Verify Fabry-Perot operation

 Next you should take an exposure with the calibration lamp and filter, in order to verify the operation of the Fabry-Perot system. Due to the faintness of the calibration lamp, it is best to turn off any ambient lights in the dome at this time. At the 60-inch telescope, make sure that the telescope cover has been removed, and that the telescope has been tipped over to face a blank part of the dome. At the 200-inch telescope, you will need to open the mirror cover at this time. Check with the Palomar crew to make sure this is acceptable, and to verify that the calibration lamp has been installed at prime focus. The 200-inch telescope should be left pointing at the zenith.

First, the Fabry-Perot software must be initialized. The easiest way to do this is via the fpsetup script:

This script initializes the serial connection between the control computer and the etalon controller, and also defines a number of useful logical aliases. Although the script will produce a small number of errors related to VMS privileges, the result should be the computer assuming control of the CS-100 etalon controller. This is indicated by the illumination of the disabled lamp on the front of the controller (see Figure 11). When the controller is in this state, the front panel controls, such as the operate/balance switch, are disabled.

To check that the computer can now control the etalon, set the etalon spacing to the fiducial value of 100:

Any response to the fp command other than ``operating normally'' indicates a problem with the connection between the computer and the controller. In such event, verify the serial connection all the way from the computer to the controller. Also make sure that the correct serial port is being used (TTA3: at the 60-inch telescope; TXA7: at the 200-inch telescope). John Henning may be helpful in diagnosing problems with the serial connection.

Once the computer is controlling the etalon, place the calibration filter in the optical path, turn on the calibration lamp, and take a 15-second exposure:

The resulting image should contain a number of illuminated rings. The brightness of the rings depends upon the precise manner in which the calibration lamp illuminates the dome and the choice of calibration lamp. If the rings are too faint, you may need to try a longer exposure. Typically, the Neon lamp requires a 15-second exposure, while the Hydrogen lamp requires 30-60 seconds of exposure time.

The number of rings visible depends upon the free spectral range (FSR) of the etalon in use; the Palomar ET-50 etalon will exhibit approximately 14 rings across the frame. Assuming the etalon wedge is being used, the rings should be centered on the edge of the frame.