December 20 1961 - Using Small Astronomical Telescopes

From time to time I have had complaints that I have dealt too much with observations made with very large telescopes, and which are therefore beyond the range of the amateur. This is certainly a valid point, though it is not easy to overcome. Accordingly, we decided to devote one programme to the use of small instruments, which will provide some fascinating hours of enjoyment for the users even if they cannot lead to epoch-making discoveries. On the programme, I was joined by the well-known science correspondent L. Marsland Gander.

The general idea of an astronomical observatory is of a vast domed structure, housing a telescope powerful enough to photograph objects hundreds of millions of light-years away. This is a reason­ably accurate picture of a great professional observatory. The largest telescope so far built is the 200-inch reflector at Palomar, in the United States; the most distant object recorded the galaxy 3C-295 in the constellation of Bootes, is something like 5,000,000,000 light-years away, and is receding at almost half the speed of light.

Professional astronomers are naturally concerned mainly with studies of the universe as a whole, and large telescopes such as the Palomar reflector are seldom directed at our near neighbours of the Solar System, the Moon and planets. Indeed, it is only very recently, with the development of rocketry, that lunar and planet­ary research has started to come back into the official programmes. This has meant that the amateur has had a definite part to play, and the amateur astronomer can do valuable work - always provided, I would add, that he recognizes his limitations.

The logical first step to be taken by the beginner is to learn the various constellations. No equipment is needed for this other than a star-map and plenty of patience. Once the main groups have been identified, they may be used as 'pointers' for the rest and a little practice will work wonders; it does not take more than a week or two to become at least reasonably familiar with the night sky. Winter is, moreover, a good time to start, since Orion and its brilliant retinue are at their best.

A telescope is of course highly desirable if astronomy is to become anything more than a casual interest, and this is often where difficulties begin. The unfortunate fact is that astronomical telescopes are not cheap, and it is bad policy to spend money on a very small instrument. A hand-held telescope, for example, will not be nearly as effective as good binoculars.

For proper observing it is essential to have a telescope which will give reasonable magnification. Telescopes are of two types: refractors, in which the light is collected by a lens termed an object- glass, and reflectors, in which the light-collection is done by a parabolic mirror. Generally speaking the refractor should have an object-glass at least 3 inches in diameter, while the minimum size for a reflector mirror should be 6 inches. Smaller telescopes will give pretty views of star-fields, various stellar objects, and the craters of the Moon, but are hardly adequate for the serious amateur.

If bought new, a 3-inch refractor will cost at least £30, and if it is fitted with a really good mount the cost may be as much as £50. The price of a 6-inch reflector is comparable. This represents a considerable outlay (though it must be borne in mind that the costs of upkeep are virtually nil for the refractor, and small only for the reflector, since the mirrors need not be attended to very often). Second-hand telescopes are not as common as they used to be, doubtless because of the increasing popular interest in astro­nomy, but they can be obtained now and then at low cost.

A word of warning is necessary here. A second-hand reflector, particularly, may not be as good as it looks at first sight. If the mirror is poor, the performance also will be poor, and the only course will be to have the mirror drastically modified - which again increases the cost considerably. The solution here is to have the mirror optically tested before purchase. The same holds good for the object-glass of a refractor, though bad object-glasses are probably less common than bad mirrors.

Equally important, perhaps, is the question of mountings. Stability is not only desirable but is absolutely essential - which is why any attempt at holding the telescope by hand is doomed to failure from the outset. Many small refractors are sold upon the pillar-and-claw mount, which means that the telescope has to be set up upon a table. I have nicknamed the pillar-and-claw the 'blancmange mount', for reasons which will be obvious to anyone who has tried to use it. The slightest shaking will make the Moon or a star jerk about wildly in the field of view, and it will be hopeless to obtain proper observations. Fortunately there is generally no difficulty in removing the claw and transferring the telescope, on its pillar, to a tripod. This is superior in every way, but it must of course be solid, so that there is no appreciable shaking.

If the telescope is freely movable in every direction, set on a pillar in this way, the mounting is of the altazimuth type. It is con­venient, and needs no setting whatsoever, but it has marked dis­advantages. As the Earth rotates, celestial objects seem to move across the sky from east to west (movement in azimuth), while their altitude also changes constantly. When a reasonably high magnification is used, the field of view of the telescope is naturally small, and the diurnal motion of the sky becomes painfully evident, so that the star seems to drift quickly across the field and disappear from sight. The telescope must be constantly moved so as to keep the object in view. With an altazimuth there must be two correc­tions - one in azimuth, the other in altitude - which is extremely awkward.

