User Tools

Site Tools


Telescope, etc. information

How To Track Solar System (Or Otherwise Moving) Objects

Steps 1 and 2 are advance preparation before your observing run.

1) Get raw ephemerides from JPL Horizons

These are more accurate than our in-house ephemeris program can provide. Go to the Horizons page (, Choose web-interface ( and get these data for each target of interest: timestamp, astrometric RA and Dec (airless, in decimal degrees, J2000.0), RA and DEC rates. You can get additional info if you want, but those are the ones that we really need.

Example settings for JPL ephemeris

Ephemeris Type [change] : OBSERVER

Target Body [change] : Saturn [699]

Observer Location [change] : Apache Point [705] ( 254°10'45.9E, 32°46'49.8N, 2891.2 m )

Time Span [change] : Start=2008-04-24, Stop=2008-04-25, Step=1 h

Table Settings [change] : QUANTITIES=1,3; time digits=SECONDS; angle format=DEG; refraction model=REFRACTED; skip daylight=YES; extra precision=YES

Display/Output [change] : default (formatted HTML)

Then Generate Ephemeris, which outputs something like:

Date(UT)HR:MN:SS R.A._(J2000.0)_DEC. dRA*cosD d(DEC)/dt


….. Daylight Cut-off Requested …..<

2008-Apr-24 02:00:00 C 154.3057447 12.5867617 -2.52 0.61

2008-Apr-24 03:00:00 A 154.3050264 12.5869290 -2.52 0.60

2008-Apr-24 04:00:00 154.3043103 12.5870917 -2.51 0.58

….. Daylight Cut-off Requested …..<


2) Massage ephemerides into the format you want

Write a script to convert the ephemeris numbers into the correct command syntax. The BIG, IMPORTANT difference is that JPL gives dRA and dDec in arcsec/hour, whereas our software wants deg/sec. That means YOU HAVE TO DIVIDE THE SPEED BY 12960000 or you will get no data and track the telescope into the ground. For objects other than the Moon, this step is also where you would substitute Jupiter's velocity in place of the moon's velocity. The timestamps aren't needed in the command, but it's useful to make a file with multiple commands, each one marked by a timestamp so as to pick a command depending on what time one actually issues the slew command. Here's an example slew command:

tcc track 136.72666, 17.43467, -0.00000146, 0.00000046 Fk5=2000.0 /Rotangle=108.5 /Rottype=Object

        ^           ^          ^            ^

where the first four numbers are: RA(decimal degrees), Dec (decimal degrees), dRA (degrees/sec), dDec (degrees/sec), and rotation can be specified with the /Rotation=rotation, where rotation should be calculated as 90 - PA, where PA is measured N thru E.

Here are a couple of example lines: 12:00: tcc track 136.72666, 17.43467, -0.00000146, 0.00000046 Fk5=2000.0 /Rotangle=0.0 /Rottype=Object

12:15 tcc track 136.72478, 17.43524, -0.00000146, 0.00000046 Fk5=2000.0 /Rotangle=0.0 /Rottype=Object

From the JPL ephemeris (example above) this line would convert to:

2008-Apr-24 04:00:00 154.3043103 12.5870917 -2.51 0.58 tcc track 154.3043103, 12.5870917, -0.000000194, 0.000000047 Fk5=2000.0 /Rotangle=0.0 /Rottype=Object

3) Slew to the planet with desired rate and rotation

Notify the observing specialist that you are about to issue a non-sidereal tracking rate to the telescope and get their permission to do so before doing the following. If the slew rate is wrong it can cause the telescope to slew too fast, and the observing specialist can stop the telescope before it goes out of control. The telescope does not check your numbers. When in doubt, ask the observing specialist to double check your numbers and they can issue the tcc track command for you as well [].

Paste your pre-prepared commands into the TUI Log window. When in doubt about anything (if your offsets get tangled up, for example), reslew. This is the motivation for making all those pre-prepared commands. They WILL come in useful.

4) Take images often to see what's going on

This is just generally good policy. It should be safe to run the guider even if you have a non-stellar tracking rate.

Last updated 12-20-2007 JMD

Some useful observing object catalogs

Exposure time calculators

Rough IDL exposure time calculator

Other exposure time calculators for the 3.5m?

Useful Observing Resources

3.5m.txt · Last modified: 2022/03/11 16:35 by holtz