STUDY SUMMARY - IPMU

SUMMARY REPORT
WIDE FIELD FIBER-FED OPTICAL
MULTI-OBJECT SPECTROMETER (WFMOS)
Figure 3.12-3: A time sequence showing the system reconfiguring between the DELZ mode
(top left) and the GAHR mode (bottom right). Note that camera, the filter wheel and the grating
exchanger all move at the same time.
3.13 Detector System
A mosaic of two close-butted CCDs will be located at the prime focus of the reflective f/1.07
spectrograph camera of each of the three WFMOS spectrographs. Four hundred spectra will be
projected on each CCD with fibers spaced at the slit to avoid the small gap between CCDs. A
plano convex field flattener in close proximity to the CCDs will serve as the window to a very
compact vacuum housing surrounding the CCD mosaic connected by a slender duct to a low vibration
Joule-Thompson cooler placed outside the spectrograph beam. The design minimizes
beam obstruction on axis and is tapered to avoid additional beam obstruction off axis. Since the
spectrograph is gravity-stable and vibration isolated, the CCD cabin is supported only by the rectangular
duct. The entire vacuum enclosure with the exception of a Kovar window frame will be
constructed from stainless steel, electropolished and cold coated on the interior for low emissivity.
The exterior finish will be matt black and have a stepped profile to eliminate most grazing
incidence reflections. Image ghosting is minimized by the very low reflectivity of the multi-layer
coated CCD, by AR coatings on the field flattener, and by spacing the field flattener 1.7 mm
from the CCD. The use of stiff materials and minimization of cantilevered mass pushes the lowest
resonant frequencies to 27 Hz and 83 Hz and 184 Hz. These figures can be significantly improved
by further tuning of the design so that significant attenuation will be achieved by the
spectrograph’s vibration damping mounts, which will have 2 pole roll off above 1.5 Hz.
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