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Field acquisition

The first step to do when performing fibre optic multi-object spectroscopy is to select a field and to put the observing fibres on the aperture plate. To insert the fibres in the corresponding holes is an easy task but it must be performed with caution in order to avoid damaging the fibres ends. Also, it must be taken note of to which star in the field each fibre corresponds. Although it is not strictly needed it can be useful as well to order the stars by magnitudes, in such a way that --to avoid contamination-- two adjacent spectra do not produce large luminosity differences onto the detector. 10 to 15 minutes are required in order to insert 61 fibres in the plate.

Briefly, the field acquisition consists of pointing of the telescope towards the coordinates of the centre of the field to be observed. It is then expected that the 3 guide stars will appear correctly centred on the TV screen. Step by step this process is as follows:

• Point the telescope towards the RA and Dec of the field centre.

• Perform an aperture offset to aperture APE1.

• If the rotator is well centred and oriented, the image of the guide stars for the coherent bundle and two patterns for the two semi-coherent ones, will be seen on the guiding monitor.

• In order to correct the telescope pointing, the telescope must be moved in RA and Dec or X and Y until the semi-coherent bundles illumination patterns are well centred. When the seeing is good ( 1.5"), this centering is easy to obtain as the 7 central fibres of each bundle must appear uniformly illuminated. If the seeing is between 1.5 and 4", the centering can be performed by means of the uniform illumination of the external ring in each bundle. If the seeing is worse, the centering becomes more difficult although basically it still relies on an uniform illumination of the fibres in the semi-coherent bundles.

• If an optimal adjustment has not been obtained for both semi-coherent bundles, the telescope rotator must be moved until a better centering of the three guide stars is achieved.

• Finally, when the field has been properly acquired, guiding can begin on the TV image of the star on the coherent bundle with the GUIDE or IGUIDE functions of the TV system, and Autoguiding can be started with the TCS command TVGUIDE ON. The first time this is done it is necessary to verify that the X and Y handset functions of the TCS drive the star along the correct X and Y axes of the TV image. If they do not, then some adjustment will have to be made. Field reversals can be corrected with the Scan Reverse switches on the Westinghouse control panel on the control desk; and the angle of the coherent fibre bundle can be rotated at the TV end. It is only necessary to set this angle to an accuracy of about 5 degrees.