Install etalon

 In order to install the etalon in the instrument, first remove the dewar plate and the camera lens plate from the end of the instrument tube. The two plates are attached to the etalon/collimator section by the same set of six screws and are not attached to each other. First lift off the dewar plate, then the camera lens plate. As you lift off the camera lens plate, you may have to unplug the shutter, although the instrument is rarely stored with the shutter installed. (As mentioned above, the internal shutter is only used with the Fabry-Perot on the Palomar 200-inch telescope. When the instrument is used on the 60-inch telescope, the ``big shutter'' in the telescope base is normally used instead.) Set the dewar and camera plates aside.

The etalon fastens to a flat plate, which in turn is usually attached to a wedge within the Fabry-Perot. The wedge places the etalon at a small angle to the optical axis. The result is that the optical axis itself is imaged at the edge of the CCD detector, thereby eliminating ghost images from internal reflections within the optical system. It is a good idea at this point to check to be sure that the etalon wedge has been installed. The wedge is very rarely removed from the instrument, so it should not need to be dealt with. If it does, however, you will need to remove the etalon/collimator section to reach the five screws which attach the wedge from underneath. Note that one screw in that set is shorter than the rest; it should be used on the thin side of the wedge.

The etalon should be fastened to the etalon mounting plate using the three metric hex screws and wrench that are provided in the etalon box. There are actually two different etalon mounting plates: one each for the ET-50 and ET-70 etalons. The latter plate has a slight recess for the etalon on one side. In the case of the Palomar ET-50 etalon, you will use the recessed mounting screw holes to attach the etalon to the opposite side of the plate. An ET-70 etalon uses the three silver slot screws to attach to the other plate.

Before attaching the etalon to the mounting plate, you must be sure to check the orientation. It is important that the etalon be oriented such that the etalon connectors point out of the cable boot hole in the side of the instrument. There are a pair of guide pins on the etalon wedge that constrain the rotation of the etalon mounting plate to one of two orientations. The etalon must be mounted on the plate such that the plate can then be fastened to the etalon wedge in an orientation where the etalon connectors are accessible through the cable boot hole. You will have to think about the geometry to get it right. Note that the etalon is a symmetric device; it does not matter which face of the etalon is mounted to the plate.

To mount the etalon, first place it on the table. Remove the plastic cover on the upper side of the etalon and blow off the exposed surface with dry nitrogen. Align the appropriate mounting plate properly on top of the etalon and attach it with the appropriate screws: either the silver metric hex screws for an ET-50 etalon, or the silver slot screws for an ET-70 etalon. In either case, the screws should be found in the etalon box, and may, in fact, be used to attach one of the protective covers to the etalon. The metric wrench in the ET-50 etalon box should be used and then returned to the box.

Leaving the other cover on the etalon for protection, next place the etalon and mounting plate inside the instrument, on the wedge. Use the guide pins to ensure proper positioning of the etalon plate. Confirm that the etalon connectors point out the cable boot hole. The etalon mounting plate is attached to the top of the wedge by five screws. One screw is slightly shorter and should be used on the thin side of the wedge. This screw is marked with a dab of white paint.

Please be careful to not lose the metric screws or wrench that are kept in the etalon box. The screws must be specially ordered if they are lost. (Postscript: As of March, 2001, the metric screws and wrench are missing from the etalon box. In their place are a set of three silver phillips-head screws which may be used for attaching the etalon to the etalon plate.)