Star testing

(a) Perfect optics: inside focus, in focus, and outside focus.

(a) Perfect optics: inside focus, in focus, and outside focus.

(b) Same, in unsteady air (the usual situation).
(c) Perfect optics with a thermal air current inside the tube because the telescope has not finished cooling down.
Figure 5.13. Star tests of a good telescope under a variety of conditions. (Images generated with Aberrator.)

Figure 5.13 shows what a star test looks like. The first picture shows a star as seen with a perfect telescope, inside focus, perfectly focused, and outside focus. The second picture shows a star in turbulent air - the usual situation -and the third shows the effect of a tube current, an air current inside the telescope tube. (Besides bashing in one side of the round image, tube currents can also produce "pie-slice" deformations.) The last picture shows miscol-limation. All of these are problems that are easy to remedy; let the telescope cool down to match the surrounding air (or wait for better conditions) and adjust the collimation.

(a) Spherical aberration (relatively common).

(a) Spherical aberration (relatively common).

(b) Astigmatism (uncommon, usually caused by decentered element).
(c) Mirror pinched by three supporting clamps or screws.
Figure 5.14. Star tests that indicate optical problems. (Images generated with Aberrator.)

Real optical defects are shown in Figure 5.14. Spherical aberration makes the out-of-focus image look different depending on the direction in which it is defocused. Small amounts of spherical aberration are common in mass-produced telescopes and have very little effect on image quality. Spherical aberration is often temperature-dependent, influenced by the expansion and contraction of the main mirror.

Astigmatism is what happens when an element has elliptical rather than circular symmetry. It is a relatively rare problem in telescopes but common in human eyes, so the first thing you should try is tilting your head to see if the astigmatism tilts with you. If so, it's in your eye, and you should wear glasses while observing. Otherwise, look for a decentered element (p. 72).

A triangular kind of astigmatism happens when a mirror is pinched by its three mounting screws or clips, and small amounts of this problem are sometimes visible even in very well-made telescopes. A tube current can mimic this problem.

The images in these pictures were generated with Aberrator, a free computer program distributed by its author, Cor Berrevoets (http://aberrator.astronomy. net). You can determine the exact condition of your telescope's optics by finding the parameters that enable Aberrator to match it. The definitive manual of star testing is Star Testing Astronomical Telescopes, by H. R. Suiter (Willmann-Bell, 1994), which includes numerous computer-generated illustrations.

But a word of caution is in order. Star testing may be too sensitive. It can reveal paltry defects that have no visible effect on the image quality. Moreover, you are at the mercy of atmospheric conditions and thermal equilibrium. A single star test can (with luck) prove that a telescope is good, but a single test cannot prove that a telescope is bad, unless a defect is very prominent. Normally, tests must be repeated over and over at different temperatures, under different atmospheric conditions, before a verdict can be given.

There are no perfect telescopes, only telescopes whose defects have not yet been measured. Some large observatory telescopes have notorious defects. The relevant question is not, "Is it perfect?" but rather, "Can you do astronomy with it?" The answer is almost always yes.

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