Seeing

Bob Marriott

	Seeing refers to definition dependent on atmospheric conditions and disturbances, while transparency is a description of clarity and apparent brightness of a star or other object. Seeing can be affected by the following conditions.
	High seeing Low seeing Ground seeing Observatory currents Tube currents Mirror currents	Caused by air currents at heights of 20,000–40,000 feet Originating at heights up to a few hundred feet Caused by radiation from the ground where the telescope stands Caused by currents throughout a roofed or domed observatory Affect reflectors particularly although refractors are not immune Troublesome with larger reflectors of more than about 18 inches aperture

Seeing can be diagnosed by defocusing the image of a bright star or planet. With high seeing, parallel lines appear moving straight across the field in one particular direction – the direction of the wind high in the atmosphere. They are caused by two layers of air, moving at about 100–200 mph, at different temperatures, creating an ‘optical surface’ between them and changing pressure and density. The lines are due to persistence of vision. The effect on a star is to blur it to an apparent diameter of perhaps 2–3 arcseconds. Low seeing is more or less local, and may be recognised by the presence of large waves moving slowly across the field, producing a flickering affect. This is shown in the accompanying real-time video. The silhouette at the centre is the shadow of the secondary mirror of the reflector. Ground seeing is important to solar observers. When a telescope is in the open, the surrounding ground is heated, and during the daytime hot waves of air upset the seeing. The trouble is worst near midday, but may continue for at least half an hour after sunset. Observatory currents can be obviated with a large observatory shutter and good ventilation. Tube currents are very serious. Reflectors are are worst affected, but refractors are not immune. There is a longstanding dispute about the merits of different types of tube – open or closed, or of metal or other material. Mirror currents affect only large telescopes. A mirror up to about 12 inches aperture generally takes about an hour to cool. However, a 20-inch mirror takes five to six hours, and a larger mirror does not really cool down entirely even after a complete night. Overall, it is important that the telescope and observatory be opened and the temperature equated well before observing commences. For visual observations, seeing is recorded with the Antoniadi scale:

1 Steady for all or most of the time.

2 Frequent periods of steady seeing.

3 Mostly unsteady but generally good.

4 Frequent periods of poor seeing.

5 Poor throughout.