and it is often approximated by a Gaussian function. The atmosphere distorts the planar wavefront coming from distant objects. Wavefront distortions are introduced on all scales. On the largest scales, these distortions called tip and tilt shift the centroid of the PSF around on timescales of 0.25 seconds. These are the distortions corrected by SIS's fast-moving mirror. Wavefront distortions on smaller scales increase the width of the PSF, and they must be corrected with faster, higher order systems such as the CFHT AO Bonnette.

SIS requires a relatively bright (V 18) guide star. The tip/tilt corrections are measured using a quadrant detector mounted on a moving guide probe. The probe can be moved anywhere in the SIS field of view, but, in practice, the guide star must lie near the corner of the field. Otherwise, the probe mount can occult close to 1/3 of the field of view. SIS fast guiding yields typical resolution improvements of 00and routinely delivers images with 0seeing. A seeing FWHM of 0corresponds to scales < 1.5h kpc at z = 0.35 (q = 0.5).