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The National Association of educational broadcasters with the cooperation of the California Academy of Sciences and radio station KPFA in Berkeley presents a program in the recorded series astronomy for the layman on today's program. Leon E. Sullivan a lecturer in astronomy at the Morrison Planetarium of the academy and John Hopkins an inquisitive layman discuss telescopes and accessories. The astronomers toolkit. Mr Hopkins begins the discussion. Yeah I felt a little sort of almost at a loss after discussing spectra scopes without discussing really the place we get the light to put into a spectroscope telescope and if you don't mind I'd sure like to discuss telescopes. Well a telescope as the name implies is simply as something to look far away with Cali for in the distance and scope to you. The original telescope was invented probably by a Dutch spectacle and the apparent spectacle
maker named Liberace who is said to have discovered that if a convex type is but the lens that is simple magnifier held up at a distance from the eye and then just at the eye a concave type such as is used for near-sightedness. This would produce a device that made something faraway look as if it were nearer. It must have been an astonishing thing to discover. Yes it must have created quite a sensation and you know little neighborhood the very lives of. For some reason he never saw all the truly great possibilities of his invention. And as the story goes if it can be believed Galileo great Italian scientists merely heard about this as a rumor immediately perceive what a wonderful thing it would be in his studies to have such a device and quite independently from his knowledge of optics. He was able to reinvent the telescope which is I think one of the greatest tributes to the great mind of Galileo. In the case
he succeeded in making a little uptick too but it was called in those days and in a short space of time. Early in one night at sixteen hundred and ten he made a number of very remarkable discoveries. He discovered the. But the spots on the moon and they were light places were various. The rough and smooth areas Rocky crater e looking things he didn't see any fine details but at least he found there were some round holes on them when he discovered that the sun had spots. That of course was a tremendously provoking thing for traditional philosophy it held the sun must be spotless as it was perfect. What Galileo showed that it had dark markings. You found the satellites of the planet Jupiter. He couldn't quite detect the ring of Saturn however. He did find that the Milky Way was not made of M. of stars and he found that the planet Venus went through all the phases just like the moon. All this a little telescope but I think it's safe to say any child could obtain by
sending a box top and 25 cents to a cereal company. Well that's a tremendous number of thing for one person to have done. Oh well you see a telescope and large is the power of human vision. We traditionally speak of a telescope power and we're talking about magnification that something looks so and so big. But that's only the beginning of it. That's the most obvious thing which a telescope does because that's the way binoculars are marked. True in general there are so many power binoculars. Yes on the other hand though the diameter of the lens 759 seven power with a 50 millimeter diameter lens and the diameter of the lens has to do with how much light passes through. That's a different kind of power and that would be sort of like gathering power. The area of the lens represents how much light can be pulled through to us so to speak and contribute to the image. Well that's a definite distance the diameter is perhaps than some other definite
distance like the length thing which has to do with the magnifying power. Yes the magnifying power can be calculated from the length of the tube of the telescope speaking very roughly and a rough rule is you divide 10 into the length of the telescope tube and then multiply by whatever as in the fine power of your eyepiece. For example you have a spyglass with a 40 inch tube and you divide that by 10 years before using it with a 10 power magnifier you have 40 power course then your right to the determination of the power of the magnifier. Yes well that can be generally found by looking closely at the night of firings pretty little or in some place. But there is a third type of power of the telescope which is extremely important for astronomers. It has to do with the ability of a telescope to show fine detail. This is called resolving power. It may
express how closely together two stars are in the sky and still be seen as two stars. Or it may express how fine and thin a dark line may be and still be seen as a line against a bright background in general these things are hard to compute or at least the background needed to understand the computations rather than us. But experience shows that a telescope with a lens four inches in diameter will show two stars which are only one second of Arc a part in the sky. Well I've noticed the same sort of thing in driving on the road at night I guess my eyes must be like a telescope because when a car is coming toward me from a great distance the headlights appears one after a while leave they are resolved I believe you said. Into two. Yes the eye has an opening in it. The diaphragm certain by matter and the ion has in it a lens and the sensitive detecting device called retina. Resolving power enters but it is very complicated
