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Two peripheral wavefront sensors (PWFS1 and PWFS2) are part of the Acquisition and Guidance (A&G) system located within the instrument support structure (ISS) cube. Both PWFS1 and PWFS2 perform primary mirror active optics control in addition to directing the tip/tilt secondary mirror.
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, although they can be moved into the center of the field for engineering purposes, they will vignette the science field unless positioned at sufficient distance from field center.
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.
Approximate limiting magnitudes for effective use of PWFS1 and PWFS2 are given in the following table.
|R MAGNITUDE LIMITS OF
PERIPHERAL WAVEFRONT SENSORSa
(a) For clear skies, good seeing, partial moon (CC50, IQ70, SB80), and tip/tilt running at 200 Hz. Limits are worse for worse seeing, clouds, more moonlight. Limits are one magnitude fainter if tip/tilt is run at 50 Hz. When chopping, the limits are roughly one magnitude brighter.
(b) At Gemini South, 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.
In general it is recommended that one use PWFS2 rather than PWFS1 and that one select the brightest PWFS star within the usable annulus that is fainter than ~7th magnitude.
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.