Sky Events

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Telescopic Optics

So you've unwrapped that brand new telescope that you got as a present during the holidays, you point it at the Moon, and say, "Yuck! What a fuzzball. I'm taking this thing back from whence it came." Are the optics really that bad? Here's a simple test that anyone can do that will determine if it's the optics or something else.

This test is known as the star test. As the name implies, a star is used and it is especially appropriate at this time of the year with all those bright wintertime stars readily visible in the evening sky. Before we test your telescope, first your need to make sure that the optics are at least roughly collimated or aligned. Alignment for closed optical systems like a refractor (big lens in front) or a catadioptric (stubby tube with the eyepiece on one end) telescope is usually not a problem, but Newtonians (big mirror at the bottom, opening on top with the eyepiece holder on the side near the top) are another matter. If you have a Newtonian see http://zebu.uoregon.edu/~mbartels/collimate/collimat.html for instructions on how this is done.

Now that your scope is properly aligned, take it outside and let it acclimate to the ambient temperature. Wait at least an hour before you start the star test. Point your scope at a bright star. Sirius, the really bright bluish-white one in the southern sky, is an excellent choice. With your high magnification eyepiece of at least 100x, center the star in the field of view and bring it to focus. Now crank the focuser such that you move the eyepiece ~1/8th inch toward the telescope such that the star expands into a small disk. Now take your time and examine that disk very carefully. If you have reasonably good optics, it will show a series of round concentric rings of similar brightness with the outer most being the brightest. If you have a secondary mirror in your scope, you should also notice its dark shadow at the center of the pattern. Crank the focuser such that you go back through focus and beyond it by the same amount (~1/8th inch) as when you were viewing the intrafocal image. In a perfect telescope, which does not exist by the way, the extrafocal image will appear identical to the out of focus image that you viewed when the eyepiece was inside of focus. If the images are similar, then your optics are of reasonably good quality. If the out of focus star image breaks out into an elliptical shape instead of round, there could be a problem with the alignment or in the optics themselves.

You may obtain more information on star testing by getting your hands on the book "Star Testing Astronomical Telescopes" by Richard Suiter, ISBN 0-943396-44-1.

Forrest Hamilton

(Submitted: 3/6/2000)

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