Astronomers have long used the star test as a comforting touch with reality. A quick turn of the focuser is often enough to confirm that the telescope is aligned, cooled off, and ready for operation. Making such a determination requires no reference to numbers, but occasionally you'll want to use the star test to estimate the strength of aberrations on the glass. You must then know exactly how far to defocus the telescope in terms of quantities that can be compared with wavefront deformations.
Also, convenient use of the star test often has nothing to do with "stars." In some cases, atmospheric turbulence, apparent stellar motion, or waiting for a clear night makes using true stars too difficult. To conduct daytime or earthbound tests, you can no longer rely on the great distances and small angular extent of actual stars. When using an artificial source, you must figure the minimum distance at which it can be placed and the maximum size of the pinhole. If you are sloppy about these points, you could unfairly decide that a good telescope is bad.
This chapter deals primarily with the following three topics:
1. Translating defocusing aberration to the more familiar but less general topic of eyepiece movement.
2. Sizing, making, and placing artificial sources.
3. Setting up and doing an "official" star test, as opposed to the usual check of the operating conditions of the telescope.
Important results of this chapter are listed in tables. For completeness, the derivations are sketched either here or in the Appendix. Those who are interested can see where some of the concepts in this chapter originated, but performing an effective star test requires no more than careful use of the tables.
Figures appearing in this book avoid any reference to how far you must turn the focuser forward or backward to obtain a certain amount of defocus. Instead, any defocus amount refers to defocusing aberration measured in wavelengths on the aperture pupil (see end of Chapter 4). Why use defocus in such a peculiar manner, when the straightforward method of eyepiece motion can be easily understood by everyone?
The answer is simple; telescopes are too different from one another. The exact amount the focuser must travel to show a given pattern varies with the focal ratio of the telescope. Telescopes with equivalent aberrations and obstructions will show identical patterns, but they all do so for different eyepiece motions. If you specify focuser motion, you must also give the focal ratio. The result is a muddy picture of a truly simple concept.
Defocusing aberration, as measured on the wavefront, is a sort of universal coordinate system that classifies identical behavior in many different telescopes. In an effort to reduce the multiplicity of patterns, defocusing aberration is used as a generic variable. It has features that transcend telescope type. More importantly, it is easy to go the other way and calculate how far you must move the eyepiece to yield a given defocus aberration.
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