STUDY SUMMARY - IPMU

SUMMARY REPORT
WIDE FIELD FIBER-FED OPTICAL
MULTI-OBJECT SPECTROMETER (WFMOS)
3.0 Instrument Description
3.1 Rationale
The overriding factors in designing WFMOS for the science applications discussed in Section
2 are the need for survey speed and positioner reliability.
With the 8.2-m aperture of Subaru, emission line galaxies in the redshift range 0.7 < z < 1.6
can yield sufficient signal to noise for a reliable redshift in only 15 minutes. This places stringent
demands on the overhead for each exposure, and particularly that associated with reconfiguring
2,400 fibers for a subsequent field.
In the course of conducting the surveys discussed earlier, WFMOS will reconfigure itself
over 3,000 times. Reliability of positioning and robustness of fiber units are key considerations.
In this section we present an instrument design that we believe represents a major improvement
over that discussed in the Feasibility Study. The key advantages are:
• A positioner system that can reconfigure itself to the required precision in under 40 s.
Each fiber is maneuvered at its tip, eliminating the need for tilting. The stout design of
each unit leads to a robust system.
• A set of three high-efficiency spectrographs based on a Schmidt optical design, which
offers an unrivalled combination of light beam area and acceptance solid angle at the
detector (the A-Omega product). Recognizing the long-term utility of WFMOS, our
spectrograph design can accommodate a range of gratings, each of which can be tilted,
offering full flexibility in survey design.
• State-of-the-art, deep-depletion CCD detectors, ensuring optimal efficiency at red
wavelengths for extending redshift surveys beyond z ~1.3 and measuring stellar dynamics
using the Ca II triplet.
3.2 Instrument Overview
WFMOS is a fiber-fed spectrograph designed to be mounted at the prime focus of the Subaru
telescope located on the summit of Mauna Kea, Hawaii. The instrument allows simultaneous
low-resolution spectral observations of up to 2,400 astronomical targets. In high-resolution
mode, WFMOS can observe 600 simultaneous targets.
The instrument system comprises several components. A block diagram appears in Figure
3.2-1. We summarize here the basic function of the components and their interactions; individual
components are described in greater detail in following sections.
WFMOS comprises components mounted in several locations. System elements required to
be at the prime focus of the telescope make up the Prime Focus Instrument (PFI) and are housed
within a Prime Focus Unit (PFU). The three spectrographs, along with their detectors and support
systems, will be located in a spectrograph room adjacent to the telescope, above the Nasmyth
focus.
WFMOS will use several Subaru provided elements that are being developed for the Hyper-
SuprimeCam (HSC) instrument. These include the HSC Field Rotator, Hexapod, structure,
and a Wide Field Corrector (WFC).
18