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Adjustment of the rotator axis and orientation.

The rotator axis - aperture plate alignment and the orientation of the rotator are crucial when fibre optics are employed. An error in the rotator orientation translates to a difference between the North direction of the focal plane and that of the aperture plate. For multi-object spectroscopy, this, in turn, means a bad field acquisition, together with a drastic efficiency decrease. In extreme cases, the field can even not be seen. For this reason it is very useful to perform these adjustments as accurately as possible before using the instrument on the first night. It is also advisable to check them at the beginning of every subsequent observing night.

The aperture plates have several holes to allow connection of the semi-coherent bundles at 400" North and 400" South from the plate centre, and the coherent bundle at the field centre. The North-East orientation of plate is fixed at the time the holes are manufactured. In order to adjust the rotator, then, the following steps are required:

• Place the coherent bundle in the centre of the aperture plate.

• Because of the orientation of the Auxiliary port with respect to the ISIS slit, the nominal sky position angle of the Cassegrain rotator to align plate and sky axes is 315 degrees, and this the Sky Position angle should be set to this as an initial estimate.

• Acquire a bright star with the telescope and place it on the coherent bundle. Determine the rotator centre, which should be on the coherent bundle. It may be easiest to determine the rotator centre on the direct TV with the ACQCOMP mirror in place, then to place the star on the rotator centre, move the LARGEFEED mirror in and mark that point as the rotator centre. It is very important to determine the rotator centre accurately, as otherwise target acquisition will be extremely difficult.

• Calibrate the telescope pointing with the TCS procedure CALIBRATE. Again this can be done either on the coherent bundle, or with the direct viewing TV.

• Move the star to the centre of the aperture plate and define a telescope Aperture there.The coherent bundle centre and the plate centre do not coincide. The centre of the aperture plate (origin of the coordinates for the plate holes) should be X=71 microns and Y=559 microns (0.32 arcsec W and 2.52 arcsec North on the sky for a plate drilled with North at the top) from the centre of the coherent bundle.

• The telescope is moved South 400", so that the star drifts 400" North away from the field centre. In general, the pseudo-image of the star does not appear at once in the semi-coherent bundle placed there. This is due to the fact that the telescope's North and the plate's North do not coincide. The rotator angle must be adjusted until the star appears in the semi-coherent bundle. This way, a preliminary adjustment of the rotator angle is performed.

• If the field centre is not perfectly determined, the star will not appear perfectly centred in the semi-coherent bundle. In order to centre the star, the telescope must be moved in RA and Dec (or X and Y).

• After centering of the star in the semi-coherent bundle, the telescope is moved 400" North. The star appears in the coherent bundle. A new aperture (APE1) is created to define that point as the new field centre.

• The telescope is moved 400" North and the same process, as described above, is repeated for the other semi-coherent bundle.

All these steps can be repeated as many times as desired, in order to obtain a very precise adjustment of the rotator.

As a reference, if the plate is manufactured at sky position angle zero then the cassegrain rotator angle fibres are employed at the auxiliary focus is 31457'36", and typical offset values between APE0 and APE1 are X= 4.2 and Y= -4.2.