by the small amateur in the eye and the structure of the retina. Well I'm sure from having read about telescopes of that kind the kind of Galileo had with the limbs isn't the same kind as the large telescopes like Wilson. I know all there are two distinct types of telescopes the refractor using the lens to gather the light so to speak and bring it to a focus or image and then the reflector which uses a mirror and the mirror is concave. The lens is convex both optical devices have the property of bringing light to converge parallel beams are converted by such devices and where these beams intersect. Speaking roughly an image is formed a star or a planet may be imaged in this way. After that it's a matter of magnifying with an eyepiece and holding the whole thing together suitably in some kind of a framework
and you have a telescope an interesting story behind the invention of the two types is it. As history shows the refractor was first and then some fifty years later the great Isaac Newton was seeking to analyze why the refractor type was so ill and satisfactory colored fringes reproduced around images form and viewed by refractors telescope for made a fantastically long dimension which in photography language you need to stop way down compared to their aperture focal length of a small lens the small lens compared to the length of the tube in an attempt to overcome this. Well he correctly saw that it was a matter of the prismatic effect in the in the glass which we talked about under a spectroscope lens is basically a sort of continuous prism. It's been on the edges and thick in the middle and round cross-section through it would look like two prisms. Will you go base to base to base with the corners rounded off.
Greg Newton decided in his time that no amount of contriving could overcome this difficulty. History was to show him wrong one of the few cases when Isaac Newton was wrong. But to overcome the difficulty he invented a different kind of telescope using the principle of reflection from a metal surface in those days the mirrors were made of metal. Nowadays glass with metal deposited on the front. Why is that better why the change. Well glass can be worked more exactly and got into the right shape to get there and the metal coating can be removed when tarnation put it back without altering the shape of the glass. The notion here is that in reflection there is no dispersion beam of light no matter how many colors it's composed of will be reflected as it continue as a complete beam or as your fraction will be broken up. And so a reflector telescope has none of the color fringe troubles of the refractor you mention that Newton was shown to be wrong later. However what happened unless less than a hundred years
later an Englishman John Dahl and discovered that all glass wasn't the same and by making a lens of two kinds of glass known in those days and even now as crown and Flint suitably using any curves in the forms of the lens the greater part of this dispersion effect could be cancelled out without removing all of the property of the lens to form an image. And so a so-called achromatic or lens without color was invented and proved to be quite satisfactory. No lens can ever be so completely free of of the color effect called chromatic aberration as a mirror but a good refractor designed properly and sometimes including even a third kind of glass comes very close to perfection. Does either type them have any particular advantage today over the question of advantages. Is quite a subject and we probably ought to say just a few words about
it. Many people are confronted with this question when they decide they want to buy a telescope or to make one question of refractor versus reflector. The first thing to be said is that a small refractor is somewhat hand hears and a reflector of the same size or one of the larger the refractor. Well let me put it this way in respect to the light a little heat distortion on the lens isn't so bad as the same amount of distortion from a mirror so a telescope has to be pulled out of the attic or closet set up in the backyard for a quick look at the moon to almost be a refractor to stand such treatment and a reflector needs to be set up and allowed to achieve some sort of balance with the outside temperature before you it that isn't so true now with high Rx glass for mirrors as it used to be with a plate glass. But it's still true that the refractor is a little more rugged. On the other hand
refractors a very expensive thing that I don't know what current prices are but a 4 inch diameter that is 4 inch aperture refractor restaurant I can use and they run the buyer anywhere up to $500 which includes a proper mounting but not an elaborate one whereas a reflector of the same power and efficiency may be constructed home for something less than $100. Quite a bit less as a matter of fact. So there isn't any question and respect the price. The chromatic aberration is not objectionable in the good refractor but anything less than excellent is quite bad whereas it's never present in the reflector so you might say workmanship or workmanship. You might come out little better with the reflector. Well I can see another feature there you mention that in order to eliminate chromatic aberration it was necessary to have what amounts to two separate lenses and since each one has two surfaces you
would then have force glass surfaces which had to be finished. I presume to some specified curve and some what I know to some degree of smoothness. Yes that really accounts for the great expense. Also the fact that the light must pass through the glass of a reflecting telescope lens while it's maybe reflected off the front of the reflector so the glass has to be an exceptional pure purity for a reflecting telescope where as all it has to be is free from strains and the tendency to bend under temperature change for reflector. I must say though the thing is a little more complicated. Not quite so simple as you indicated because we can tolerate about three times the distortion in the lens surface as a mirror surface for the same fault in the image of the final result. So. Actually the way it works out the reflector is the more sensitive to departure from perfect form.
