Seeing

W.H. Steavenson

Journal of the British Astronomical Association, 70 (5), 204, June 1960

With regard to the word ‘seeing’ itself; seeing has nothing to do with a star’s brightness, but refers to definition. The apparent brightness of a star is a measure of transparency, and is entirely different from seeing, though often confused with it. Leaving aside telescopic faults, it may be said that poor definition is due to atmospheric disturbances. These disturbances come under six headings. Bad definition can in fact be diagnosed, and it is always desirable to find out just why seeing is bad instead of simply accepting it. I want, therefore, to deal mainly with the diagnosis of bad seeing conditions. Let us take the six headings as follows:

1 High This type of bad seeing is due to air currents at heights of, say, 20,000–40,000 feet.

2 Low Disturbances originating at heights up to a few hundreds of feet.

3 Ground Disturbances caused by radiation from the ground upon which the telescope is standing.

4 Shutter/observatory currents Of course, these affect only the observer who has a dome.

5 Tube currents These affect reflectors particularly, but refractors are not immune.

6 Mirror currents These become troublesome only with the larger reflectors, say above 18 inches aperture.

The way to diagnose the cause of bad seeing is to take a bright star, such as Capella or Vega, use a high-power eyepiece, and rack out. If the trouble is high, you will see parallel lines, like telegraph wires, moving straight across the field in one particular direction – the direction of the wind high in the atmosphere. These were first observed by Douglass in 1893 or 1894; they were described in Popular Astronomy and later in Amateur Telescope Making. I once had the curiosity to try to find out what they were, so I used a Thornton-Pickard shutter, exposure 1/20 second. I saw moving bright and dark patches; you would see five or six at a time, for instance, with a 20-inch aperture. Since then they have been photographed with the Palomar 200-inch reflector. They are caused by the fact that two layers of air at different temperatures create an ‘optical surface’ between them: there is an analogy with what happens in the case of a stream of water in contact with the air. You also get roughly similar effects just before totality in a solar eclipse (shadow bands). This, then, explains the moving lines; they are produced by the different densities and temperatures of atmospheric layers, and they move at from 100–200 mph. The ‘line’ appearance is due to persistence of vision. The effect on a star is to fuzz it, and give it an apparent diameter of perhaps 2–3 seconds of arc. Things may be even worse than this. At the Cape, which can produce the worst seeing in the world, I have seen Arcturus looking just like Mars. (I have heard it said that there are three kinds of seeing: good, bad, and Cape!) When this trouble is experienced, nothing can be done about it. Neither can it be regarded as ‘local’ in any spot in a small country such as England. For instance, we cannot claim that the seeing at, say, Scarborough is generally better or worse than at Wolverhampton. The cause is due to alternations of weather systems such as cyclones and anticyclones, which produce changes of pressure and therefore of density. So if you want to avoid the trouble, do not change your altitude; going up a mountain is quite useless. Change your latitude instead. W.H. Pickering first pointed this out, and it was probably the most important thing he ever said. Near the equator, of course, pressure is more constant.

Next we come to low seeing. This really is more or less local, and some parts of England are worse than others. For instance, you would be more bothered with it on the lee side of a city such as Birmingham. It may be recognised by the presence of large waves moving slowly across the field. The waves move so slowly that they may take as much as one second each to cross the field, and with a telescope up to 3 inches in aperture a wave may occupy the whole aperture for a fraction of a second. This gives a flickering effect, and you may get periodical flashes of good seeing. You can avoid the worst of it simply by moving away from areas that are particularly badly affected.

Ground seeing is very 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 only real remedy is to use a ‘tower’ telescope, as has been done at Mount Wilson. However, do not put your telescope on stone or concrete – as has been done at Herstmonceux, where they have done their best to ensure the worst possible seeing by surrounding all the instruments with slabs of stone. Instead, put your domes on grass, or green stuff of some kind. The trouble is worst near midday, but may continue for at least half an hour after sunset.

Now for shutter currents. The remedy is to avoid too narrow a shutter – and do not have too many hot people inside the dome at the same time. I know what has happened at Cambridge, when I have had as many as twenty people in my dome at once, all hot and all talking. You can diagnose the trouble by putting the aperture across the edge of the shutter; a wide shutter more or less cures it. When the 6.75-inch Sheepshanks telescope was set up at Greenwich about 1837, it was given only a 9-inch diameter shutter, and bad currents were experienced, but this sort of thing is not done nowadays.

Shutter currents are not serious, but tube currents are very serious indeed. Reflectors are worst affected, but as I have said, refractors are not immune. A few things may be done to reduce the trouble. First, do not have a metal tube. As heat is radiated off, cold, dense air falls to the bottom of the tube, and currents are produced. Unfortunately, telescope-makers tend to be conservative; most old telescopes had cylindrical metal tubes, and the practice still goes on. At any rate, do not have a cylindrical tube, whether it be made of metal, cork, asbestos, or anything else. Square tubes are better, since the currents tend to go out of the top. With a cylindrical tube, the currents hit the side and produce gyrations reminding one of spiral nebulae. Every cylindrical tube does this – even if it is not made of metal, though metal tubes are naturally the worst. Sensible people therefore use lattice tubes, and the lattice should go right down to the mirror. It is wrong to have a closed-in section close to the mirror, and indeed this is just where you do not want it. A mistake was made here with the 72-inch Victoria telescope, but the 100-inch and 200-inch reflectors are really well designed. The best plan is to have a skeleton structure all the way down, and this may be either cylindrical or square. If you are bothered with tube currents, install a fan to suck the air down (not blow it up). This helps very considerably. Note that tube currents go round and round, irregularly, and this gives a means of diagnosis. If the disturbance is irregular, it originates inside the tube. If it takes the form of straight waves crossing the field, the trouble lies outside. Refractors are not immune from tube currents, though they are less affected than reflectors. Dawes noted this, many years ago, when he described an occasional triangular deformation of the star image. The worst time is during daylight, for observations such as last year’s occultation of Regulus by Venus. (You do not get the same effect in solar work, since the Sun is not shining on the side of the tube.) I made some experiments years ago, when I was testing telescopes on artificial stars – as I still do. When I put the telescope in the shade of a tree, the seeing steadied at once. I do not know how anyone managed to find Regulus for this particular observation if the Sun was shining on the side of the tube.

Mirror currents affect only users of large telescopes. A metal mirror will cool quickly, and a glass mirror up to, say, 12 inches aperture, takes generally less than an hour. However, a 20-inch glass mirror takes five to six hours, and a larger one does not really cool down entirely even after a complete night. Consequently you get a ‘boundary layer’, with numerous small ‘flames’ not unlike the solar chromosphere. A fan is no good in dealing with these crawling, maggot-like things, which adhere closely to the surface of the mirror. One method of testing is to take a bicycle pump, and plough a furrow through them! The result of the trouble is, of course, a fuzzy image. I have proved this often at Cambridge. Under good seeing conditions I have had poor results with my 30-inch reflector and excellent ones with the 25-inch Newall refractor on the same night. The moral is, of course, never to do visual work with large glass reflectors, unless you can afford £10,000 or so for an elaborate building or a refrigeration system. I think I shall leave matters there. As I have said, I have been most concerned with telling you about the importance of diagnosing the cause of bad seeing on any particular occasion.

Extracted from the report of the Ordinary Meeting of 24 February 1960. Steavenson’s talk opened a discussion in which participants included F.M. Holborn, B.M. Peek, H.E. Dall, A.C. Curtis, G. Fielder, D.G. Hinds, E.J. Hysom and J.R. Smith.