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Observing coordinate determination.

The (X,Y) coordinates of the targets to be observed with multi-object spectroscopy, can be obtained in two ways:

• From the astrometry of the field. Transformation of the equatorial coordinates of the targets onto the corresponding aperture plate coordinates (X,Y), is performed with the LAPLATE programme, which is described in Appendix F and which is a modification of the APLATE programme of the STARLINK package, for the geographical coordinates of the ORM. LAPLATE takes into account the following parameters: telescope focal plane scale, thermal dilation coefficient of the aperture plate material, astronomical coordinates for the centre of the field, and possible proper motions of the targets. These are all considered by the programme in order to obtain the appropriate corrections of the X,Y coordinates.

• By taking photographic plates of the field in advance. The positions can be measured with a microdensitometer or other coordinate measuring machine.

Several error sources can contribute to the field acquisition, for example poor astrometry (the drilled holes for the stars do not coincide with the apparent positions) or bad centering of the field and rotator adjustment. Taking direct plates eliminates some sources of astrometric error, at the expense of requiring extra observing time.

Besides the coordinates of the targets, at least three other field stars must be chosen for telescope guiding. These stars must fulfil the following requirements:

• They must have magnitude 12 to 13 in V. Brighter stars are close to the Earth and can have large proper motions, so that it is in general difficult to obtain good astrometry of them. Less bright stars are difficult to observe.

• They must have similar brightness. If not, simultaneous observation on the monitor could be prevented as the brightest ones can saturate the camera while the weakest remain unseen.

• They must be as separated from each other as possible in the field, and, preferably along the East-West direction which is free of atmospheric refraction. This helps in obtaining a more precise field acquisition.

Finally, some positions free of stars must also be chosen, in order to perform sky corrections.