AMATEUR ASTRONOMERS’ TELESCOPES

Wanganui Chronicle, 3 December 1946, Page 8

AMATEUR ASTRONOMERS’ TELESCOPES

ENCOURAGEMENT TO OWN THEIR OWN . .. (By “Sky Pilot") Once we become really interested in the .starry sky, maybe through the un- | aided eye or perhaps through reading about it, there is a great desire Io visit an observatory to see the wonders of the sky for ourselves through a large telescope. For this reason applications Io Melbourne Observatory for an evening’s observation, which arc many, meet with appointments even a year ahead. Despite this opportunity many aspiring amateur astronomers are anxious to make or buy a telescope for themselves. Every year there are many applications from new members ot the NewZealand Astronomical Society seeking help in making a telescope from the director of the amateur telescope section. The writer years ago did the same thing. From time to time small telescopes are purchaseable and these reveal many glories of the sky and provide the amateur with no end of interest. In the United States there must be thousands who have constructed their own instruments and thousands who have saved their money and purchased some of the splendid instruments offering. Fortunately the United States has a great variety of telescopes offering to the amateur, but every society encourages amateurs to make their own, have the joy of making it and enjoy many happy hours with an instrument the product of their own hands and brain. A friend of the writer in Auckland by persistence, interest and care had made several splendid telescopes and is quite an authority on certain makes to-day. The late Mr. J. T. Ward, of Wanganui, also manufactured a number. TWO TELESCOPES With a view to showing what can be accomplished by amateurs this article is planned to give some information to | encourage even the youngest to know that the making of a telescope is not I beyond them. The chief three parts of the visual telescope are the light-gath-ering objective, the eye-piece and the human eye. The chief two types of telescopes are the refractor and the reflector, the former using lenses and the latter a mirror. In the refracting telescope the objective is a lens situated at one end of the telescope tube with the eye-piece at the otter end, a magnifying glass which makes possible an examination of the image. The principle of the telescope is therefore quite simple, lens and eye-piece held together by a tube. Certain difficulties and high cost of the refractor, however, cause the amateur to turn his attention to the reflecting telescope which has at the lower end a concave mirror known as the pricary mirror which forms an image of the celestial object in the eye-piece at the top end of the tube, a flat or secondary mirror at an angle of 45 degrees, being used to throw tile image into the eye-piece. Whereas the light in a refractor is focused at the bottom, of the tube, in the reflector it must travel the length of the tube twice before reaching focus, which means that a smaller tube is necessary. The amateur may make either kind for himself, but he will certainly find the reflector with say, a six inch mirror for the start, the simplest and much the cheapest. COLOUR DIP FICL’LTIES Let me say a few words on the chief parts of small telescopes, considering those of the reflector first. To overcome the colour dispersion unfortunately found in the refractor, the object glass is made of crown and flint lenses “cemented’’ together with Canada Balsam. This glass is almost achromatic (without colour effects) being about 95 per cent, which is perfectly satisfactory for small telescopes. The remaining 5 per cent, however, is very troublesome with large telescopes, especially with photographic cameras. The eye-piece permits a much closer approach to the subject. With a magnification of 100 seen through the telescope, it is equivalent to seeing the object with the naked eye from a point one hundredth of the distance. A boy a mile distant viewed through a telescope under a magnification of 100 appears to be but 50 feet away whereas a boy 100 feet away viewed through the same telescope appears as large and distinct as if he were a foot away. Stars are so far away that the highest magnifications allowed by the atmosphere and the telescope combined will not reveal them as more than points, certainly never as discs. Planets and satellites on the other hand can be considerably enlarged. A crater on the moon when viewed through a large telescope with a power of 2000 appears to be only 120 miles distant.

The tube or framework of a telescope is its backbone. Visual telescopes as a rule have focal lengths

between 15 and 20 times their aperture so that in practice we speak of F/15 or F/20 telescopes. Concerning tile reflector telescope, the mirror may be even of poor glass but it must be brought to a proper curvature, carefully ground, figured, tested, and filmed with silver or aluminium. When silvered, the mirror surfaces tanish in time by gathering dust, and pinholes appear in the film, which means that these must be resilvered every six months or so. The aluminium film is much the better proposition as it lasts much longer. REFLECTOR IS CHEAPER. The reflector mounted simply costs about one half to a quarter the refractor of equal light grasp and is much easier to observe with, the refractor being “a neck wringing instrument” for altitudes above 4b degress especially when the telescope is very small. On lire other hand the refractor is portable. Whereas the focal length in the refractor is 15 times the diameter of the lens, in the reflector it is usually only five or six times, which means that the refractor is three to four times as long as a reflector. Since the mirror has only one surface to be brought up to proper curvature and the lenses have four, obviously tne cost is much increased in the case of the refractor. A mirror, too, is perfectly achromatic whilst the lens can never be. Except for very small instruments the best kind of mounting is the equatorial where the main axis (polar) is set parallel to the earth’s axis while the declination axis is set at right angles to it. The right ascension and declination circles are set accordingly and if possible it is advisable to have a driving mechanism (actuated by a weight), which will turn the polar axis at just the rate necessary to follow the object in the sky. The other mounting is the altazimuth, which is turned in azimuth about a vertical axis and in altitude about a horizon lai axis. A good shelter is also required for the amateur’s telescope especially if it is a reflector fixed to mother earth. This house may run back on rails, leaving the telescope free for observations or there may be a dome or sliding roof. Many devices can be employed. From amateurs all over the world have come men who have been associated with some of the largest astronomical enterprises in the world, Clark, Swasey, Porter; all ot whom began as amateurs.

In the least expected places of the country, its cities, villages, and open country, these amateurs are to be found, some of them doing a fine job specialising in variable stars, comets, or double stars. There are also groups such as the Hamilton Society and the New Plymouth Society. Ingalls, of Springfield Group, in the United States, from the great interest which he had in the amateur and in making telescopes was led to produce one of the finest books ever written on the subject, “Amateur Telescope," and no finer help could | be recommended to the keen amateur anywhere. in this age when hobbies are the order of the day, 1 would like to commend this very fascinating one, make a telescope or save and buy one. It will bring untold and unmeasured joy to you for all the years of your life, and also to your guests and friends. If you can use tools and are handy with your hand, and possess a reasonable amount of reasoning power and

stickativity there is no reason in the world wliy you should not have an instrument on your own lawn.