Ken Tapping, January 31, 2013
On a clear, dark night, our unaided eyes can see up to 2000 stars, and possibly the Milky Way as a silvery arch. If we then observe using binoculars, we will see many more stars, and the Milky Way will resolve into countless faint stars and silvery clouds of gas and dust. This has nothing to do with the magnification offered by the binoculars, but simply because they collect more light, making fainter things visible. Depending upon our age, health and other factors, our dark-adapted eyes may have pupils opened up to maybe 5mm or so. If we then observe using a pair of 7x50 binoculars, which have a magnification of seven times and objective lenses 50mm in diameter, you will be collecting about 100 times more light. This is contributed to the disaster with the Titanic, because the lookouts did not have their binoculars with them on that fateful night.
Most of the astronomical objects we observe are faint, so our telescopes are designed to collect as much light as funding and technology allow. Many amateur astronomers have telescopes with objective lenses up to 15cm in diameter. The largest objective lens used in astronomy has a diameter of almost a metre. However, making large telescopes using lenses as objectives runs into a serious problem. They have to be convex, which means thicker in the middle and thin around the edges. Big lenses tend to be very thick in the middle. This makes them heavy, and this weight has to be supported at the edges, where they are thin. Lenses larger than about a metre in diameter can break under their own weight. Moreover, because the lenses have to let the light go through them, we cannot add supports to take the weight. Fortunately, Isaac Newton showed us a solution. Instead of collecting the light and forming an image using a convex lens, we can do it using a concave mirror. This offers huge advantages. Firstly, because the light does not go through, we can sit the mirror on a firm support structure. Secondly, whether a lens or mirror, the objective is the heaviest component in the telescope. Lenses need to be at the top end of the telescope; mirrors can be at the bottom, where the weight is less of a problem. Mirrors make it possible for amateurs to have large telescopes that are affordable.
Mirrors made of a single chunk of glass or similar material can be as big as 5m or so. Above that size they get very heavy and tend to sag out of shape under their own weight. However now we have solutions to that problem. We can make mirrors thin and light, and use computer-controlled actuators to continually keep them in shape. We can even make the mirrors out of segments which together add up to a big mirror. These developments have pushed the size limit up to the point where other issues such as cost determine what we can do. Telescopes with mirrors up to 10m in diameter are now in use and an instrument with a 30m mirror is under development. If only Isaac Newton, and pioneer astronomers like the Herschels, Cassini and Galileo could see the “Light Buckets” we have available to us now!
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