Other Solar System Objects

For minor bodies ATCASCHED does not have a built-in ephemeris. However, in absolute time mode, ATCASCHED treats source names beginning with the @ symbol (e.g., a source name of @hbopp) as pointing to an external ephemeris file (eg a file $ATCA_EPHEM:halley.eph). You must generate needed ephemeris files before running ATCASCHED. This is readily done using JPL's Horizons on-line system. This contains ephemerides for more than 15000 asteroids, comets, natural satellites and several dynamical points.

Horizons is available on the World Wide Web at the address:

http://ssd.jpl.nasa.gov/?horizons

Only the telnet (terminal-based) access method is described here, there is also a web-interface method available from the web-page above which is self-explanatory. Horizons asks the user a series of questions. To generate an ephemeris file for ATCA observations,

• First select an object by giving the source name (followed by a semi-colon for minor bodies) or by source number.
• In response to prompts, select Ephemeris, then Observe, then Geo to select geocentric RA/DEC ephemerides.
• Then give the start time of interest, in UTC, in the format suggested by the prompt. On the same line (after the time) add UTC. For example,

  1997-May-07 00:00 UTC

  Then give the end time, and then the time increment (typically 1 hour is more than adequate - ATCASCHED uses simple linear interpolation of ephemeris values). The ephemeris you produce must start at least two time increments before your observation, and go to beyond your end time.

• Accepting default output, and select ``table quantities'' 1,20 (which gives RA, DEC, distance and velocity of the object).
• Horizons will then list the ephemeris and then either e-mail it to you or allow you FTP access to it (you could also capture the screen output). You need to retrieve the ephemeris using one of these ways.

The ephemeris file produced by Horizons needs to be massaged into the format that ATCASCHED requires. The resulting file should be placed in the $ATCA_EPHEM directory (the place expected by ATCASCHED).