Tools for Automated Observing
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  System requirements
Getting Started
  Modeling slew times
  Obtaining slew time
  Modeling slew time
  Preparing slew time
  Measuring camera
  download times
  Specifying filter
  names and numbers
  Modeling the local
  Creating user profiles
  Initializing target
  Customizing the
Daily Operation
  Starting observatory
  control software
  Updating target
  Generating a list of
  potential targets
  Preparing a list of
  observation requests
  Running the
  Starting scheduled
Image Acquisition with
the MU Script
  Customizing the
  Starting MU
  Sequence of events
  during an observing
  run using MU
Timing Refinement
  Collecting timing
  Analyzing timing
  Adjusting the
  empirical timing
Other Tools
  Slew time
  measurement script
  Minor Planet Checker
  query script
  Regression program
  Software updates
  License agreement
  Contact the author
Getting started

Step 2: Modeling slew times

In order to accurately simulate the timing of telescope motions, the TAO scheduler needs information on the time it takes for the telescope mount to perform a slew between two arbitrary points in the sky. This information is provided through a mount slew time model, which is a set of equations which express an empirical relationship between slew time and displacements in right ascension and declination. If your telescope is enclosed in an automated dome, an analogous dome slew time model (an empirical relationship between dome slew time and dome displacement in azimuth) is also required.

The scheduler assumes that slew times are reproducible, that is, that repeated slews between the same two sky positions (or between the same two dome azimuths) will take essentially the same times. It also assumes that the telescope slew times in right ascension and declination depend only on the angular displacements of the right ascension and declination motors, respectively. This means that two 30° slews in declination, say, should take the same time, even though one starts at declination = -10°, hour angle = +1 h, and the other starts at declination = +30°, hour angle = -4 h. Similar assumptions are made about the dome slew behavior. Such assumptions are satisfied for many automated telescopes. If your telescope does not satisfy these assumptions, the timing of scheduled observing runs will be less accurate, but it may still be acceptable.

Obtaining telescope slew time data

  • ACP users should use the SlewTime script to produce a data file containing measured telescope slew times in right ascension and declination for a wide range of angular displacements in both telescope axes. The output of the SlewTime script will be a text file similar to the one below:
      dRA    Slew Time
     (deg)     (sec)
      0.0537    2.0
      0.1074    2.0
      0.2148    3.0
      0.4296    5.0
       ...      ...
     90.0000   46.0
     dDec    Slew Time
     (deg)     (sec)
      0.0012    2.0
      0.0024    3.0
      0.0036    3.0
      0.0072    4.0
       ...      ...
    128.0000   37.0

    In the above table, dRA and dDec are the angular displacements in right ascension and declination, respectively. It is a good idea to run the SlewTime script several times to verify that the slew times are reproducible, and to obtain a larger sample of measurements for the modeling of slew times.

  • Orchestrate users might set up a simple Orchestrate script which slews the telescope by widely different distances, first only in right ascension and then only in declination:

    SlewToRADec , 18.00000h +00.00000d ,
    SlewToRADec , 18.00500h +00.00000d ,
    SlewToRADec , 18.01500h +00.00000d ,
    SlewToRADec , 18.03500h +00.00000d ,
    SlewToRADec , 22.00000h +00.00000d ,
    SlewToRADec , 18.00000h +00.05000d ,
    SlewToRADec , 18.00000h +00.15000d ,
    SlewToRADec , 18.00000h +00.35000d ,
    SlewToRADec , 18.00000h +89.00000d ,

    This script should be executed in "step" mode (that is, one command at a time), and the time spent on each slew could be measured with a stopwatch. The results of these measurements could be tabulated in a form similar to the output of the SlewTime script (see above).


  1. If the telescope is enclosed in an automated dome, dome motion should in principle be disabled during the telescope slew time measurements. This is important for some telescope control software which will only report that a telescope slew is over when both the dome and the telescope have finished their respective slews. If the dome moves during the telescope slew time measurements, the measured telescope slew times will be longer than the actual telescope slew times for those slews in which the dome takes longer than the telescope to complete its motion.

  2. The algorithms currently implemented in TAO are able to accurately compute slew times for a variety of equatorial mounts, both of fork and German types (support for altazimuth mounts will be added soon). However, certain mounts and telescope control software may violate some of the assumptions made in these algorithms, leading to inaccurate timing of telescope motions even though the relationship between telescope slew time and angular displacement in each axis is well determined. If you encounter such a problem, it may be necessary to adapt the slew time computation algorithm in TAO to take into account certain peculiarities of your mount's geometry and behavior. The same remarks apply to automated domes. The author will be happy to assist you in dealing with these problems.

Obtaining dome slew time data

A script analogous to SlewTime is not yet available to automatically measure dome slew times in azimuth. To measure dome slew times, one could use the dome control software to order slews in azimuth by widely different angles (from near zero to 180 deg), and measure the slew times with a stopwatch. The resulting measurements could be tabulated as follows:

  dAz    Slew Time
 (deg)     (sec)
   5.       2.0
  10.       4.2
  20.       7.9
  30.      12.5
  45.      18.0
  60.      25.3
  80.      34.7
 100.      42.2
 125.      53.6
 155.      66.6
 180.      80.2

Note: The algorithms currently implemented in TAO assume that the telescope housed in the dome has an equatorial mount, either of fork or German type (support for altazimuth mounts will be added soon). Some other assumptions are also made, which may not be valid for all domes and mounts:

  1. When the dome slews between two different azimuths, it moves through the shortest angle between them.
  2. When a slew begins, telescope and dome start to move simultaneously; if the telescope finishes the slew before the dome, the telescope control system waits until the dome has finished its motion.
  3. When the dome is not slewing, the projection of the telescope optical axis intercepts the center line of the dome's shutter.
  4. The polar axis of the telescope mount (or its projection) intercepts the dome's rotation axis (in other words, the polar axis lies in a vertical north-south plane which contains the dome's rotation axis).
If some of these assumptions is not valid for your telescope/dome combination, it may be necessary to modify the dome slew time computation algorithm in TAO to adapt it to your equipment. The author will be happy to assist you in dealing with these problems.

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© 1999-2004 Paulo Holvorcem