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Wavefront Sensor Overview
All observations with the Gemini telescopes require the use of wavefront sensors (WFSs) assigned to one or more stars to provide image motion compensation (fast guiding) and/or higher order correction. This is achieved by articulation of the secondary mirror, manipulation of the primary mirror figure (active optics, aO) and/or adaptive optics (AO). See an evaluation of the Galactic stellar surface density for expected WFS star availability.
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 distance to avoid vignetting depends on the science field of the instrument.
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 6.7 arcmin of the science target if possible, although separations of up to 6.9 arcmin are useful in some circumstances. The available patrol annulus to avoid vignetting thus is the region between the relevant inner radius and the 6.7 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 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. Thus, in good conditions R~12 can be reached for PWFS1 and R~14 for PWFS2 at the optimum guiding frequency, and these limits can be extended by a magnitude or so by guiding at a lower frequency. When chopping the limits are roughly one magnitude brighter.
In general it is best to select the brightest PWFS star available within the usable annulus. 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 (see the characteristic WFS performance curve). 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.
Some of the Gemini facility instruments have on-instrument wavefront sensors. In the near-infrared instruments (e.g. NIFS) the OIWFS is itself an IR detector. For the optical instruments (e.g. GMOS) the OIWFS is a CCD. The mid-IR instruments and visiting instruments have no OIWFS and use the PWFSs.
As they are located very much closer to the instrument focal plane than the PWFS, the OIWFS have a much smaller vignetting footprint, albeit with a more limited patrol field. See the specific instrument pages for availability of the OIWFS and details of patrol field restrictions and sensitivity.
Altair is the adaptive optics system of the Gemini North telescope. Currently it is used with NIRI and NIFS. It uses a 177 actuator deformable mirror and a separate tip/tilt mirror to correct the wavefront in visible light at up to 1 Khz. The fast, high-order, corrections can be done using either a bright natural guide star or the sodium laser guide star. When the laser is used a natural guide star (which can be quite faint for low-strehl modes) is also required for tip/tilt corrections. The NIFS OIWFS can also be used with Altair to provide flexure corrections.
The wavefront sensors may be configured in various ways. In most observations, without adaptive optics, only one PWFS or the OIWFS is selected for use. The estimates of delivered image quality assume normal operation. However, if the observer desires accurate guiding on a distant sky position a second WFS may be used. Currently Michelle and T-ReCS only guide on one side of the chop, but 2-sided guiding, requiring 2 guide stars and both PWFSs may become available in the future.
Last update October 30th, 2007; Rachel Mason