Astronomy Hacks

Eliminate Astigmatism

Pinching can turn good optics bad and your pinpoint stars into fuzzy lines.

Astigmatism is an optical aberration that may be present in your eyes, in your telescope optics, or both. In a telescope that suffers from pure astigmatism, slightly defocusing a star image in one direction results in the circular star image becoming a short, straight line. Defocusing in the other direction yields another straight line, but oriented at 90° to the first line. Such pure on-axis astigmatism is rare; most telescopes that suffer from astigmatism also suffer from other aberrations and miscollimation, all of which can cause bewilderingly distorted images.

Tip

Unless you are an optics expert, looking at a defocused star won’t tell you much except perhaps that your telescope is not providing very good images. An optically perfect telescope that is perfectly collimated displays stars as Airy discs, a bright center spot surrounded by symmetric rings of decreasing brightness. There are so many possible causes of imperfection—from unstable atmosphere to air currents in the telescope tube to miscollimation to mediocre optics— that it’s very difficult to determine not just what’s causing a problem, but what the problem actually is. Accordingly, unless your scope already star-tests perfectly, we recommend that you follow the steps described in this hack. They can’t hurt, and they may help.

If you want to learn about all of the technical details involved in testing optics, ...