Driving the 40m telescope

Stowing the telescope

The ability to stow the telescope is built into the drive-system hardware. This allows one to stow the telescope via a physical switch in the telescope teepee, even if the control computer isn't working. The telescope can also be sent to the stow position, by entering the stow command into the telviewer command window. This tells the computer to hand control over to the hardware that actually stows the telescope.

When using the stow command to stow the telescope, the operator should first suspend the currently running schedule, if any. If this isn't done, then the current schedule is likely to immediately cancel the stow and resume observing.

Sending the telescope to the service position

Beware that the service position is at the hard lower elevation limit. At this position, the drive hardware does not allow computer servo control of the elevation drive. So if you attempt to enable the drive servos and slew the telescope away from this elevation, then the result will be an underspeed error, which will disable computer control of the telescope drives, and require an explicit reset of the drive servos, once the telescope has been moved away from the limit. So to move away from the service position, it is best to first initiate a stow, which is done under hardware control, then wait until this moves the telescope above the 7 degree software elevation limit, before attempting to enable the drive servos.

Preparing the telescope for observing

Manually enabling the drive servos

1. If you enter observing motion commands such as the slew and track commands, then these commands usually automatically enable the drives for you. However this is not done if a previous drive error disabled the drive servos. In such cases the drive servos must be enabled explicitly, using the axes command. This precaution is to prevent a schedule that doesn't notice that the drive servos went offline, from continuously entering motion commands that attempt to re-enable the drive servos, since this could damage the telescope if the drive error is serious.
2. The axes all command attempts to enable the drive servos, and then restart whatever motion command was running when the drive servos were previously disabled. After a drive error, this command is the only way to force the control system to try to re-enable the drive servos.

Automatically enabling the drive servos

The correct way to enable the drives automatically in schedules, is to invoke the enable_drives command, which is a script command defined in schedlib.sch. This does the following:

1. Wait until the $drives_available() function indicates that the drives are online and available for observing.
2. Next, if the $drives_enabled() function indicates that the computer drive servos are not currently enabled, then the following is done to enable them.
   • A halt command is issued, to ensure that when the axes are enabled, the telescope is left halted.
   • The axes all command is used to enable the computer drive servos.
   • The completion of the axes command is awaited.
   • If a drive error caused the axes command to fail, then an error message is output to the log, and the enable_drives command waits indefinitely until the operator manually enables the drive servos. It does this by watching for the $drives_enabled() function to indicate that the computer drive servos have been successfully enabled.
3. Control is returned to the caller of enable_drives, indicating that the drives are now ready for observing.

Handling observing interruptions

The following is an example of an observing schedule that handles interruptions.

  # Assign true to the following variable once the observations are completed.

  Boolean finished = false

  # Attempt to perform the observations at least once, repeating the attempt
  # if the telescope gets wind-stowed etc, before it has been finished.

  while(!$finished) {

    # Forget any previous reports of the schedule being suspended.

    signal/init suspended

    # Enable the computer drive-servos as soon as this is possible.

    enable_drives

    # Reset the receiver configuration.

    setup_rx

    # Stop observing if drive control is lost, or the schedule is suspended.

    catch !$drives_enabled() | $signaled(suspended) {

      ...perform the observations...

      finished = true
    }

    # Rate-limit the retry loop.

    until $elapsed > 0:0:1
  }