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As noted in section 16.2, the beam from the telescope to the CCD autoguider comes to a focus in the reflected focal plane below the autoguider mirror. In that reflected focal plane are two autoguider probes. Each probe consists of a small prism feeding a fibre image-guide, which transfers an image to the CCD autoguider, mounted on top of the A&G unit. The two probes, normally referred to as the 'coarse' and 'fine' guide probes, give different scales and field of view. The coarse probe provides a field of 13 arcsec (3 mm) and a scale at the CCD autoguider of 0.27 arcsec/pixel. The fine probe provides a field of 4.4 arcsec (1 mm) and a scale at the CCD autoguider of 0.13 arcsec/pixel. The fine fibre is UV transmitting, for guiding in the UV, the coarse fibre is not. The larger field of the coarse probe means that this is more useful for most applications. The fine probe is only useful for acquisition and guiding in the UV, or in conditions of superlative seeing.
The autoguider probes are mounted on an (x,y) table, making it possible to move them around the reflected focal plane in order to pick up the image of a potential guide star. The probes can be positioned in steps of 20 m (about 0.09 arcsec). The probes can also be moved in the z direction in order to focus the image. The stepsize for the focus movement is 53 m; one z--step gives a change in image-size of about 0.02 arcsec.
N.B. Remember-when filters are used in the main beam, and the WHT
focus is adjusted to take account of this, the autoguider focus must
also be adjusted to compensate for the change in WHT focus.
It is planned to fully automate this process of focus compensation before 1995, but for the present it must be done manually.
During object acquisition, the coarse probe is used to view the field at the rotator centre, with the autoguider mirror positioned so that all the light from the telescope is directed down into the reflected focal plane. During an exposure, either guide probe might be used for autoguiding on an offset guide star in the outer part of the field.
The light from the autoguider probes can be independently filtered, by means of a 12 position filter wheel. The autoguider filters are permanently mounted, and are summarised in Tab. 9. All filters are 2 mm thick, including the fused silica filter in the clear position. Transmission curves are shown in Fig. 13.
The optical path which transfers the image output from the fibres to the CCD autoguider is fairly complex. It is summarised here for information only; see the nasmyth A&G Unit optics manual for a more detailed description. The output from the coarse fibre is first redirected by a 45 degree mirror, then bought to a focus in an intermediate focal plane by a pair of achromats. The outputs from the fine fibre and the slitviewing fibre (see section 16.4) also appear in the intermediate focal plane, injected into the lightpath by means of prisms on the ends of the fibres. The intermediate focal plane therefore contains images of all three fibre outputs, slightly offset from one another. This combined image is then passed through the autoguider filter wheel, and finally reimaged onto the CCD autoguider by means of two off-axis paraboloids.
Which guide probe is used is determined by reading out only a small window of the CCD autoguider, containing the image of the desired probe. The CCD autoguider can be used in two modes: