6.2 Acquisition

6.2.3 Positioning NTFS on the T/FS (LGS Facility)

Step 1. Sequence 6.2.1, "NGS acquisition (NGS Facility)" and sequence 6.2.2, "LGS acquisition (LGS Facility)", should have already been performed.

Step 2. Once all acquisition settings are satisfactory, click on the APPLY button at the top of the acquisition GUI screen. This is the event that will trigger the OBS to position all devices appropriately. Given the information gathered on the acquisition screen, check the state of each of the following devices, and re-position as necessary:

• WFS Field Selection Mirrors

The correct position is a function of guide star coordinates on the acquisition camera.

• T/FS XYZ stage

The correct position is based on the calibration between T/FS position and SI focus.

• Image Rotator

The correct position is a function of azimuth, elevation, and desired orientation of the SO on the SI.

• IR ADC

The correct position is a function of azimuth, elevation and rotator angle.

• Visible ADC

The correct position is a function of azimuth, elevation and rotator angle.

Clicking on the APPLY button also signals the OBS to remove the APOM from the beam.

Step 3. Bring up the "Verify T/FS Ready" GUI screen. It is accessible from the "Loop Control" screen.

Step 4. Verify that light is falling on the T/FS by looking at the intensity strip chart derived from 9.2.1.7.3, "Subaperture intensity Vector" on the "Verify T/FS Ready" screen. If the graph indicates no significant light on the sensor, then go to the appropriate engineering support GUI screen.

Step 5. If necessary, depending on the total intensity, the WFC may automatically insert a neutral density filter in front of the T/FS.

Step 6. Manually or automatically move the T/FS XY stage to a position such that the intensity of the four quadrants is balanced. We expect that the GUI will show numeric and graphical centroid feedback during this process.

TBD, We expect to allow the T/FS to move in the Z direction also to compensate for field curvature (focus variations across the field). The Z position would be a function of XY and field curvature.

Step 7. Look at the Focus error strip chart provided on "Verify T/FS Ready" screen. If the time averaged trend is above a threshold, (based on the DM stroke available to correct focus) then the user would click on the "AUTO FOCUS" button. In this case auto focusing would require:

• The telescope secondary should piston to focus the telescope, and thereby minimize the focus error sensed by the T/FS.

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