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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.

  • 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.