The difficulty is overcome by means of the equatorial mount, in which the main axis is pointed toward the pole of the sky, and is therefore parallel to the Earth's axis of rotation. Azimuth must still be allowed for, but the altitude changes will look after them­selves, and the procedure is very much more convenient. If the telescope is driven by a clock drive, the star will remain in the field of view once the telescope has been clamped and the motor started. All large telescopes are equatorially mounted, and indeed could not be properly used otherwise.

An equatorial mount naturally increases the cost of a telescope, particularly if fitted with clock drive, but for the beginner's 3-inch refractor or 6-inch reflector it is by no means essential. Moreover, it means that the telescope will be hard to move about; the weight will be increased, and each time the telescope is shifted it has to be re-set, or the advantages of the equatorial will be lost. Generally speaking, any refractor with an object-glass larger than 4 inches in diameter must be regarded as non-portable. With a reflector the limit is about 6 inches for a Newtonian, though instruments on the Cassegrain principle are shorter, and the limit for portability is 8 inches or so.

If the telescope is to remain in a fixed position, perhaps mounted on a concrete pillar, it will have to be covered up when not in use. Car covers may be pressed into service, but an observatory is clearly better. The form need not be that of a dome; there is much to be said in favour of the run-off shed. The 125 inch reflector at my own observatory in East Grinstead is covered by a shed mounted on rails. The shed is built in two parts, and when the telescope is to be used the two sections of the shed are rolled back in opposite directions, leaving the telescope in the open. During winter observing has therefore to be a chilly process, but even a full-scale domed observatory does not help much, since it cannot be heated - otherwise the warm air inside the observatory will cause so much turbulence that seeing and definition will be ruined. (This is also why it is hopeless to try to use any astronomical tele­scope from indoors, by directing it through an open window. The oft-described 'roof-top observatory' is also most inefficient.

The choice of eyepieces is of vital importance, since trying to use a good telescope with a poor eyepiece is rather like trying to use a good record-player with a bad needle. In theory, at least, all eyepieces are interchangeable, so that any eyepiece may be used with any telescope, but there are obvious limitations. A common beginners' fault is to try to use too high a magnification. This means that the resulting, image is hopelessly faint, and there is moreover a fatal lack of definition. It is better not to tolerate the slightest blurring; a smaller, sharp image is always far better than a blown- up, hazy picture. A magnification of 100 on a 3-inch refractor is usually satisfactory, though a lower power is desirable for objects such as star-clusters and the Milky Way.

The photographs published in books and articles are, in a way, slightly misleading to the beginner, who may expect to see - say - Saturn looking the size of an orange. In fact the pictures are smaller than sometimes expected, but practice means everything; the more you look, the more you will see.

Let us suppose that our beginner has acquired a small telescope, such as a 3-inch refractor, and is ready to begin observation. What is he to look at? The obvious first choice is the Sun, but great caution is necessary here: if the Sun's light and heat is focused on to the observer's eye, even for a second, the results will be disastrous, and will probably result in permanent blindness. The dark caps, which, it is said, may be fitted over the eye-piece for direct solar views are highly dangerous, and should never be used; they afford inadequate protection at best, and are always liable to crack or splinter without warning. To observe the Sun, the telescope should first be directed at the solar disk, an opaque cap being placed over the object-glass. Then remove the cap, and project the Sun's image on to a white paper or card screen. If this is done, the picture will be satisfactory, and any sunspots which may happen to be on view will be clearly seen. The temptation to have 'just one quick look' by direct vision should always be resisted, even when the Sun is masked by fog or mist and looks deceptively.

In the case of the Moon there is no danger at all, even though the observer's eye may be temporarily dazzled. Curiously enough, full moon is the worst time to start lunar observation; the craters lack shadow, and details are lost in the general glare. Far better views arc obtained when the Moon is crescent or half, when the craters along the terminator (the boundary between light and darkness) are filled with shadow, and appear very prominently. Here again it is not difficult to learn one's way about, and lunar maps may be obtained quite cheaply, but it is worth noting that the appearance changes strikingly from night to night. For instance the large crater Eratosthenes, at one end of the Apennine mountain chain, is a splendid object when near the terminator; but at full moon, when the sun is shining straight down upon it and there are no shadows, it appears as a confused medley of light patches and streaks, so that the unpractised observer may have trouble in identifying it at all.

