When making high precision AO observation of a target that is different from the guide star that the wavefront sensor sees, it is necessary to locate a pair of stars instead of a single star in order to to determine the PSF at the target position. This is due to the anisoplanatism of the AO corrected field of view. In general, the difference between the PSF at the AO guide star and at the target position increases with separation of the target and guide star and decreases with wavelength of the passband.

This search tool will help the observer locate a pair of stars in the vicinity of the target that matches the separation and position angle of the target and guide star. The program uses the USNO2.0 catalogue. The magnitude referred to as V magnitudes is the actually the average of the USNO2.0 R and B magnitudes.

NB: The search may take several minutes. If you experience timeouts, reduce the search radius to limit the number of stars.