The Developing Telescope

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Galileo's first astronomical observations demonstrated how even a small telescope can exceed the capabilities of the human eye in many respects. The telescope collects much more light than the eye. This makes it possible to see much fainter objects than by the naked eye. For example, Galileo saw in the direction of the Pleiades 36 stars instead of the usual 6. Photographs by modern telescopes show hundreds of stars in this stellar group. The big lens also makes resolution much better. This means that while two close-by stars are seen as one dot of light by the naked eye, the telescope shows them as separate. The ability to collect more light than the eye and the improved resolution allow one to see much more structure and fainter objects in the starry sky. The improved resolution also makes the measurements of stellar positions (their coordinates) more accurate. This proved crucial for the determination of stellar distances as we will discuss in Chap. 8.2

The first telescopes suffered from poor image quality. Simple lenses are hampered by a color error (chromatic aberration), which means that rays of light of different colors do not focus onto the same point and hence the image of a star is an indistinct spot surrounded by colored circles. The lens acts a bit like a prism. This problem was greatly improved in the eighteenth century with the invention of achromatic lenses. Before that a remedy was to make very long telescopes. When the ratio between the diameter of the objective lens and the focal length is small, the rays of light are only slightly refracted, the color error is smaller and the image sharper. Figure 7.4 shows such long telescopes in Paris Observatory.

Christiaan Huygens also built telescopes, the biggest of which had a length of 123 ft. or 37 m. It was not possible to make such gigantic solid tubes and one had to put the objective lens on the top of a pole or on the edge of a roof and to control its movements with a long rope, while standing oneself on the ground and keeping the ocular before one's eye. It must have been quite inconvenient to follow the revolving starry sky with such instruments, but nevertheless interesting observations were made. For example, Huygens found that the curious appendages of Saturn, which Galileo had noticed, were actually a thin flat disc around the planet in the equatorial plane.

2 Kepler improved Galileo's telescope with a design still used today. In the "Keplerian" telescope, a large objective lens forms an image of a celestial object at a large distance from the objective. The detail and brightness of this image are then examined by a magnifying convex eyepiece lens.

Fig. 7.4 "Aerial telescopes" of Paris Observatory in the seventeenth century. Even though inconvenient to use, such instruments led to new astronomical discoveries (photo credit: Georges Paturel)

Another famous observer during the era of the long lens telescopes was the Polish Johann Hevelius (1611-1687) who had his own observatory in Danzig, the first one in the world complete with a telescope. His wife Elisabeth made observations too. Hevelius' record-sized instrument was 150 ft. or 45 m long. Its complicated system of ropes and long rods reminded one of the rigging of a sailing boat and certainly required seaman's skills to handle! With his telescopes Hevelius studied the surface of the Moon and drew fine maps of it. Our habit of speaking about the "seas" on the Moon goes back to Hevelius. We now know these to be depressions filled with solidified lava.

The development in the eighteenth century of achromatic lens telescopes in which color fringes are greatly reduced ended the era of the long lens telescopes. Large diameter objective lens telescopes up to about a meter in diameter were built through the 1800s but another kind of telescope was developed that gradually came to dominate the research field today. In 1671, Isaac Newton built the first reflecting telescope where a concave curved mirror gathers the light, instead of the lens as in the refractors. His experiments with glass prisms and refracted colors had led him to the conclusion that the color error in refracting telescopes is here to stay. And this led him to consider an alternative way to focus rays of light into one point by reflection which is the same for light of all colors. The image formed at the focus of the mirror does not show color fringes. The concave mirror surface must be a parabola so that all the rays, close to the center of the mirror as well as near its edge, will converge into the common focus. Newton's telescope, built with his own hands,

Fig. 7.5 The 3.5-m mirror made by the Finnish optical firm Opteon for the European Herschel Space Telescope, together with the team of specialists. The mirror surface had to be polished to make it so extremely smooth that its "bumps" are smaller than a few thousandths of a millimeter. This is the largest space telescope built up to now. From left to right A. Sillanpaa, T. Lappalainen, D. Pierrot (Astrium), T. Korhonen (the director of Opteon), M. Pasanen, P. Keinanen (Credit: Opteon)

Fig. 7.5 The 3.5-m mirror made by the Finnish optical firm Opteon for the European Herschel Space Telescope, together with the team of specialists. The mirror surface had to be polished to make it so extremely smooth that its "bumps" are smaller than a few thousandths of a millimeter. This is the largest space telescope built up to now. From left to right A. Sillanpaa, T. Lappalainen, D. Pierrot (Astrium), T. Korhonen (the director of Opteon), M. Pasanen, P. Keinanen (Credit: Opteon)

has survived. Its mirror, made from metal, had a diameter of 3.5 cm. Newton used a small flat secondary mirror to direct the light to side through a hole in the tube of the telescope where the eyepiece then magnified it.

Large modern reflecting telescopes often have a hole in the center of the main mirror, through which the light reflected from the secondary mirror goes into a detector. The detector is nowadays, instead of the eye or a photographic plate, a highly light-sensitive CCD camera or a spectrograph. This so-called Cassegrain typereflec-tor was invented by a Frenchman, G. Cassegrain (of whom little is known) shortly after Newton's reflector.3

An important plus for the reflecting telescope is that its main mirror can be made much larger than the glass lens of the refractor, allowing a large light gathering power and observations of very faint and distant objects. The mirror can be supported over its entire back while the objective lens can only be supported at the edges. Once mirror silvering and later aluminizing was developed, glass could be

3 In fact, Cassegrain's telescope was an improvement over one suggested by James Gregory before Newton. Gregory did not actually build his version. In the Cassegrain telescope, the secondary mirror is convex which results in a short telescope.

used rather than the metal Newton used. The glass does not even have to be transparent. Overall a color-free and larger reflecting telescope can be built for the same price as a smaller lens telescope.

Even though the reflecting telescope started to dominate astronomy in the nineteenth century there were still many important tasks left for the lens telescope. It was better for accurate measurements of the positions of stars, once the problem of the chromatic aberration was reduced. This finally made possible the dream of measuring the distances to stars.

Today, telescopes are still more sophisticated. Besides visual light, they operate at x-ray, ultraviolet, radio, and infrared wavelengths invisible to the eye. Some orbit in space, thus leaving behind the atmosphere which blurs optical images and absorbs radiation at most wavelengths (excepting visual light and radio waves). Figure 7.5 shows a big mirror made for a space telescope. For radio telescopes, one has a concave reflecting dish rather than a mirror as part of the telescope with a radio receiver at the focus. The long wavelengths of radio waves make their resolution much worse than that for the same size visual telescope, so the radio dishes are typically much larger, perhaps 100 m in diameter or more, much larger than the 10 m size of the largest visual telescopes today. Radio astronomers have learned to combine signals from separate dishes simulating a single dish comparable to the size of the Earth. These are called interferometers. With modern electronics even optical astronomers are doing this with several telescopes at the same observatory.

Finally, some modern telescopes are hardly recognizable as such. Devices have been constructed which have detected subatomic neutrino emissions from the Sun and a supernova. Gravitational wave detectors have been built to detect field variations from orbiting black holes or their formation in supernovas.

Indeed, this explorative spirit is strong in astronomy - one wants to look deeper and deeper in space, to see what nobody else has seen before. The discovery and further study of all those unexpected celestial bodies and cosmic phenomena require larger and larger telescopes.

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