AO PSF Star Search Tools

AO PSF star search tools

Three tools are supplied for aiding AO observers in finding appropriate stars for AO correction or PSF determination.

AO Guide Star Finder
This tool is relevant for observers who has a target or a list of targets and wants to locate possible AO guide stars within the NAOMI wavefront sensor field of view. Note that the default 90 arcseconds corresponds to the entire WFS field of view, but the field of acceptable AO correction is much smaller. See the NAOMI documentation for details on useful field sizes in different bands.
AO PSF Star Finder
This tool is made for observers who are studying a target that is bright enough to be used as the guide star for the AO correction (i.e. brighter than about V=13.5). If the objective of the observations is to search for faint emission or objects around this target, it is essensial to have a PSF star that is known to be a point source for comparision. This is due to the fact that the PSF when working close to the diffraction limit of the telescope is normally quite complicated as compared to normal atmospherically blurred PSFs. It can also change with time and with elevation of the telescope, so it is important to locate a PSF star not more than a few degrees away from the target.
AO PSF Pair Finder
Targets fainter than the sensitivity limit of the wavefront sensor can not be used for AO correction, and in these cases a bright star offset from the target might be used. In this case 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.

These three programs uses the USNO2.0 catalogue. Note that the catalouge is divided into zones every 7.5 degree from declination -90 to +90, and solutions are only retrieved from one zone. This means that if the target coordinates given is just on the boundary of a zone, the effective search area will be only half of the area of the full circle.

Note also that any returned objects may not be a useful as a PSF star, since the USNO catalogue contains many unresolved binaries or unidentified extended sources.

The following notes are written for the AO PSF Pair finder, but since the program invoked is actually the same for all three tools, some of the notes are relevant for all the tools.

Target coordinates

The target coordinates can be given either in sexagresimal format, i.e. "hh mm ss.ss" and "dd mm ss.ss", where the RA is given in hours (0 to 24) and the declination in degrees (-90 to +90). Alternatively, decimal notation can be selected and the coordinates are both given in decimal degrees notation, where the RA is from 0 to 360 and the DEC again from -90 to +90.

Sky search radius

The sky search radius should be as small as possible to limit the search time and large enough to ensure a few solutions. The default value of two degrees is usually good enough to find a few solutions. If the search fails, try to increase this parameter in small steps until a good solution is found.

Magnitude limits

The magnitude limits corresponds to the faintest star considered in the search. Both limits should not be significantly fainter than the target and guide star for the target itself.

Guide star distance

The distance as measured from the separation of the target/guide star pair. The tolerance for this value can be adjusted to allow better pairs to be returned.

Position angle of the pair

The PA should also be as measured from the target/guide star pair. The tolerance can be quite relaxed since the main degradation is with separation, not with PA.