ABBEY’S EYEBALL TEST

(March, 1986)

At the December meeting, we were treated to a really first rate discussion of the different types of telescopes and their individual characteristics. We learned how to analyze the diffraction patterns of out-of-focus star images to detect defects in a telescope's optical system.

This reminded me of a little test I discovered about 15 years ago which is a little more difficult to perform, but is much easier to analyze.

This test is much like the familiar Foucault test which is used by every amateur telescope maker. The idea is to use a star for our light source and the iris of the eye for the knife edge.

First pick a moderately bright star which you can comfortably observe through the telescope without having to assume an awkward position. Very bright stars such as Sirius are not suitable because of the great amount of atmospheric turbulence that they seem to always be able to find. Remove the eyepiece and if you wear glasses, take them off. Rack the focusing tube in as far as possible and look through it, resting your eyebrow against the end of the tube.

If the telescope is aligned properly, and if you are very lucky you. will see the mirror or objective fully illuminated by the star. if this not the case, you will have to move the telescope tube around a little to find it. Once the mirror is fully illuminated, move the telescope tube, along with your eye (which should remain immobile against the focusing tube) towards your right. You should see the black edge of your imaginary "knife" moving across the mirror from the left, as in any Foucault test. Now rack the focuser out a little and try again. If your "knife edge" moves in from the right, you have moved outside the focus, and must back up a little.

When you finally locate the exact focus, and it does take quite a bit of work, you will see the mirror darken all-over at once, and just before the light completely disappears, every detail of the mirror’s figure will stand out in amazing relief.

There are two words of caution that you must bear in mind. First, since the light rays are parallel instead of diverging, the apparent figure of a perfect mirror will be absolutely flat. And last of all, be careful of your footing. You do not want to undergo enucleation. Enucleation is the removal of the eyeball by surgical means (or otherwise)!