About the Subaru Telescope

Observational instruments

188-Element Adaptive Optics (AO)

188-Element Adaptive Optics(AO)

Subaru's 188-element AO system includes laser guide star technology, which allows observers to use a laser as an artificial star when there is no bright guide star illuminating an object-star. The development of the 188-element system was a product of continual improvements to Subaru's earlier 36-element AO system, which was designed, produced, and then made available to astronomers worldwide in 1990. The current 188-element AO system successfully passed its first test on October 9, 2006 and replaces the older system. This 188-element AO mounts at the infrared Nasmyth focus.

Fast Facts

the 188-element deformable mirror, the diameter of which is 3.5 inches (90 mm)

Photo:the 188-element deformable mirror, the diameter of which is 3.5 inches (90 mm)


  • Second-generation adaptive optics system
  • Main components
  • A wavefront sensor that analyzes light from a guide star
  • A beam splitter that channels the light to the wavefront sensor and the science instrument
  • A 188-element deformable mirror capable of tiny adjustments to correct for the effects of turbulence
  • Optimized for use with infrared cameras and spectrographs and being tested for use with optical instruments


  • Compensates for atmospheric turbulence.
  • Selection of a guide star (a point of light) near the target object to use as a reference for measuring and correcting for the effects of turbulence, e.g., blurring, twinkling.
  • Passage of the guide star's light beam from the telescope's primary mirror to the wavefront sensor, which measures the wavelengths around targeted objects.
  • The wavefront sensor sends the light to a beam splitter, which channels part of it to the science instrument and part, to the control system, which measures the degree of turbulence.
  • Based on the control system's measurements, it adjusts the surface shape of the 188-element deformable mirror, which makes very fine corrections for the effects of turbulence, thus facilitating sharp, high-resolution images
  • Because air is moving, the AO system is a dynamic one, always adjusting to current conditions, almost in real time.
Laser beam sent into the night sky

Photo:Laser beam sent into the night sky

Optics that produce the laser beam

Photo:Optics that produce the laser beam

  • Can operate in two modes
  • Natural guide star (NGS) mode, when a "natural" star is used for analysis of atmospheric turbulence.
  • Laser guide star (LGS) mode, when an "artificial" star is created near the target object. When AO 188 is used with the LGS, it is referred to as LGSAO (laser guide star with adaptive optics).


  • Always used with an instrument, e.g., IRCS, HiCIAO
  • When used with another instrument, it contributes to:
  • Studying objects that were previously unobservable, e.g., the detailed structure of faint distant galaxies, stellar populations of nearby galaxies
  • Imaging the precise structures of disks around stars
  • Detecting exoplanets


  • Size and weight
  • AO188 optical bench/base plate: 2100 mm x 1720 mm
  • Weight: 4872 lbs. (2210 kg)
  • Placement: Nasmyth focus (IR)
  • Wavelength: .6 to 5 microns (optical to infrared)


Specialized Information about the Instrument and Observing: