SKYMAP 3. Plotting Objects on the Sky

Stars and Other Point Sources

skymap can plot sources from two separate catalogs, in addition to the Guide Star Catalog, on a map. The SAO Catalog, the Yale Bright Star Catalog, the IRAS Point Source Catalog, and the Position and Proper Motion Catalog (PPM) are available in a rapid-plotting binary format. Catalogs also may be plotted from the simple ASCII format,
catalog name
one-line catalog description
number RA(hh.mmsssss)  Dec(dd.mmssss) magnitude name
A program, starcat, exists to translate files in this format into thebinary format for faster plotting in skymap and faster searching using star. These files are then machine-, or at least byte-order-dependent, and contain the right ascension and declination in radians as 8-byte floating point numbers, magnitude with two decimal places, and optionally, the source number.

Magnitude and flux limits may be set, and the size of the plotted source scaled by its flux. Foreground objects may be labelled and plotted as open crosses or half-cross as well as circles. The star program may be used to select interesting objects or print listings of positions and flux. Sources may be identified using the cursor. A table of source size vs. magnitude may be plotted in an arbitrary location on the display. Sources may be labelled with their number, or if plotting from an ASCII file, their name. The location of this label is specified in display (Tektronix) coordinates, where coordinates less than 100 are used as offsets from the plotted source position. The font types and sizes of the star labels may also be specified. An optional log file may be kept which lists all of the stars plotted to the current region.

The HST Guide Star Catalog

If the Hubble Space Telescope Guide Star Catalog CD-ROM set is mounted on the computer as a Unix file system, skymap can plot sources directly from the CD-ROMs. If only one CD-ROM drive is available, only the hemisphere of interest need be mounted. As with other catalogs, magnitude limits may be set. In addition, objects may be selected by whether or not they have been flagged as non-stellar. Guide Stars may be plotted in the same way as the other catalogs. If the stars are labelled, the number of the region is omitted. An optional log file listing all Guide Stars with region and source number may be kept.


Positions of many solar system objects, including the sun, the moon, the other eight planets are read from auxiliary binary files. These files are defived from the current JPL DE-130 ephemeris and are the most accurate available. There is one file per year, so no more space is used than is neccessary, and they reside in the directory whose pathname is specified by the parameter EPHDIR.Planets may be plotted for any time between 1950 and 1999. Pluto's position is made more accurate by the inclusion of software to compute the position of its companion Charon. More than one object can be displayed, and variation in position over time can be plotted as a series of points or a line with tick marks. Planet characteristics are tabulated in a file called planet.db. Rings may be plotted by selecting the appropriate data base entry. Additional objects may be added to the planet database if ephemeris files in a simple ASCII format
Julian date right_ascension declination distance_from_Earth
exist. In this file, is in fractional hours, is in fractional degrees, and is in Astronomical Units. Planet positions and distances for any object in the planet database (see Appendix F) may be printed using the program planet.

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