Wavefront Sensor (WFS) Star Availability

The two diagrams below show the surface density of stars as a function of (a) optical R band and (b) near-IR composite J+H 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 WFS overview; the performance of the instrument OIWFS and adaptive optics AOWFS are included with the details of the relevant instruments.

Note that the likelihood of finding a suitable star within the NIRI OIWFS field with the f/6 (0.12 arcsec/pixel) camera is small. Any star found will likely be fainter than the OIWFS sensitivity limit (see the NIRI pages for more details when available) and, moreover, would often move out of the accessible field when executing a telescope mosaic or dither pattern. Thus the OIWFS cannot be used with the f/6 camera. However, the OIWFS is expected to be available for use with the f/14 and f/32 cameras.

Several on-line guide star catalogue servers are available.

Last update September 22, 1999; Phil Puxley and Doug Simons