But as far as most of our amateur astronomer friends are concerned the possibility of having a telescope at all rests with it being economically possible and there is a reflector come then. There are many people who make a very very fine telescopes and there are even clubs. People manage together for that. To that end in their professional realm the use of Telus large telescopes almost demands a refractor reflector type and the biggest refractor in the world as a lens not to have a 40 inch diameter lens. But as far as anyone can see is about the top and efficiency use of a large lens and the possibility of using plastics to make larger lenses. I suppose so for non astronomical use. But from what I know about the demands made upon an astronomical telescope I can't see where plastic could ever measure up to it. Being flexible and subject to deform the. Importance of analysis is a spectrum analysis also makes the reflector are very good for
professional work because the light is fed into the spectroscope slit without any preliminary dispersion so to speak and somewhat simplifies the analysis of the spectrum. At least it makes it easier to get a good spectrum. It seems to me there's another difficulty with reflectors as I think about it if the light is reflected back along the telescope to be detected. So stick your head up to the edge of it some are over just to see the image which is being formed and also William Herschel actually did that he tells of the big mirror a little bit and stuck his head over the edge of the tube at the top and then look down with an eyepiece but the Newton form regional and form. He was a small mirror to throw the light off at a right angle and the hole in the side of the tube allowed the eyepiece to be set in to be within the unified image. That does cut out a little of the light. A small mirror at the upper end generally will cut out about 15 percent of the incoming light. No images formed however many people wonder why you don't see
that little black spot in the middle of the moon. You see when you focus on the moon you're focusing on something with light coming from a quarter million miles away. And if you just stick somethings in Italy you know a line of vision nearby you're not going to see that you'll lose some light but you won't see it any more than you would see a mosquito or fly that satellite telescope lens while you're using it. What about a bird flying by maybe a mile away. Well that would begin to be and I have seen birds insects fly across the moon by being far enough away they can be in focus at the same time produce a small silhouette on the surface of the thing being looked at mine and in photographic terms and the telescope has a large depth of focus. Oh yes yes anything from several miles to infinity. There are other modifications of the telescope principle that are used for astronomy the
focal length is important in securing a cation long focal lengths behind the unification. There is more to the power of a telescope and just one a following but simple fact that objects are shown on a large scale is advantageous and the other hand that long focal length will make the tube and the mounting of the telescope clumsy so very often the caster grain form is used in which a mirror with a convex surface is thicker in the middle and the edges whereas the main There is thinner in the middle and the edges. Well this convex mirror is set at the upper end of the tube and sends the converging light back toward the main mirror and through a hole in the middle of it. This has the effect very much as an OT telephoto lens to use in photography. The distance through which the light beam travels before it's focused is increased. But they affect you
focal length is increased much more so that the telescope with a tube only five or six feet long may have the unifying properties of a telescope with the focal distance of 15 20 or 30 feet. You can be chosen more or less at will. That is always a form in which the big reflector telescopes can be used for certain kinds of work. Of course this is power in the sense you use it first for magnification. But I think you mentioned that the largest refractor was 40 inches in diameter. Therefore I suppose it really doesn't compare in light gathering power although it may be equal in magnifying power. That's important point to realize the magnifying power can be pretty well chosen and will particularly by just using more and more powerful eye pieces but the on a certain identifying power of the images becomes very washed out and weak light gathering power is much more important for many purposes. It's more important for example that the
