Return true if the telescope drives are available to be enabled for observing.

Arguments:

(none)

Example:

  # If the drives are currently unavailable, wait for them to become available.

  if(!$drives_available) {
    print "Waiting for the telescope drives to become available"
    until $drives_available
  }

  # Enable the drive servos, and wait for the completion of this operation.

  axes all
  until $acquired(source)

  # Keep an eye out for the drives becoming disabled again, while observing.

  catch(!$drives_enabled) {
    ...observe sources...
  }

Context:

Whenever the telescope is wind-stowed, or the drive-power or computer-enable switches are turned off, or a drive error has occurred, the $drives_available() function returns false, to indicate that the drives are not available for observing. When the drives are again available, the $drives_available() function returns true. Schedules can call this function to determine when it is okay to attempt to re-enable the drive servos after an outage, and resume observations.

Note that the $drives_available() function doesn't indicate that the drives are currently usable for observing. That is the job of the $drives_enabled() function, which indicates when the drives are not only available, but when their servos are also enabled. So to detect when a drive outage occurs, schedules should use !$drives_enabled() in their catch statements, rather than !$drives_available().