The peripheral wavefront sensors (PWFS) vignette a large part of the GMOS science field. Furthermore, it has been found that there is significant flexure between GMOS and the PWFSs. At this time it is therefore recommended to use the on-instrument wavefront sensor for all GMOS observations with the exception of non-sidereal tracking, since the OIWFS implementation is purely electronic and limited to a small field of view.

At a later time when the flexure between the PWFSs and GMOS has been modeled and can be corrected, the PWFSs may be used for long-slit and IFU observations, where it is possible to select guide stars such that the slit or IFU is not vignetted. An example of a set-up for long-slit spectroscopy is shown below.

Schematic diagram showing the vignetting caused by the PWFSs. In this example the circle corresponds to the 7 arcmin outer radius of the PWFS patrol area. The white horizontal line in the center shows a GMOS long-slit. The large, shaded irregular boxes are the vignetting patterns of the two PWFSs; PWFS1 in the top of the image and PWFS2 in the bottom of the image. PWFS2 is not positioned on a star.

In original form; Isobel Hook
Previous update February 22, 2002; Inger Jørgensen
Last update August 30, 2006; Kathy Roth