NIRI OIWFS and Guide Stars

The NIRI on-instrument wavefront sensor (OIWFS) is currently unavailable, and all users should instead use the Peripheral Wavefront Sensors.

The OIWFS has a field of view (FOV) which depends on the camera selected because the science channel pick-off mirror, past which the OIWFS light shines, is sized to match the science field. See the cameras section for illustrations and more details. The OIWFS is not usable with the f/6 camera because its FOV is extremely restricted, but it is usable with f/14 and f/32 cameras.

The performance of the NIRI OIWFS has not yet been measured. It is predicted that the limiting guide star magnitude will be H~17 mag, depending on wind speed. To date, the OIWFS has only been tested on stars as faint as H=14 mag. Proposers who request OIWFS guide stars should be conservative and assume the limiting magnitude is between 13 and 14. The NIRI OIWFS now has a Shack-Hartmann prism dividing the light into four spots, so the sensitivity might be lower than previously estimated.

The OIWFS nominally uses a broad-band J, H, or K filter. See the modelled availability of guide stars at these wavelengths, which depends on the WFS patrol field of course.

Use of the OIWFS is expected to improve the delivered image quality. See the observing condition constraints for an overview and further details about the expected performance at J (1.25µm), K (2.2µm) and L (3.4µm) bands including estimates of the minimum guide star brightness.

Last update 2006 September 1; Andrew Stephens, Tom Geballe & Joe Jensen