STUDY SUMMARY - IPMU

SUMMARY REPORT
WIDE FIELD FIBER-FED OPTICAL
MULTI-OBJECT SPECTROMETER (WFMOS)
4.2 Reconfiguration Time
In addition to instrument throughput, the other driver of observing efficiency is observational
overhead. In order to minimize overhead, parallel activities are planned. Immediately after the
previous exposure is complete, we begin reading out the three science detectors. Simultaneously
we begin slewing the telescope to the new target location and repositioning the fibers. This involves
illuminating the fibers, imaging the fiber tips with the Metrology camera, computing the
centroids of the imaged fiber tips, and moving the fiber tips. The repositioning sequence is repeated
until the fibers are within tolerance of the object coordinates. The time budget allows six
iterations for fiber positioning.
The fibers are back-lit from the spectrographs and are arranged in the focal plane so that no
two adjacent fibers are illuminated from a single spectrograph. This allows unambiguous identification
since there are overlapping patrol regions. Table 4.2-1 illustrates the activities happening
in parallel. Once the metrology cycle is complete, the Acquisition and Guide System acquires the
guide targets. When the A&G system notifies the Control System that it has a lock, the instrument
starts a new exposure. The entire process will take less than 40 s.
Table 4.2-1: Positioner Configuration Timeline
Start Time Duration (s) Steps being Taken in Parallel
0 1 Move A-1
1 2 Image A-1 Move B-1
3 2 Compute A-1 Image B-1 Move C-1
5 2 Move A-2 Compute B-1 Image C-1
7 2 Image A-2 Move B-2 Compute C-1
9 2 Compute A-2 Image B-2 Move C-2
11 2 Move A-3 Compute B-2 Image C-2
13 2 Image A-3 Move B-3 Compute C-2
15 2 Compute A-3 Image B-3 Move C-3
17 2 Move A-4 Compute B-3 Image C-3
19 2 Image A-4 Move B-4 Compute C-3
21 2 Compute A-4 Image B-4 Move C-4
23 2 Move A-5 Compute B-4 Image C-4
25 2 Image A-5 Move B-5 Compute C-4
27 2 Compute A-5 Image B-5 Move C-5
29 2 Move A-6 Compute B-5 Image C-5
31 2 Image A-6 Move B-6 Compute C-5
33 2 Compute A-6 Image B-6 Move C-6
35 2 Compute B-6 Image C-6
37 1 Compute C-6
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Read out of Science
Detectors
Move Telescope to
New Position
Acquire Target w/
A&G system
4.3 Allocation Efficiency
An important aspect of any positioner scheme is the efficiency with which the individual
positioner elements can be allocated to the targets of interest for spectroscopy. This allocation
efficiency is a measure of how effectively the spectroscopic capability of the instrument is being
used. WFMOS has been designed to provide high allocation efficiency for both the low and high
spectrograph modes. If the number of targets is very large compared to the number of positioner
elements, then every element will have many targets available to choose from and the positioner
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