200 inch telescope has a mirror about 17 feet in diameter than that its focal length is something like six hundred sixty six inches. By the way our photographic friends over perceive that the 200 inch telescope on Palomar has a ratio of aperture to focal length of three and a third the one which makes it highspeed in anybody's language. Sounds like a good lens. I mean mirror. And by the way many people talk about the great lens of this or that telescope when actually they're speaking of a mirror. The telescope is a special modification and she combines principles of reflection and refraction. I'm afraid we won't have an opportunity to go into that in detail here but just briefly before covering a wide area of the sky photographic lenses have been used by strong armers for a long time. That is to say lenses such as were used for portrait photography years ago and then they had their own types
designed for them. But there are always certain disadvantages to be found no matter how carefully those great photographic lenses are made. Very clever German Bernard Schmidt about 20 years ago invented a completely new principle in which a mirror and I very week correcting lens were combined. And these telescopes are able to cover with perfect definition three areas of the sky like a whole constellation of such a Terry as one shot or as a big pair refractor or reflector would only cover one star or star cluster. The same exposure. So these so-called cameras are of very great importance in modern astronomical work. I've noticed in pictures of telescopes the telescope always seems to be surrounded by or mounted in or mounted on quite a complicated apparatus. It is this necessary and if so how will amateurs do about it how do they get these
fancy mounting. The best way to see how necessary it is is to take a telescope magnifying perhaps 100 times and point it to the moon and see how far you'll get by us holding it. If you can't hold it steady with your hand you might try sitting in a rocking chair or holding it against the door post conclusion to soon reach that something better is needed. The difficulty is that the rotation of the earth causes objects in the sky to turn apparently over our heads and we must continually re point our telescope. This is done by a so-called equatorial mounting which has on an axis or axle aligned with the earth's axis of rotation and turn in opposite direction at the same rate. So any telescope mounted to this in the proper way will point continually to one object say a star group of stars. As long as the driving mechanism or Clockwork is kept gauged the mountings are have various forms depending on the type of telescope. We have the
usual crossed type that is to say one where there are two axles at right angles is called a German type of mining it's the most commonly seen and measured probably by from whole Fearn called the German knowing ever since then there are types of markings where the telescope is cradled between two supports called a yoke type of mounting. Yes very very rigidity rigidity is important you see with the have a high and then a fine power there can't be any vibrations. This is certainly true I at one time owned a pair of binoculars which were only 8 power and I could hardly hold them still nuff to make any use any real use of them. A good telescope is of no use whatever on unless properly mounted especially if a certain accessory devices are to be used. I'm thinking particularly of the type we call photographic For example one then want to photograph the moon. Why not just point a camera into the end of the telescope
and snap a picture. Well that can be done and some result obtained but it's far better to have a well mounted telescope with some kind of a tracking device. You can mount your camera then a certain way that gives the best result in the hollow move along when you see the conditions set up just right everything said Just so you can snap your shutter. Then of course in photographing faint star story bodies clusters of stars and nebulae a snapshot of all do you have to expose minutes or hours in which case the. Martin must keep the alignment on the photographic film precise with a little help from the side of the astronomer looking to a guiding telescope. These exposures ever run longer than can be done in one night. Yes and sometimes the astronomer will simply cover up the film but everything said next day or next night rather a report and start all
over again and accumulate several nights of exposure on one film. Is it difficult to report to have a telescope aimed exactly the same point of the sky where it was before. It's not difficult to get it in the general vicinity and pick up the same field of stars but then to get the so-called guide star which is the reference for keeping everything straight exactly on the same part of the film it is quite a delicate task but it can be achieved when it's done regularly where long exposures on faint objects are required. It sounds like the kind of thing which would be very difficult. Yes because the image of the star is just a few hundred to the millimeter in diameter that much on the finished film. And you can't have much of a displacement between exposures without producing a double image. You mean even with all that magnification it's still only that large. Well a star though a very large body physically is that at enormous distance and the telescope produces a very tiny point of light. You spoke of accessories and mentioned photographic accessories.
