Non-sidereal tracking (guiding)

The script nst.py will, after a guider is activated, repeatedly offset the guiding fiducial to compensate for the non-sidereal motion of a source on sky.

Note that this method allows non-sidereal sources to be tracked as long as they do not move more than ~10" during the observation (due to the limited field of view of the guider cameras). The FoV is ~20" but we assume you start from a well-centered guide star. For HPFACAM setups the actual range available may be even smaller due to the required fiducial offsets in the setup. Consult your local TO for more details/insight on optimizing this procedure.

To make this work, do the following:

1. Download the ephemerides for the time period you plan to observe the target: ​https://ssd.jpl.nasa.gov/horizons/app.html#/
   1. Ephemeris Type: Observer Table
   2. Target Body: [pick your target, like Titan or Europa or whatever]
   3. Observer Location: McDonald Observatory, Fort Davis [code: 711]
   4. Time specification: Start=today's UT date, Stop=tomorrow's UT date, Step=1 (minutes)
   5. Table Settings: Need the Astrometric RA & Dec, other quantities optional.

2. Simplify the complicated ephemeris file by removing the header/footer and all columns except date/time and RA/Dec. You should have a file like this:

 2023-Dec-29 00:00   02 13 50.76 +12 10 15.9
 2023-Dec-29 00:01   02 13 50.76 +12 10 15.9
 2023-Dec-29 00:02   02 13 50.76 +12 10 15.9
 2023-Dec-29 00:03   02 13 50.76 +12 10 15.9
 2023-Dec-29 00:04   02 13 50.75 +12 10 15.9
...

3. Use these coordinates to also fill in a TSL file with the target and its coordinates at whatever time you plan to observe it.

4. Assuming you are observing a bright target, a DirectACAM (or HPFACAM) setup will probably be possible, and that is the best option. Do that.

5. During setup, before a guider is activated, run the python script with the simplified data file as a positional argument:

   nst.py ganymede_20231229_simple.txt
   

By default it will make fiducial updates once per minute. You can change that with the command-line argument --step which accepts decimal values in minutes (0.25, 0.5, 2, whatever you want).

6. When the TO activates a guider, the script will report that it has identified which guider is active and will begin making automatic fiducial updates to that guider's fiducial until you stop it.

7. To stop the script, touch a file called "stop" in the same directory/location where you are running the script.

This webpage contains more details about this script and some of the testing we did with it: ​https://www.het.astronomy.utexas.edu/HET/hetweb/nonsid/nonsidereal_guiding.html