Beginner's Corner
March, 1940

NOT ALONE SOME beginners but some 1, others also appear to think that the best seeing, through the telescope, will be had when the stars look brightest and sharpest to the naked eye. It could be, but generally isn't. Some recorded oral comment by the widely known advanced amateur observer, Dr. W. H. Steavenson of England, reprinted from the ever interesting Journal of the British Astronomical Association, is instructive.

"The word 'seeing' should be taken as referring merely to the quality of the telescopic image. Many beginners have the impression that bad seeing means everything that was described by the late T. W. Webb as 'an impediment to distinct vision.'

"I will deal first with the question of transparency, which is really quite distinct from the question of seeing. Transparency is, of course, primarily of importance to those who work on faint objects in towns, on account of scattered light. The inexperienced is apt to get a misleading impression of the effect of a slight increase in transparency, since an extension of 0.3 magnitude in the limit of visibility nearly doubles the number of stars to be seen.

"I will now deal with the question of seeing proper, which means the definition of the telescopic image. It is often best under conditions of bad transparency. There are two possible sources of disturbance, the true atmospheric conditions, due to air currents at great heights, and local disturbances low down, often in the tube of the telescope. These latter are the ones which can often be avoided. The best to find out where the trouble lies is to put a bright star out of focus, racking the eyepiece out and using a high power. The out-of-focus image may often be seen crossed by parallel lines or streams moving with great speed. These are due to air currents at great altitude, often as high as 50,000 feet. They are generally most evident just before changes of weather begin, but they are consistent with quite calm conditions at sea-level.

"At other times the focused image may be quite sharp, but it goes slowly in and out of focus. All slow changes mean disturbances inside the telescope. There are two forms of these disturbances, mirror currents, which usually are only serious with large mirrors of twelve inches or over, and tube currents. Mirror currents, which consist of air currents in close contact with the surface of the mirror, show themselves in the out-of-focus image as wavy lines slowly creeping across the image in a direction at right angles to their length. Tube currents are much more common, and give rise to a series of vortices in the out of focus image. It should be noticed that any disturbance far outside the tube is generally rapid and is seen in the form of straight lines in the image. It is well to remember that refractors may also show tube currents, and the fact that they are not commonly recognized is due to the fact that large instruments of this type are not generally used under the same exposed conditions as reflectors of similar aperture."

Beginners who want to go a bit deeper into these matters will find that a chapter entitled "Atmosphere, Telescope and Observer," in "Amateur Telescope Making- Advanced," companion volume to "Amateur Telescope Making," is instructive.