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Guiding and Wavefront Sensors

Essentially all observations with the Gemini telescopes require the use of either a peripheral wavefront sensor (PWFS) or an on-instrument wavefront sensor (OIWFS) assigned to a star near the science target, and in some cases an adaptive optics wavefront sensor (AOWFS) assigned to a star or artificial laser guide star, in order to:

(1) remove low order aberrations of the primary mirror;

(2) compensate for image motion due to telescope tracking inaccuracies, windshake, and atmospheric effects; and

(3) for some near-infrared observations to correct for blurring of images by the earth's atmosphere.

These enhancements to the image quality are achieved respectively by:

(1) manipulation of the primary mirror figure (adaptive optics, AO) on one-minute timescales;

(2) articulation of the secondary mirror typically on ten millisecond timescales; and

(3) adjustments to the shape of a small deformable mirror on millisecond timescales (adaptive optics, AO).

When adaptive optics is not in use the PWFSs are employed by instruments that do not have their own built-in WFSs. Instruments with built-in WFSs usually do not use the PWFSs, although a PWFS may be used.

The region of sky in which a suitable guide star must be found for the WFS is limited to an small area close to the science target. In almost all, but not all cases a suitable star can be found; see this page for some indication of likely guide star availability.

The wavefront sensors may be configured in various ways.


  • For most observations without adaptive optics, one PWFS or the OIWFS is selected for use. The estimates of delivered image quality assume normal operation.
  • At Gemini North, for normal natural guide star adaptive optics, ALTAIR itself provides the tip/tilt and focus correction (using either the optical light from target itself or that from a very nearby point like object).
  • For laser guide star adaptive optics (at Gemini North) normally a very nearby tip/tilt and focus star is used with ALTAIR. If a nearby star is not available PWFS1 can be used to improve the natural seeing FWHM by a factor of 2-3; see here for further information.