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Pointing and Focus - Dynamic Corrections

Note: IARDS cannot be used to process focusSubreflector obervations when dynamic corrections are enabled. You must either use GFM, or disable dynamic corrections.

The Dynamic Corrections system provide real-time corrections to the local pointing and axial focus offsets, to compensate for thermal gradients in the antenna structure. The system appears to be working well, and we now encourage local staff scientists to start using it for observations at C-band and above.

Please contact RichardPrestage for a brief overview before you first try using the system.

In order to use Dynamic Corrections, you need to:

• start Monitoring the dynamic corrections.
• Enable use of the dynamic corrections via the Antenna Control CLEO screen.
• Optionally, monitor the values being applied by the Antenna Manager.
• Use GFM to process the Peak and Focus Offsets
• What to do when you are done.

Monitoring the Dynamic Corrections

Typical values of dynamic corrections are shown on the PointingFocusDynamicCorrectionsExamples page.

The dynamic corrections are calculated by the AntennaCharacterizationSpring2004 Manager every 10 seconds, even though use of these is disabled by default. Therefore you can monitor the current value and time history of the corrections regardless of whether or not use of the corrections is enabled. Monitoring while they are disabled allows you to see the proposed corrections you are missing. This might be appropriate for example during benign night-time conditions, if you prefer to leave the corrections disabled, but want to confirm that any proposed corrections would be small.

Since the dynamic corrections system is still rather new, you should always monitor the dynamic corrections periodically (at least once every half an hour) when they are enabled.

There are two mechanisms which you can use to monitor dynamic corrections:

CLEO:

• start CLEO chart-recorder widgets on the values being displayed in the Antenna Manager window (see below).

Use the DynamicCorrections application DCMON. For now, this must be run stand-alone, in the future it will be available via Astrid. As of 29th March, DCMON appears to suffer problems when left running for long periods of time; CLEO is recommended until this is fixed. To use the DynamicCorrections monitor:

• Invoke the DynamicCorrections monitor by typing dcmon at a shell prompt.
• a graphical display will appear, with three tab panels for focus, azimuth and elevation.
• DynamicCorrections will not plot until two samples have been received, so in the worst case it may take up to 20 seconds for the plots to start updating.
• The local time of the last correction, and it's value in mm (focus) or arcmin (az,el) is shown as the (x,y) value in the top left corner.
• The "S" and "P" buttons will save or print the plot, respectively. Experiment with the other buttons to determine their effect!
• to stop dcmon simply close the window.

The typical magnitudes of the dynamic corrections are a few mm to an extreme of ~30mm for focus, and a few tenths to an extreme of ~0.5 arcminutes for azimuth and elevation. The values should not change significantly on timescales of less than ten minutes, but fairly large gradients may occur over timescales as short as half an hour, especially during the day. Before enabling use of dynamic corrections you should ensure the values look sensible; while using dynamic corrections you should monitor their values to check for possibly spurious corrections.

Enabling dynamic corrections.

When the state of dynamic corrections is changed, a transient message will appear in the CLEO window informing you that this has happened, and suggesting that a new "peak" or "focus" should be performed.

Monitoring the values being used by the Antenna Manager

Performing "peak" and "focusSubreflector" observations while using dynamic corrections.

For "peak" observations, once dynamic corrections are enabled, operation is totally transparent (except that the resultant LPCs should be closer to zero, and change more slowly). As you change from not using to using dynamic corrections (or vice-versa) the residual LPC pointing error will change discontinously. So, you should always perform a new "peak" immediately after enabling or disabling corrections.

For "focus" observations, since the focus is measured relative to the subreflector absolute encoder values (albeit corrected by the focus tracking curve), the measured local focus offset always includes both the dynamic focus correction, and any residual error. Thus when dynamic corrections are enabled, the thermal component must be subtracted from the local focus correction. This subtraction is reported in the GFM message window.

When you are done...

-- RichardPrestage - 25 Feb 2004