Optical Telescopes
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Research telescopes that use ordinary (optical) light are basically light funnels. Their purpose is not so much to make things look bigger or closer, but to make them look brighter. Many astronomical objects are big enough to see with the naked eye; they just aren't bright enough. Therefore the most important feature of an optical telescope is the size of the area over which light is collected - the bigger the size, the more photons are collected, the brighter the image becomes. The first astronomical telescope was designed and used by Galileo Galilei. He used two lenses, one an objective lens to focus the light, and one an eyepiece. Such a telescope is called a refracting telescope. OUR IMAGE OF A REFRACTING TELESCOPE WILL GO IN HERE. An alternative was designed by Isaac Newton, which uses a mirror to bring the light to a focus. Such telescopes are called reflecting telescopes. OUR IMAGE OF A REFLECTING TELESCOPE WILL GO IN HERE. There are problems with both kinds of telescope. Refractors have the difficulty that lenses refract different wavelengths of light by different amounts, so the focus is at a different point for blue light than for red light - this is called chromatic aberration. Also, the objective lens must be held by the edges. This means that bigger more powerful lenses become difficult to deal with, since a huge piece of glass has to be held by a very thin edge. The biggest refractor ever successfully used for astronomical observations was the Rosse 72" telescope (needs a link). Almost all astronomical research telescopes are now reflecting telescopes. These can be supported all across the back of the primary mirror, and this has many advantages as the mirrors get large. The problem with a reflector is where to put your head (or photographic plate or detector). If you stare down at the mirror, you get in the way of the light coming from the star. The solution is generally to put another smaller "secondary" mirror in front of the primary mirror, to deflect the light to some other position where detectors can be placed. Various configurations of secondary and additional mirrors are in general use, such as prime, cassegrain and nasmyth focii. Very big telescopes sometimes use a prime focus cage where the secondary mirror ordinarily sits - the observer (or instrument) then rides around in this cage at the front end of the telescope while observations are being carried out. HERE WE PUT OUR FIGURES OF DIFFERENT TELESCOPE FOCII For almost half a century the most powerful optical telescope in the world was the Mt Palomar 200" (5 meter) telescope (needs a link!). This telescope, built in 1947 is a marvel of mid-20th century engineering. But within the past decade, technical advances have made it possible to make bigger mirrors. This has set off a huge burst of new telescope construction. There are now many telescopes bigger than the Palomar 200". One of the first of the new technology telescopes was the WIYN 3.5m described in the video included in your course material. While this telescope is actually smaller than the Palomar 200", it served as a proving ground for many of the technological innovations now in common use. Interestingly, the famous Hubble Space Telescope is not a particularly large telescope - it has only a 2.4m mirror, about half the diameter of the Palomar 200". The advantage of the HST is that it is above the atmosphere, and therefore does not have degraded resolution due to the Earth's atmosphere. The incredibly sharp images produced by Hubble (link) are due to its location. But there are many astronomical projects for which Hubble is too small - for these projects, astronomers continue to rely on ground-based optical telescopes. Another crucial element in carrying out observations is scheduling. For some kinds of observations, including many of the studies of black holes we are going to be talking about, observations must be carried out repeatedly to watch how the object changes with time. Since most telescopes schedule different projects are on different specific nights, it can be very difficult to arrange a half-hour observations once per night for several months. We carry out such observations with the YALO observatory, a relatively small (1m) telescope in Chile specifically designed for long-term monitoring projects. |
Astronomers measure resolution by a quantity known as seeing. The Hubble Space Telescope, which is above the atmosphere, has seeing about 10 times better (standard Hubble slide of a Hubble image of globular cluster next to similarly sized ground-based image) than a typical ground-based telescope (although in the early stages of the mission that performance was compromised by the infamous primary mirror problem). Recent advances in techniques such as adaptive optics have improved ground-based seeing to the point that under certain conditions IMAGES ALMOST AS GOOD AS THE SPACE TELESCOPE (get images from HST and WIYN of Ring Nebula at http://www.noao.edu/wiyn/wttm) can be obtained. |