We've discussed spectroscope so I presume spectroscope were also used in conjunction with the school. Yes and they're called spectrographs generally because rather than looking through the spectrum we simply photograph the dispersed light pattern of spectrum lines with some kind of a comparison spectrum for reference and that constitutes a very important activity. You see we're talking about the astronomers toolkit basically and his toolkit consists of telescopes of various kinds photographic attachments and accessories and spectroscopic devices over spectrographic as they should be called the spectrographs become very large when certain kinds of work have to be done such as analysis of the light of very bright stars. But the simplest of all spectroscopic accessories is merely a prism not in front of the telescope the light of the star is dispersed into a spectrum which is not white enough however to show any lines by just letting the spectrum go
back and forth. At right angles to the direction the light is spread out by the prism a little spectrum band is produced and on it can be seen spectral lines the star image acts as its own slit. This is called the objective prism and has the advantage of giving stellar spectra wholesale a whole cluster of stars can be analyzed. It has the disadvantage of allowing no comparison spectrum for chemical analysis. Well I suppose also it has in a sense a disadvantage in the sense that any prism which could be placed in front of the lens of a telescope would have to be pretty large and it couldn't be very thick then. Well a come large biggest one I know is 24 inches in diameter and it does not have a very steep angle so this is not a dispersion a small that is to say not much detail can be seen in the spectrum. But for certain kinds of work it definitely has great use. So all all must be available to the astronomer. Whether he's working on a bright star or a faint star will depend whether he uses one kind of a
spectroscope or spectrograph or another. Well I can see that if if some stars or some star fields require exposures of several nights if you then take the light from such a star and spread it out into a spectrum the exposure time is either going to be lengthened still further or else you're going to have to use something like a photoelectric cell which will measure very very faint amounts of light even much less than a photographic plate. Yes before we just dimension the electric cell I would say that all sorts of ingenuity has been expended in making the spectrographs more and more efficient so that these very faint objects can be registered without running an exposure time into weeks or months. Well how's a done I can think. Well you know I make more sensitive film more sensitive film and using the Schmidt principle which makes it possible to get out area very very high speed camera effect and such things as that. The full electric principle is extremely important as
part of the astronomers toolkit for electricity simply involves getting off of the electrons by some metal film. You know vacuum when light strikes it and the amount of electricity emitted is a measure of the intensity of the light. This is the beginning of a wonderful series of things at the most important being the detection of a very very small changes in the light of a star so called variable stars can be detected where they would not be known. If all we had were photographic or visual methods of comparison to getting now so that qualified astronomers also the good radio technician because certain parts of it for electric attachment telescope looked like nothing more or less to me than I very complicated radio. You've been listening to a discussion between Leoni Sellon a lecturer in astronomy at the Morrison Planetarium in San Francisco and John Hopkins an inquisitive layman on telescopes on excess arrays. The astronomers toolkit
this there is astronomy for the layman was recorded at radio station KPFA in Berkeley with the cooperation of the California Academy of Sciences. This is the on AB network.
Astronomy for the layman
Telescopes and accessories
Producing Organization
pacifica radio
KPFA (Radio station : Berkeley, Calif.)
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University of Maryland (College Park, Maryland)
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Episode Description
This program looks at "Telescopes and Accessories: The Astronomer's Tool Kit."
Series Description
Six programs on astronomy featuring Leon E. Salanave, lecturer in astronomy at Morrison Planetarium in San Francisco, and John Hopkins, interviewer. Produced with cooperation of California Academy of Sciences.
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Interviewee: Salanave, Leon E., 1917-
Interviewer: Hopkins, John
Producing Organization: pacifica radio
Producing Organization: KPFA (Radio station : Berkeley, Calif.)
AAPB Contributor Holdings
University of Maryland
Identifier: 55-20-4 (National Association of Educational Broadcasters)
Format: 1/4 inch audio tape
Duration: 00:29:26
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Chicago: “Astronomy for the layman; Telescopes and accessories,” 1955-06-12, University of Maryland, American Archive of Public Broadcasting (GBH and the Library of Congress), Boston, MA and Washington, DC, accessed April 25, 2024,
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