Stellar objects are, on the contrary, very plentiful. A good first step is to use a low magnification (say 30 diameters) to scan the Milky Way with its rich star-fields. Then there are various star- clusters; pride of place goes to the Pleiades or Seven Sisters, in Taurus, in which the telescope will at once reveal many extra 'sisters'. The Hyades, round Aldebaran, are more scattered, and the lowest available magnification should be used, so that the field of view is made as large as possible. Look also at the Sword-Handle in Perseus, not far from the W of Cassiopeia, which consists of two separate clusters in the same field. Between the two clusters lies a faint reddish star; this may be seen with a 3-inch refractor.

Different in character is the Sword of Orion, below the three bright stars which make up the Hunter's Belt. This is a gaseous nebula, and is particularly important to astronomers because it seems to be one of the numerous regions in which fresh stars are being born out of interstellar material. In the nebula lies the famous multiple star, Theta Orionis, commonly nicknamed the Trapezium; all four main stars are easy objects with a moderate power.

Double stars also are common. One of the most spectacular is Mizar, the second star in the tail of the Great Bear (or the handle of the Plough). Alcor, the faint star close to Mizar, is easily visible with the naked eye on a clear night; our small telescope will reveal that Mizar itself is made up of two, while between this pair and Alcor lies yet another faint star. Castor, the senior but fainter member of the Twins (Castor and Pollux) is another fine double, though less easy to separate with a low magnification. The Pole Star, Polaris, has a faint companion which may be glimpsed with a 2-inch refractor, and is easy with a 3-inch. These are only a few of the many double stars visible in the winter sky.

It is also true to say that useful astronomical work may be carried out with a small telescope. A 3-inch refractor is barely adequate for lunar studies, and is of very limited use on the planets, but telescopic variable stars are well shown, and observations of long-period and irregular stars are of great value. A more detailed star-map is of course needed, together with a good knowledge of the sky, but anyone who is anxious to make himself (or herself) useful will find a rich field of opportunity here.

It may often happen that the beginner is disinclined to spend £30 or so on a telescope, and is unable to find anything suitable second-hand. In this case it is always worth making a start by con­structing a small refractor out of cheap lenses and cardboard tubes. The lenses may be obtained from various opticians' shops, though again they are less common than formerly and some hunting may have to be carried out: the object-glass should be 1J to 2 inches across, with a focal length of about 2 feet, and the eyepiece lens smaller and with a shorter focal length. (Jewellers' eyepieces will often serve here.) The eyepiece tube should slide in and out of the object-glass tube, to allow for focusing, and the whole instrument should be mounted, preferably upon a wooden tripod. The con­struction is easy enough once the lenses have been obtained. Since the object-glass will certainly be of poor quality by astronomical standards, the results will be imperfect; there will be considerable false colour, and the field of view will be inconveniently small. Nevertheless, such an instrument will be far better than nothing at all.

Lens-grinding is beyond any but the very experienced amateur with proper facilities, and if the would-be observer wants, say, a3-inch refractor he has no alternative but to buy it. Mirror-making, however, is a different matter, and anyone who has enough patience, together with reasonable skill, can make a perfectly serviceable 6-inch or even 8-inch mirror. The mounting is a sheer problem of mechanics. The main requirement is that it should be firm, so that shaking of the telescope is eliminated as far as possible. The tube may be square and wooden; there need not even be a solid tube at all, since for reflectors the skeleton form of tube is quite satisfactory.

Lastly, it is always worth while for the beginner to join some society. Many towns have their own local societies, and on a national basis there is the British Astronomical Association, which is predominantly amateur. In this way the newcomer to astronomy will meet others with interests similar to his own, and will be able to compare his observations with those made by others.

In any case there is always a tremendous amount of enjoyment to be gained from making a hobby out of astronomy; the skies are always changing, and there is always something new to see. More­over, astronomy is one of the few sciences where the amateur is still able to contribute something of value, even if he has had limited technical training and can use only relatively modest equipment.