About the Subaru Telescope

Observational instruments

FMOS (Fiber Multi-Object Spectrograph)

FMOS (Fiber Multi-Object Spectrograph)

FMOS is a powerful, fiber-fed, wide-field spectroscopy system that enables near-infrared spectroscopy of over 100 objects at a time. It is composed of three subsystems: 1) an infrared unit at prime focus (PIR) that includes a wide-field corrector lens system and fiber positioning system ("Echidna"), 2) a fiber bundle unit of 400 optical fibers, and 3) two spectrographs. Echidna can precisely position all 400 fibers in just 15 minutes. This high speed for repositioning allows observers to reconfigure Echidna, observe multiple fields during a night, and rapidly observe hundreds of faint targets that can be compiled as data for statistical analysis.

Fast Facts

The FMOS System

Photo: The FMOS System


a wide-field spectroscopy system composed of three subsystems

  • The Prime-Focus Unit for Infrared (PIR)
  • Fiber bundle unit of 400 optical fibers
  • Two spectrographs


  • Uses 400 optical fibers for fast, effective simultaneous observations of faint objects.
  • Echidna positions the fibers by using a unique “stick and slip” motion driven by a piezo electric actuator that moves without gears.


Rapid observations of dim objects ranging from substellar objects like brown dwarfs to large-scale studies of the distant Universe (with redshifts of more than 1).


  • Size and weight
  • PIR-height of 2.5 m (8 ft) and weight of 2,600 kg (5,700 lbs)
  • Spectrographs-5 X 2 X 2 m each (17 X 8 X 7 ft. each)
  • Placement:
  • PIR: on the top unit at prime focus
  • Fiber bundle unit: 400 optical fibers are bundled into two main cables wrapped around a central strength cable and then snake 60 m (200 ft.) down the telescope and through the enclosure to the two spectrographs.
  • Two spectrographs: on a special platform within the enclosure.
  • Wavelength(s): near-infrared between 0.9 and 1.8 microns (J and H bands)
  • Detectors used: Hawaii-II 2K X 2K array
  • FOV: 30 arcminutes
  • Pixel scale/spatial resolution: fiber diameter 1.2 arcseconds.
  • Spectral resolution: R=600 (low resolution mode) and 2200 (high resolution mode)


  • Developers
  • FMOS is a product of collaboration by scientists, engineers, and other staff in Japan, Australia, and the United Kingdom.
  • PIR
  • The Kyoto University group began its development.
  • The Subaru Telescope (NAOJ) assembled and upgraded it.
  • Echidna: Anglo-Australian Observatory (AAO, now named the Australian-Astronomical Observatory)
  • Fiber cables: Durham University group
  • Spectrographs
  • One from Kyoto University group
  • One from a joint team of Oxford University and Rutherford Appleton Laboratory
  • Both were integrated, commissioned, and characterized in collaboration with Subaru Telescope (NAOJ).

Specialized Information about the Instrument and Observing: