What is an astronomical telescope
The astronomical telescope
But now back to our actual topic, the astronomical telescope. Probably most of the amateur astronomers of my generation experimented with self-made refractors when they were teenagers: They bought cheap collective lenses for a few marks from the optician, put them in cardboard tubes and ultimately connected them more or less without wobbling to a tripod made of dad's camera equipment.
But even the first look through this simple telescope revealed serious optical errors: there was no question of a sharp, high-contrast image. The objects observed had an intense, bluish-red color fringing. But that's not all of the iniquity: If the image in the center of the field of view was still reasonably usable (if you got used to it), catastrophic blurring was noticed at the edge.
So how do these two most serious optical defects of simple converging lenses come about? The color error is also known as "chromatic aberration". The core of the matter here is white light, which is known to be composed of light of different wavelengths, which manifests itself in the form of different colors, the spectral colors.
Everyone has seen a rainbow before. It comes about when water droplets act like prisms in the atmosphere and break the sunlight down into its colors. And our converging lens apparently does this too, at least mainly for the colors red and blue.
Why? It was said earlier that the converging lens reunites the refracted rays of light, that is, deflected from their original direction, at the focal point. Unfortunately, the different wavelengths of light also have different focal lengths, so that, strictly speaking, there is no focal point, but a kind of "focal line". And since this problem is most serious for the colors blue and red, the so-called "secondary spectrum" occurs.
The second major error, the edge blurring or "spherical aberration" (which literally means "spherical error"), has practically the same cause as the color error: In plain language this means that a light beam that penetrates through the center of the lens causes something has a longer focal length than the light that passes through the lens near the edge, which in turn leads to blurred images near the edge of the field of view.
Of course, the pioneers of astronomical observation with optical amplification wondered how best to counteract lens defects. It soon became clear that both optical errors decrease significantly the longer the focal length of the lens is chosen compared to its diameter.
What does this mean in concrete terms? So do I have z. B. a refractor with a lens diameter of 10 centimeters (a perfectly acceptable amateur instrument) and a focal length of 100 centimeters, this will have a considerably better image quality than one that has a focal length of only 50 centimeters with the same size optics. In other words: the smaller the ratio between lens diameter and lens focal length ("aperture ratio"), the better the image quality. With our telescope with a 10 centimeter lens diameter and 100 centimeter focal length, the aperture ratio would be 1/10 (i.e. 1 to 10) because the focal length of the lens is 10 times longer than its own diameter. With a focal length of only 50 centimeters, the aperture ratio would be considerably larger, namely 1/5.
Anyone who may still have an old SLR camera where all settings had to be made manually can still observe the effects described "live": The image created by the camera lens becomes sharper the higher the f-number is selected, i.e. the more the camera lens is stopped down .
But no rose without thorns: stopping down the lens leads to darker images, and this is of course also the case with our refractor. Thus, with increasing focal length, the light intensity of the lens also decreases, i. H. no faintly glowing objects can be recognized, which is not particularly useful for observing star clusters and nebulae, which are not among the brightest celestial objects anyway.
In the 17th century there were quite adventurous forms of the lens telescope. Due to their long focal lengths of up to 20 meters (!) With a lens diameter of perhaps 20 centimeters, it was difficult to find a long enough tube. In many cases, this was completely dispensed with: the objective was simply mounted on a long mast so that it could be adjusted vertically and laterally, while the eyepiece was attached to another device behind the focal point. Those were the famous "aerial telescopes", but observation with these, as one can easily see, was not particularly fruitful.
It was therefore necessary to find suitable means of eliminating the optical defects of the lenses. Nevertheless, it took almost 150 years after the invention of the telescope until the error was finally overcome.
But what happened in the meantime? The imperfections of the lenses literally cried out for alternatives. And here we meet the previously mentioned English astronomer Isaac Newton. He is best known for his discovery of the law of gravitation and its three axioms. But he was also the first who in 1668 replaced the objective lens with a concave mirror (i.e. a concave mirror that was curved inward on one side) and thus invented the reflecting telescope.
The concave mirror (shaped roughly like the shaving mirror in the bathroom at home) collects light rays just like the lens and therefore also has a focal point (apart from a special type that will be discussed later). The subtle difference, however, is that the mirror makes no difference between the wavelengths of light: All are united in one point, so that there are no color errors, because the principle of image creation in the mirror is reflection. Therefore one speaks of "reflectors" in the case of reflector telescopes. However, spherical aberration is also known in mirrors.
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