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Mechanical alignment of etalons

The orientation of the etalon can be altered by adjusting the 3 Allen screws which locate the etalon in its kinematic mount. You will first need to gain access to the etalon from above, by moving both it and the large hole of the pupil-plane filter wheel into the light table. Then follow the following procedure to change the etalon orientation:

  1. Loosen the bolts holding the etalon to the etalon wheel, and slacken off the lock-nut on one of the adjusting screws.
  2. Use an Allen key to turn the adjusting screw by a well-defined amount (e.g. a quarter of a turn)
  3. Tighten the lock nut and bolts

The reason for changing the mechanical alignment of the etalon is normally to deal with the problem of ghost images (section 19). The aim is usually either to throw the ghost image well away from the real image, or to align it with the real image. Which approach to take depends on circumstances. The former approach is much easier, but may cause problems when the object being observed has emission over the whole field. In general, the latter approach is to be preferred, except perhaps when the scientific aims of the project require the detection of faint structure close to a bright point source.

It is not easy to tilt the etalon by a precisely repeatable amount, thus the process of aligning the etalon is rather iterative. You will have to take a series of images of the pinhole mask, illuminating it with the tungsten calibration lamp, with the etalon in the beam. Each of the pinhole images will have an associated ghost image, offset from the real image by the same amount. The procedure is then to remove the etalon from the beam, adjust one of the alignment screws as described above, return the etalon to the beam and take another image. Changing the tilt of the etalon should shift the ghost images with respect to the real images. It normally takes a few iterations to establish how much of an adjustment, of which alignmnent screws, should in principle produce the required shift. Then a few more iterations to get it right.

Thu Apr 7 00:29:52 BST 1994