STUDY SUMMARY - IPMU

SUMMARY REPORT
WIDE FIELD FIBER-FED OPTICAL
MULTI-OBJECT SPECTROMETER (WFMOS)
These components are commercially available with glycol cooling, which may be modified to
remotely locate the field joint on the secondary ring.
One of the major integration phases occurs at JPL where the positioner assembly, Acquisition
and Guide Cameras, and Fixed Fiducial are integrated onto the PFI structure using a temporary
structure for the PHSC. As a risk-mitigation strategy, a fit check of the Rotator Interface Ring
and the PHSC Rotor is done before it is integrated into the instrument. A turnover/rotation fixture
is used to build up the PFI starting from the rotator interface. As soon as the alignment system
and positioners are integrated onto the Rotator Interface Ring, the fixture is used to test performance
in multiple orientations with respect to gravity.
3.10 Positioner
One of the key features of WFMOS is the positioner assembly, which places the tips of the
fiber optics at specified locations on the focal plane to route the light from the astronomical objects
of interest to the spectrograph. Previous generations of multi-object spectrographs have allowed
simultaneous spectra of many hundreds of objects. Techniques employed for these spectrographs
have included humans placing fibers by hand in automatically machined plates (e.g.,
SDSS), and robotic pick-and-place of magnetically attached fibers (e.g., 2dF). As our ambitions
rise to simultaneously measuring many thousands of simultaneous spectra, a reconsideration of
the full array of viable techniques is required.
After examining a number of alternatives (Detailed Technical Design, Section 2.3.16), the selected
approach, the “Cobra” positioner system, populates the focal plane with 2,400 positioners
that fill a hexagonal field (Figure 3.10-1). Each positioner moves a single fiber to its target position
in its planar patrol region. Translating the fibers through the patrol region using these actuators
allows rapid, direct placement of the fiber tip with a mechanism that is robust in terms of
stability, accuracy, and life. Its planar movement eliminates losses due to fiber tilt and tests replicating
the fiber path through the positioner confirm that there is no loss due to fiber twist or
bend.
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