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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 (active 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.