Seeing

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An Airy disk surrounded by several stationary diffraction rings is, alas, a rare telescopic sight - the presence of the Earth's atmosphere sees to that. In addition to absorbing the incident starlight, it also causes the star images to change in size (seeing), move about (wander) and to change in brightness (scintillation). Another significant effect which is better seen in larger telescopes at high magnification is the appearance of speckles, which are diffraction-limited images of the Airy disk and explained in more detail below.

Essentially, in a small telescope, aperture limits the resolution. With a large aperture the seeing limits the

resolution. Many observers try to quantify conditions of atmospheric steadiness and clarity by reference to a numerical scale. There are several scales of seeing and whether the numerical value of seeing increases or decreases as seeing gets better is purely a matter for personal choice. Aitken and van den Bos, for instance, each used a scale of 1 = worst to 5 = best with the occasional use of a + sign to indicate "slightly better than" as in 2+. It is difficult to justify a scale that goes from 1 to 10, for instance, because it would be difficult to be that specific about what is, after all, a very subjective parameter.

The performance of a telescope on double stars can be improved by considering some of the following points:

(a) Don't take a telescope out of a warm house into a cold garden and expect to see point-like images straightaway. The telescope must be given time to reach the temperature of the night air. This goes for the eyepieces as well.

(b) Don't be put off by a little mist or haze or even thin cloud. The atmosphere on these occasions is usually calm and can result in good seeing.

(c) If housed in an observatory, open the dome as soon as is practicable. Just after sunset is not too soon. Keep the dome closed during the day but allow a little air circulation if possible.

(d) Don't observe from surfaces which absorb a lot of heat. Grass is more preferable to concrete.

(e) Don't use a magnification which is clearly too high for the state of the atmosphere. If the images do not show disks, wait until things have improved. If the star you are after cannot be resolved, switch to a backup programme of wider pairs but always be prepared to take advantage of good seeing when it occurs.

(f) Plan your observing so that your target stars are as close to the zenith as possible when you observe them.

(g) Make sure that the telescope optics are as well adjusted as possible. For reflector users see Chapter 11 for advice on how to improve optical performance.

References

5 Lord, C.J.R., http://www.brayebrookobservatory.org

6 Lord, C.J.R. (see Haas, S., 2000, Sky & Telescope, 102, 118).

7 Sidgwick, J.B. (rev. Muirden), 1979, Amateur Astronomer's Handbook, Pelham.

8 Nicklas, H., 1994, Compendium of Practical Astronomy (ed. G.D. Roth), Springer.

9 Couteau, P., 1981, Observing Visual Double Stars, MIT Press.

11 van Albada, G.B., 1958, Contributions of Bosscha Observatory, 7.

12 Simonow, G.V., 1940, Annals of Bosscha Observatory, IX, Pt 1.

Chapter 11

Reflecting Telescopes and Double-Star Astronomy

Christopher Taylor

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