Two peripheral wavefront sensors are part of the Acquisition and Guidance (A&G) system located within the instrument support structure (ISS) cube. PWFS1 consists of a 6x6 lenslet array feeding 2D array detector and PWFS2 has a 2x2 lenslet array feeding its array detector. A selection of filters covering the optical and near-infrared (e.g., V,R,I) is available for each array. Mounted on rotary stages, and as their name implies, the PWFSs patrol an annulus of sky around the science field. The PWFSs are upstream of the science instruments and, whilst they may be moved into the centre of the field for engineering purposes, will vignette the science field unless positioned at sufficient distance.

The minimum angular distance (i.e. inner patrol radius) of the PWFS1 and PWFS2 stars from the field center differs for various instruments and their configurations. For example, the PWFS2 guide star must be at least the following distance off-axis to avoid vignetting the science field: NIRI imaging f/32 camera - ~4.2 arcmin; f/14 camera - ~4.6 arcmin; f/6 camera - ~5.0 arcmin; NIRI spectroscopy - depends on slit width and orientation, but can be as small as ~4.0 arcmin; check the individual instrument pages for more details. For PWFS1 the minimum distances are typically 0.5' larger than for PWFS2; Acquisition Camera - ~6.1 arcmin. The outer patrol radius depends on the location of the star in the patrol field and the amount of motion needed for focus corrections. Guide stars should be within 6.9 arcmin of the science target if possible, although separations of up to 7.0 arcmin are useable in some circumstances. The available patrol annulus thus is the region between the relevant inner radius and the 6.9 arcmin outer radius. PIs are urged to use the Position Editor feature of the Observing Tool for an accurate view of the focal plane, instrument field of view and vignetting, when setting up their observations to be sure that the WFS does not vignette their field of view. Please also be sure to check that any offsets in the sequence that are intended to be guided do not cause the guide star to fall outside the usable range.

Both PWFS1 and PWFS2 perform primary mirror active optics control in addition to directing the tip/tilt secondary mirror. Because of its larger number of lenslets, PWFS1 is capable of making more accurate measurements of the primary figure. However, the larger number of lenslets means that it must observe brighter stars than PWFS2. The limiting visual magnitudes for effective use of PWFS1 and PWFS2 for tip-tilt guiding depend on seeing, moonlight, cloud cover and wind speed and direction, but are typically 11-13 mag for PWFS1 and 13-15 mag for PWFS2. In good conditions (clear skies and good seeing) R~12 can be reached for PWFS1 and R~13.5 for PWFS2 at the optimum / maximum guiding frequency (200 Hz). These limits can be extended fainter by a magnitude or so by guiding at a significantly lower frequency (~50 Hz). When chopping (e.g. when using MICHELLE or T-ReCS) the limits are roughly one magnitude brighter.

In general it is recommended that one select the brightest PWFS star available that is fainter than ~9th magnitude, within the usable annulus and to use PWFS2 rather than PWFS1.

Guide stars are usually acquired using a red filter, so magnitudes as close as possible to R band are most relevant (other filters can be used if, for example, the only available guide star is very blue; in cases like this please alert your NGO/contact scientist at Phase II). Note that the simple PIT search (required for phase I proposals) usually selects the brightest guide star available, but it may not be bright enough for the WFS. The delivered image quality degrades very rapidly for fainter stars. Guide stars must also be real, single, point sources as opposed to galaxies, double stars or diffraction spikes around bright stars, for example; some guide star catalogues contain many such objects.