Verify etalon parallelism

 As a final check on the instrument, verify that the rings have similar widths on all sides. If there is any noticeable azimuthal variation in the ring width, then something in the optical path is probably not seated correctly. Commonly this means that either the dewar is not properly seated in the flange, or the camera lens is not properly seated in its bayonet mount. If this appears to be the case, check the CCD dewar first, then remove the camera lens focus arm plate, the shutter control cable, and finally the dewar and camera lens plates, and check the seating of the camera lens. Follow the instructions given above when reassembling the instrument, being sure to keep the camera lens wide open and other optical components clear of the optical path.

You can also now verify the parallelism of the etalon plates. Poor parallelism causes a similar effect to a bad camera focus: rings appear wider than they should be. However, bad parallelism causes the rings to be degraded proportionally to their widths, while a bad focus degrades all rings to the same extent. It is therefore easier to spot parallelism problems in the inner (broader) rings, while focusing problems are more evident in the outer (narrower) rings.

In the unlikely event that the the etalon exhibits poor parallelism, turn the operate/balance switch on the controller (Figure 8) back to balance, cycle the controller power, then switch the controller back to operate. Apart from this, the problem is unlikely to be parallelism unless someone has mistakenly adjusted the controls. Verify again that the etalon controller settings are appropriate for the etalon being used (see Table 3 and Figure 10). If you are sure the parallelism needs to be improved, you will probably need to adjust the X and Y fine parallelism knobs on the controller. Note that this has never been needed, so use this as a last resort. Carefully record the settings before you change them. The parallelism can be optimized in a similar manner to the focus, by adjusting the X knob slowly and taking a calibration image at each position. Repeat this procedure for the Y adjustment.