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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:
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:
The catalogue file can be read into LEXT using the command
READ/CAT namewhere name is the first part of the catalogue file name ( name.CAT). The representation of a catalogue file inside LEXT is a MASK list. Initially this contains the same information as in the catalogue file with additional default values for slit parameters supplied from the values of some of the LEXT internal parameters. These will probably be altered during the mask design phase. The command WRITE/CAT will output to a disk file the same information as you input (plus any subsequent changes) with extra information about the mask configuration.