Many sophisticated methods of evaluating the optical quality of a telescope have been developed over the years. But there’s one very simple, convenient, and sensitive test that’s easy to perform any clear night. Known as the extrafocal image test, it uses an actual star as the test source.
While any star that’s not too bright nor too faint can be used for testing purposes, a perfect choice is Polaris, the North Star. Not only is it of ideal brightness but it also offers the great advantage of not moving during testing due to the rotation of the Earth—a real plus for those with telescopes without motor drives! Using a medium magnification eyepiece (ideally one giving about 20X per inch of aperture), first place the star at the center of the eyepiece field and bring it into sharp focus. Next defocus the star, either by going inside of focus or outside of it, and examine the image. You should see a circular disk within which are faint concentric rings of equal brightness. (If using a reflector or compound telescope, you will also see the dark silhouette of the secondary mirror at the center of the disk.) Now change an equal distance on the other side of focus. Should you see an identical-looking disk and ring pattern in both positions, congratulations– your telescope has essentially perfect optics!
If optical defects are present, they will readily reveal themselves in the extrafocal image. For example, should the image be triangular-shaped on either side of focus you have pinched optics.
This usually means that either the primary mirror of a reflector or the objective lens of a refractor is mounted too tightly in its cell. If you see an elliptical-shaped image that turns 90 degrees as you reverse focus, you have a serious condition known as astigmatism. However, before putting the blame on your primary optics, make sure that this isn’t in the eyepiece or your own eye! Simply turning the eyepiece in its focusing tube will show if it’s the former, while rotating your head will show if it’s the latter—in either case by the ellipse turning with it.
Other symptoms are concentric rings that have a jagged appearance to them, indicating a rough optical surface (typically resulting from too rapid machine polishing) rather than a smooth one. Rings of varying thickness and brightness instead of being uniform in appearance indicate zones (high ridges and low valleys) in the optical surface. Rings that are bunched together and skewed into a comma-shaped image indicate misalignment of the optics—what is known as poor collimation. And the extrafocal image can even tell something of the state of the atmosphere (a rapid rippling across the disk seen on turbulent nights), the cooling of the optics (snake-like plumes moving across the image until the optics reach equilibrium with the nighttime air temperature), and the thermal environment of your observing site (heat waves seen like those rising from pavement on a warm day).
I’ve personally used the extrafocal image test to evaluate literally thousands of telescopes over the past 50 years, ranging from small "backyard" scopes to some of the world’s largest research instruments. What a joy it is to see perfect optics—especially if it’s your own telescope!
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing.