4.4 Locking the Servo System and Locating Optical ZPD

Once the laser has stabilized, the servo system can be locked. This is a two step process: using the appropriate buttons on the Error Signal/Servo Control Panel, first put the servo system in `ZPD' mode, and then enter `Fringe' mode

There are two ways to position the interferometer at optical ZPD:

Method A (using the source to be observed):

1. Open the FTS entrance aperture.

2. Select the filter you will be using with the filter selection wheel on the master control panel.

3. Position the star in the Hot Spot.

4. Run the ZPD task from the HP9000 workstation.

5. The instrument is now ready for observing.

Method B (using internal light source):

1. Turn on the white light source, and position mirror 2.

2. Select a filter (for instance, the He I filter) using the filter selection wheel on the master control panel.

3. Run the ZPD task from the HP9000 workstation.

4. After ZPD has been found, turn off the white light source and remove the injection mirror.

The instrument is now ready for observing.