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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.