coordinates -- description of coordinates for MX Spectrometer DESCRIPTION This section of the documentation describes the procedures and formats used with celestial coordinates for observing with MX. In order to record the spectra of 32 objects simultaneously, the MX observer must have accurate coordinates of these objects for the probes to point at. These coordinates are usually generated by measuring plates on the 2-D Grant machine at KPNO, or by digitizing plates on one of the automatic measuring machines. If matched correctly to the optical reference frame, radio coordinates from the VLA or assorted other coordinates could be used. The GASP package at STScI provides convenient coordinates from the Quick V Sky Survey plates. The coordinates should be good to at least 0.5 arcsecond rms relative to each other, and should be accurate in the absolute sense to a few arcseconds. The MX focal plane is adjusted for scale and rotation at the beginning of each run, and for translation at the beginning of each night, but no active alignment is currently attempted while observing a particular field. Separate files with field stars can be used to check centering and alignment on individual fields. COORDINATE OVERVIEW Before observing with the MX, it is necessary to generate a file which contains the celestial coordinates of the objects to be observed as well as the coordinates of some associated guide stars. This file is cleverly called the ``.MX'' file because it ends with the suffix ``.MX''. Target fields around particular guide stars selected as field centers are converted to Cartesian format by gnomic projection. The Cartesian coordinates are stored in the ``.XY'' file. Finally the selection of which probe will look at a particular target galaxy is carried out. The target selections and their X-Y positions in the focal plane are stored in the ``.TRG'' file. These ``target'' files are those actually used by the STARBASE-32 Z-80 when it reads the coordinates from floppy disk. Corrections for probe offsets, exact plate scale, and other instrumental corrections are performed in the ENTERPRISE Z-80 inside MX and need not worry the casual user. The ``normal'' user (We realize that no person working on astronomical spectroscopy is completely normal.) will generate an ``.MX'' file by whatever means he can devise. This ``.MX'' file will then be converted to ``.XY'' and ``.TRG'' files with the MXCOORD programs. Ambitious users are challenged to write clever target selection algorithms to generate their own ``.TRG'' files in the standard format. Of course, we take no responsibility for such actions. The MXCOORD program is able to read cardimages produced by the ASTRO, but the standard ASTRO output file does not have sufficient precision as of MAY 1986. The solution file must be used to obtain precise coordinates. The particular set of CCD frames corresponding to each distinct arrangement of the probes is referred to as a ``nucleus'' during data reduction. Each nucleus for multiobject observations has its own target file which is used to generate the file dimensional spectra. Nuclei are so named for historical reasons having to do with the Medusa aperture plates. ASTROMETRIC TECHNIQUES PROJECT reads the .MX file and precesses all of the celestial coordinates to the epoch of observation. The effects of aberration and nutation are also included. Coordinates are also corrected for differential refraction at the specified hour angle. Gnomic projection is used to project the coordinates onto the focal plane of the 2.3m telescope. Finally, the x-y coordinates are corrected to take the curvature of the focal plane into account. All corrections specific to the MX hardware are done by the Enterprise after target selection. SELECTION OF CENTER STARS The center stars which will be used for guiding during MX exposures may be selected interactively in PROJECT or they may be pre-selected and labelled with a ``C'' in the .MX files. If you specify automatic center selection, the program will look for the highest priority center stars in your .MX file. Otherwise, the program will prompt you for the numbers of the center stars you wish to use. An .XY file is created for each center star selected. Each .XY file contains the projected coordinates of all the objects within the field of view around the center star. If an .XY file for a particular star had previously been created, MXCOORD will prompt you for permission to overwrite. Multiple center stars should be used for cases where objects extend over more than one .MX field (45 arcmin) or where crowding might make some targets difficult to reach. When presented with multiple .XY files, MXCOORD will attempt to select the optimum center to use for creating .TRG files. Optimum is considered to be the maximum number of objects weighted by their assigned priorities, ie. one 60-point object is better than two 25-point objects. When using multiple .XY files in auto mode, the program chooses a set of targets from each .XY file. The set with the highest total priority is used for the .TRG file. After the first .TRG file has been written, the priorities of each of the observed objects are lowered in the other .XY files (in memory, the disk files are not altered). Then a new set of targets is chosen for each .XY file if additional .TRG files are desired. You may specify up to 30 center stars to choose from, but we recommend using only a few to speed up the selection process. TARGET SELECTION ALGORITHMS In order to select which objects will be assigned to each probe in observations of rich clusters, a complicated search procedure is needed. Each object-probe combination is assigned a priority based on the priority given by the observer and the distance of the object from the best position of the probe. The highest priority object is added to the target list for the specified probe. Probe priorities can be set in the probe priority file to discourage the program from using probes with bad fibers. This process continues until all probes have targets or the list of objects has been exhausted. After an object has been selected, it is removed from consideration by other probes. After each object has been added to the target list, the probe configuration is checked for possible collisions. If a collision possibility is found, that object is rejected, and the next highest priority object is selected. SEE ALSO mxcoord.mx_format, mxcoord.mx_spec, mxcoord.select, mxcoord.project, mxcoord.mxplot