WFS Focus Tracking
As the telescope HA changes, the distance to mesosphere where the Laser Guide Star (LGS) is created, changes. To maintain proper focus, the WFS must move to compensate for this distance change. This is done using the WFS focus tracking software, started with the command saoWFSFocusTrack.tcl from shanevnc.
Figure 1: WFS Focus Tracking GUI
When the GUI first starts the Current Airmass, Current WFS Position (in mm), and the Predicted WFS Position (assuming the LGS is at 90 km), are displayed and all other fields are blank. In NGS mode the WFS position should be 22.38mm (4475 motor encoder counts).
To start WFS focus tracking, push the WFSFocus Track checkbox at the top of the GUI. Once tracking is enabled, the program will calculate where the WFS focus stage should be based on the Current WFS position and move the WFS focus every two minutes to compensate for the changing airmass and distance to the LGS. If the WFS focus was at the NGS focus position, a warning message will appear saying that tracking is not allowed in NGS mode and what the predicted WFS focus stage position should be if switching to LGS mode.
Once WFS focus tracking is enabled, the following fields will be displayed: WFS Position Offset (mm), Desired WFS Position, Starting Airmass, and Starting WFS Position.
The WFS Position Offset is the difference between the starting WFS Position and the Predicted WFS position. Normally this number should be small (less than 0.1 mm), though could be larger if there is a problem with the equipment or the sodium layer is unusually high or low.
The Desired WFS Position is the place that the software is attempting to drive the WFS focus stage to, as determined by the Starting Airmass, Starting WFS Position, and WFS Position Offset. This number should be very close to the Current WFS Position if all is working properly (within 0.1 mm).
Starting Airmass and Starting WFS Position are the airmass and WFS focus stage position at the time that the WFSFocus Track checkbox was enabled. These are listed for reference in case there are problems that need troubleshooting.
Last modified: Wed Jul 6 08:57:31 PDT 2016