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Creating the input catalogue

In order to design masks which will allow the targets to be acquired accurately, you must provide good astrometric data for each potential target and for a number of fiducial stars that are required to help acquire the target field. Specifically, you must provide:

• Positions for each potential target object. These should be accurate relative to each other to a small fraction of the slit width. If the random positional errors are large then a large fraction of the useful light from the targets will be lost.

• Positions for fiducial stars which will be used to align the telescope with the target field. These should be accurate relative to the positions of the targets to a fraction of the slit width. At least 3 of these are required, preferably 4 or 5. There is an optimum brightness range for the fiducial stars: if they are too faint, they will be hard to see in the direct exposures required for target acquisition; if they are too bright, the acquisition algorithm will not be able to determine their positions securely and they may lead to charge bleeding during spectroscopic exposures which may obscure the spectra of the programme objects. Stars of between 15 and 17th magnitude in V work well.

• A position for a single, bright fiducial star used for locally recalibrating the telescope pointing model, the pointing recalibration star. The star must be bright enough to be easily seen by the telescope direct TV system, and it's position must be known accurately relative to the positions of the targets and other fiducial stars. This star should be as close as possible to the target field and, usually one of the fiducial stars within the field can be used.

The common coordinate system for these objects should be aligned with the celestial coordinate system to an accuracy of about 0.5 otherwise the rotation could be large enough to make the fiducial stars miss the holes in the mask. The absolute position of the field centre is not important since the pointing model will be locally reset but it should be good enough so that the pointing recalibration star can be easily identified with the direct TV system, i.e. arcsec.

Coordinates derived from CCD exposures and scans of photographs should be converted carefully to celestial coordinates using astrometric standards to avoid errors due to incorrect calibration of the pixel scale or field rotation. Coordinates derived from direct images taken with LDSS-2 can be used directly since the the same coordinate system is used throughout.

All these positions are entered into a catalogue file with a name of the form name.CAT. The file must also contain global information about the target field. Detailed instructions on the file format and examples are given in Appendix E. The most important global parameters are:

• Position of the field centre
• Equinox
• Epoch
• Position angle of the slits

• The ID of the object
• The celestial position (RA and Dec)
• A weight which, if positive, indicates the importance of the target for subsequent optimization of the mask or is zero for fiducial stars

The catalogue file can be read into LEXT using the command

READ/CAT name