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Return beam from slit to acquisition TV

The slitviewer fibre image guide gives only a very small field of view. This is adequate for verifying that the object remains on the slit during an exposure, but is not adequate for target acquisition. An additional means of viewing the entire field at a very coarse scale on an acquisition TV is therefore provided. The scale is about 1.14 arcsec/pixel in the X direction on the TV, and about 0.73 arcsec/pixel in the Y direction. This option cannot be used whilst an exposure is active.

In order to use the acquisition TV to view the field, it is neccesary to do three things:

• The autoguider mirror must be moved so that the TV pickoff mirror views the slit. The image of the slit jaws is then relayed to an ISEC intensified TV mounted on top of the A&G unit.

• Unlike the fibre slitviewer, the pickoff mirror for the acquisition TV is not attached to the slit unit, and so does not move with the slit; in order for the return beam from the slit to reach the pickoff mirror, the slitangle must be set to zero degrees.

• The return beam from the slit to the pickoff mirror has to pass through the main filter wheels; in order to make this possible, the filter wheels must all be moved to position 1, so that the beam can pass through specially cut holes in the wheels.

Even once these actions have been carried out, the field of view of the TV will still be obstructed; the slit-viewing probe obstructs both the beam from the telescope to the slit, and the return beam from the slit to the pickoff mirror. Fig. 10 is a sketch of the TV field, showing the following features:

• The slit itself appears vertical on the TV, running from top to bottom of the image.

• There is an obstruction in the centre of the field, preventing the TV from viewing the central part of the slit. This obstruction is the slit viewing probe (see below), with its 3 support vanes. Since the slitviewing probe and TV both view the slit from the same angle, it is inevitable that the slitviewing probe obstructs the central part of the TV field.

• There is a further obstruction close to the lower right-hand edge of the TV field. This obstruction is again the slit-viewing probe, but this time obstructing the beam from the telescope to the slit, rather than the return beam from the slit to the TV.

• The dekker slide appears in front of the slit jaws, and contains a dekker mask designed to restrict the slitlength. The shape of the dekker mask (normally referred to as the 'concorde' dekker) is illustrated in Fig. 2. It moves across the TV field from right to left, with the narrow end of the dekker appearing on the right--hand side. Fig. 10 shows the appearance of the TV field with this dekker mask in different positions.

In order to use the TV to view the slitjaws, it is first necessary to move the dekker mask out of the way, by moving it to close to the top end of its range of movement.

Since the central part of the TV field is obstructed, it is not possible to use the TV to centre an object on the slit directly. After pointing the telescope at a new object, the normal procedure is to apply a standard aperture offset. This will offset the telescope so that the object appears in an unobstructed part of the TV field. The object can be centred up, and the aperture offset then removed to place the object back on the slit.