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Optical alignment of etalons

The final process in setting up an etalon is the optical alignment of the etalon plates (i.e. setting the plates parallel). This should be carried out when the etalon is first loaded, and then checked during the observing run, preferably every day.

Note that before the etalon can be accurately aligned, the etalon compartment should have been flushed with dry nitrogen for at least an hour.

The etalon is aligned by moving it into the light-table position, illuminating it with an arc lamp mounted below the light-table, and viewing the Fabry-Perot ring pattern from above. The access hatches should not be opened during this process, as this will disturb the controlled environment of the etalon compartment. Instead, there are two slides above and below the light table. The slide below the light table covers a diffusing screen. Remove this slide, and bolt the calibration lamp unit to the underside of the light table. Then remove the slide above the light table, which covers a viewport. Note that in order to move the top slide, it will be necessary to use an electric screwdriver (or similar) to depress a catch in the top left hand corner of the slide.

If you switch the calibration lamp on, and the etalon is servoing correctly, you should now be able to see the Fabry-Perot ring pattern through the top viewport. As you move your head from side to side across the etalon, the ring pattern will appear to contract or expand if the etalon plates are not parallel. The basic aim of the alignment process is to minimise this variation in ring radius, by adjusting the offsets applied to the CS100 servo channels. The CS100 should be set to local control during the alignment process, as for the initial servoing of the etalon.

First move your head from side to side along the X axis of the etalon, and adjust the COARSE X PARALLELISM control on the CS100 to minimise the expansion or contraction of the rings as the head is moved. Then move along the Y axis, and adjust the COARSE Y PARALLELISM. The orientation of the X and Y axes is marked on the top of the light-table. The FINE Z control should now be used to make a ring just appear in the centre of the field and the alignment process should be repeated observing this ring and adjusting the X and Y FINE PARALLELISM controls. This small central ring is very sensitive to departures from parallelism, so provides an accurate indication of alignment. Some

Thu Apr 7 00:29:52 BST 1994