STUDY SUMMARY - IPMU

SUMMARY REPORT
WIDE FIELD FIBER-FED OPTICAL
MULTI-OBJECT SPECTROMETER (WFMOS)
Focal ratio degradation (FRD), the third major source of throughput error, is a measure of
how well the fiber preserves the angular distribution of the input light. FRD is often hard to
quantify and depends to a large extent on fiber geometry (core diameter and core/cladding concentricity)
variation and details of the routing (bends, twists, stresses on fiber). Proper NA
matching between the telescope, fiber, and spectrometer is critical to avoid losses from overfilling
or under-filling the fiber or the spectrometer. Fiber mismatch at the connector interface can
also introduce FRD. LNA has devoted substantial effort to understanding and minimizing losses
from FRD. Figure 3.11-2 shows some early results.
Besides the above major contributors there are other loss terms. Fresnel losses (~4%) at both
ends of the fiber can be substantially minimized (to ~1%) with broadband anti-reflective
(BBAR) coatings. This is relatively easy to do at the slit-block end. AR coating fibers at the positioner
end presents logistical challenges; therefore, those ends will be left uncoated. There are
also losses (evanescent field decay) from bending and twisting of fibers. We have allocated 5%
losses due to these effects.
The fibers at the positioner end are placed and bonded in a custom ceramic ferrule that is
mounted to the actuator. As the fiber is moved, it twists and coils, either of which can cause
FRD. To understand this, 15 mock-ups of the actuator assembly were built to simulate articulation
of fiber in the same manner as the proposed design. Test results indicate that twisting does
not appear to induce an increase in FRD.
To present light to the spectrometer, 800 fiber tips will be lined up in a linear array with a
pitch of 230 μm. Each “slit block” will contain 80 fibers, and 10 slit blocks will be assembled
end-to-end to build one “slit” for each of the spectrographs. In risk-reduction activities, 80 rows
of fibers were epoxied (with EPOTEK 301) to quartz blocks. Several PTFE blocks were used to
hold the position of the fibers accurately during epoxy curing. Once the fibers were bonded, the
“slit block” was lapped and polished. For WFMOS, an additional cover plate, with BBAR coating
on one side and index-matching gel on the other side, will be placed against the slit block
surface to minimize Fresnel losses.
To protect the fibers from handling and environment, they are enclosed first in a protective
tube and then are grouped into the segmented tubes which act as a protective conduit. The filled
volume of the segmented tubes is calculated to prevent any crushing of the fibers.
As mentioned earlier, the fiber cables are separated into two sections:
• The short, approximately 5-m, cables go from the positioners to the top of the PFU
through a strain relief box. The strain relief box is designed to protect the fibers from the
motions of the telescope. This short section stays with the PFU and instrument.
• The long, approximately 55-m, cables run from the connectors on top of the PFU all the
way to the spectrograph located in the adjacent room. Part of this long cable—from the
spectrograph to the telescope structure—is permanent.
The remaining section is connected to the PFU during instrument operation and stored away
(tied to the telescope structure) when not in use.
3.12 Spectrograph
Our WFMOS spectrograph subsystem design comprises a set of three identical generalpurpose
spectrographs capable of carrying out all four required surveys (see Section 2) as well as
offering the flexibility for many other types of observation. The spectrograph requirements and
basic parameters are summarized in Tables 3.12-1 and 3.12-2.
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