Perform a fast azimuth scan of specified amplitude, for a specified time.

Mandatory arguments:

PointingOffset amplitude

The azimuthal amplitude of the scan. This is half the peak-to-peak width of the scan.

Optional Arguments:

Interval duration

The length of time over which to repeat the scan. This will be rounded up to the duration of an integral number of scan cycles.

Count repeat

If no duration is specified, then this argument specifies the number of times to repeat the body of the scan. If neither this argument, nor the duration argument are supplied, then the body of the scan is performed a single time.

Example:

The following command would perform a fast azimuth scan of amplitude 2.5 degrees (peak-to-peak width 5 degrees), with the body of the scan repeated until 8 and a half minutes had past.

  azscan 2.5, duration=8m:30s

The second example shows how, instead of specifying the amplitude in azimuth, one can use the $sky_to_az() function to specify the angular throw on the sky, instead of the azimuthal throw. The commanded azimuthal amplitude thus changes with elevation. The example commands a scan of 10 degrees on the sky, at the current elevation of jupiter, again for 8.5 minutes.

  azscan $sky_to_az($elevation(jupiter), 10), duration=8.5m

Context:

The azscan command, computes the fastest possible scan of a specified amplitude, within the constraints of the velocity and acceleration limits of the azimuth axis. The way that this is done is documented in the trajectories document.

When the azscan command is invoked, any existing motion is superseded, and the telescope is brought to a halt, before the scan starts. This initial halted position of the telescope is adopted to be the center of the scan.

Once the telescope has been halted, the azscan command slews the telescope a little way from the halted position, in the negative azimuth direction. It then accelerates from there, such as to pass through the center of the scan with the coasting velocity of the scan, in the positive azimuth direction. The passage through this point is the start of the first scan cycle. It continues to move in the positive direction, initially coasting and then decelerating, until it reaches the specified scan amplitude from the center of the scan, moving at zero velocity. It then accelerates in the reverse direction, passing through the center of the scan at the coasting velocity, in the negative azimuth direction, then first coasting, then decelerating, until it reaches the central position minus the specified amplitude of the scan, moving at zero velocity. The first cycle ends as it goes through the central position again, moving at the coasting velocity in the positive azimuth direction.

At the end of the first cycle, the azimuth either continues coasting into the second cycle, if there is a second cycle. Once the final cycle has been finished, instead of coasting into a new cycle, after passing through the central position in the positive direction, the azimuth is decelerated as quickly as possible, to bring the telescope to a halt. The telescope is thus left halted, some way away from where it was when the scan was started.

Scans of amplitude less than 22/27*Vmax*Vmax/Amax, don't have enough time for the limiting acceleration, Amax, to accelerate the azimuth to its velocity limit, Vmax, before they have to decelerate to turn around.

Scans of amplitudes greater than 22/27*Vmax*Vmax/Amax degrees, accelerate to the velocity limit, Vmax, then coast for an integral number of seconds, before starting to decelerate into the next scan turnaround.