STUDY SUMMARY - IPMU

SUMMARY REPORT
WIDE FIELD FIBER-FED OPTICAL
MULTI-OBJECT SPECTROMETER (WFMOS)
Figure 3.10-2: Positioner Patrol Region Overlap. The black dashed lines indicate the patrol
regions of adjacent positioners, showing there is complete coverage of the populated field of
view. The distance between adjacent positioners, d, is 8 mm, while the diameter of the Patrol
Region, 2R, is 9.5 mm. Blue and orange represent the two stages of the positioner motors (see
text).
The positioner is designed to locate the fiber tip to within 5 μm accuracy in less than
10iterations (FPRD L3.790), with all commanded moves taking less than 1 second (FPRD
L3.800). Meeting these timing and accuracy budget allocations means the throughput loss from
the positioners is less than 10%. Fiber repositioning — taking into account the other steps such
as the metrology loop (Table 4.2-1) — will be accomplished in less than 40 seconds. This maximizes
observing time and efficiency. A prototype was built by New Scale Technologies and recent
testing at JPL demonstrated this first unit was able to converge on 90% of the targets to
within the 5 microns accuracy in 6 iterations and had 100% convergence after 8 iterations, all at
the required speed. Additional details of this test are discussed later in this section and in the Detailed
Technical Design, Section 2.3.14.
The Cobra Positioners are mounted in an optical bench at the focal plane and are placed
8 mm apart to create a 2,400-positioner field in a hexagonal pattern to allow efficient tiling of the
sky region of interest. Each positioner has a circular patrol region of 9.5 mm; this, combined
with the 8-mm spacing, ensures complete coverage of the populated area of the focal plane. The
populated area fully covers a 1.3-degree field over the area of the focal plane with higher
throughput, resulting in more efficient use of the positioners (Figure 3.10-3).
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