STUDY SUMMARY - IPMU

SUMMARY REPORT
WIDE FIELD FIBER-FED OPTICAL
MULTI-OBJECT SPECTROMETER (WFMOS)
Changing survey modes requires the following:
• Changing the camera–collimator angle. The spectral resolution R is directly related to
this angle, with the lowest R corresponding to the straight-through position.
• Changing the VPHG by moving the active VPHG out of the beam on to the selectorslide,
moving the slide to the position for the new VPHG and then moving the new
VPHG in to the beam (see Figure 3.12-3).
• Changing the band-pass filter by rotating the filter wheel.
This process is fully automatic and can be done in less than one minute.
The WFMOS design uses VPHGs for all four surveys. VPHGs have several advantages over
the ruled gratings traditionally used in astronomical spectrographs. VPHG peak efficiency is in
excess of 90%. For both the DELZ and DEHZ surveys, the gratings will be used in first order.
For the two Galactic Archaeology surveys, higher orders are used.
The design is extremely versatile and general-purpose. The survey gratings can potentially be
used at other wavelengths and resolutions by changing the camera–collimator angle. For example,
the GAHR grating gives R ~30,000 at the maximum camera–collimator angle, and any
wavelength sub-range (about 30-nm long) in the 420-nm–970-nm range can be selected by selecting
the appropriate order with a band-pass filter. The band-pass filters are small and inexpensive.
A great advantage of a VPHG spectrograph with a variable camera-collimator angle is that
the gratings can always be used so that they satisfy the Bragg condition and, therefore, work at
their peak efficiency. The design has additional flexibility in that it is possible to install new gratings
for future observations.
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