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Guide Star Availability
The two diagrams below show the surface density of stars as a function of optical R band magnitude for Galactic latitudes of 30 and 90 deg. They were derived from a Bahcall - Soneira model (e.g. Bahcall & Soneira 1984, ApJS, 55, 67) modified by Doug Simons.
The horizontal lines indicate the reciprocal patrol area of various Gemini wavefront sensors. The point at which each line cuts the stellar density curve indicates the magnitude where, statistically, one star should lie within that WFS's patrol area.
For example, one star of magnitude R = 13.5 - 15 (depending on Galactic latitude) should be available within the Peripheral WFS patrol area (~14 arcmin diameter). This implies that PWFS guide stars for most science targets can be selected from the GSC1 catalogue which has a limit of approximately 14-15 mag.
The detailed patrol field and sensitivity of the PWFS are given in the PWFS overview; the performance of the instrument OIWFS and adaptive optics AOWFS are included with the details of the relevant instruments.
Several on-line guide star catalogue servers are available.