About the Subaru Telescope

Observational instruments

FOCAS (Faint Object Camera And Spectrograph)

FOCAS (Faint Object Camera And Spectrograph)

Designed for high-sensitivity optical observations of faint celestial objects, this versatile instrument includes all of the fundamental modes of optical astronomy: imaging, spectroscopy, and polarimetry. Its multi-object spectrograph, which enables observations of the spectra of about 50 objects at once, allows astronomers to determine the distance and detailed physical proper-ties (e.g., chemical composition, mass, stellar population) of very faint celestial objects in far-distant galaxies. The data it produces contribute to an under-standing of the origin and evolution of the Universe.

Fast Facts


  • Optical imager
  • Low- to medium-resolution multiple-object spectroscopy
  • Specialied for observing faint objects


  • Allows broad- and narrow-band imaging, spectroscopy, imaging polarimetry, and spectropolarimetry of a wide range of objects.
  • Several observational modes
  • Direct optical imaging
  • Long-slit spectroscopy
  • Observes a single celestial object through an elongated slit aperture.
  • Refracts light with a prism or diffraction grating.
  • Allows different frequencies of light to fall on different parts of the sensor.
  • Multi-object spectroscopy
  • Uses several slits to observe multiple celestial objects in the field of view.
  • Uses a slit-mask changing system to position a special mask each time, depending on the target area.
  • Obtains simultaneous spectra of up to 50 objects close to each other in the area.
  • Polarimetry
  • Measures polarization of light, i.e., the degree to which light waves vibrate wholly or partially in one direction rather than at random.
  • Gives information about the orientation of the object not obtained by other observational methods.
  • Spectropolarimetry
  • Measures polarization of light from celestial objects
  • Uses spectrograph to isolate the areas for measurement
  • Flexibility of observing modes with quick, efficient exchanges
  • From spectroscopy to polarimetry to imaging
  • Of filters, grisms (a combination of a prism and grating), gratings (fine, parallel, equally spaced grooves)
  • Of the angle of the wave plate, which converts polarized light in different ways


  • Designed especially for spectroscopy of faint objects, e.g., distant galaxies, quasars, outer asteroids of the Solar System.
  • Confirms actual distances of celestial objects, e.g., supernovae, and measures distances to the most distant, faint galaxies near the edge of the Universe.


  • Size and weight
  • Size: 2 x 2 x 2 m
  • Weight: 2 tons
  • Placement: Cassegrain focus
  • Wavelength(s): 0.4-1.0 microns
  • FOV: 6 arcminutes
  • Detectors: CCDs with enhanced sensitivity (Hamamatsu)
  • Two 4,096 x 2,048 pixel 4-side buttable detectors
  • 2 x 1 mosaic pattern


  • 1991-1995 Finalized the detailed design, e.g., optical design, mechanical design, control system, grisms, wave plates for polarimetry, multi-slit exchange mechanism.
  • 1996 Construction began. The collimator lens unit and the camera lens unit were manufactured by Nikon.
  • 1999 Shipped to Hawaii. Action and control tests, adjustments, optimizations on telescope simulators at Mitaka campus in Tokyo and at the Hilo base facility.
  • 2000 (February) First light
  • 2001 Available for common-use observations after commissioning and testing most of the instrument's features
  • 2003 Polarimetry mode available
  • 2010 CCDs replaced with fully-depleted type CCDs manufactured by Hamamatsu Photonics KK

Specialized Information about the Instrument and Observing: