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Transcript
The Historical Evolution of the Telescope Diego Benavides 0241824 Abstract The telescope is a fundamental tool in the successful study of the universe. Since its inception, it has undergone many redesigns and improvements through engineering practices. This paper discusses these evolutions while considering its origins and functions. Introduction The study of the universe is observational, and depends to a large degree on the quality of a specific tool used for observation: the telescope. Without it, astronomy as a science could not exist [1]. A telescope gathers and focuses radiation from the electromagnetic spectrum, allowing a large amount of data to be compiled and analyzed from objects at large distances from the point of observation [3]. The data received may span the entire range of the electromagnetic spectrum, and thus is not limited to optical images, although such a reference is popular. Many types of telescopes are often employed in formal astronomical studies [2]. Origins and Evolution The term telescope itself comes from a combination of Greek words meaning “far-seeing”, and its primary purpose facilitates just that. It is contended that the first of such optical devices crafted for remote observation dates back more than three thousand years, desgined by the Assyrians. The credit for the first formal invention of the optical telescope, however, is bestowed upon a Dutch eyeglass maker in the 1600s, Hans Lippershey. Shortly thereafter, a telescope would be built by a revolutionary of science and used for cosmological study. Galileo Galilei constructed what is now referred to as a Galilean telescope. This telescope uses the property of light refraction to collect light at a convex objective lens and focus it on a concave eye lens [3]. Such a design finds use even outside of astronomy, in areas such as photography or ophthalmology [4]. Since the Galilean telescope, there have been various improvements in optical instruments, with the creation and refinements of refracting telescopes, reflecting telescopes (Newtonian scopes) and combinations of the two [2]. But it wasn’t until just after World War II that new types of telescopes would give cosmologists a drastically different, yet scientifically relevant, look at the universe. The radio telescope emerged in 1932, after Karl Jansky designed receivers with directive aerials sensitive enough to monitor background noise levels. The evolution of radio telescopes has allowed for finer detail resolution, assisted in part by utilizing an array of telescopes [1]. Submitted in partial fulfillment of Engineering 4H03 (Professor A.A. Harms) In June of 1962, an Aerobee rocket was sent to space containing a Geiger counter for detecting new radiation sources. Following this event, a series of satellites were developed in order to detect X-Rays, gamma rays, and UV radiation [1]. Other advances in telescope technologies include the Hubble Space Telescope and multiple mirror observatories. Launched in 1990, the Hubble Space Telescope orbits at 600 kilometers above the Earth’s surface and completes a full orbit in approximately 97 minutes. This telescope has allowed astronomers to resolve images from over twelve billion light years away [5]. Multiple mirror telescopes (MMT) are Earth oriented telescopes using an array of identical mirrors to increase their resolution while eliminating the cost associated in construction. Research and development in MMTs have allowed arrays to overcome the drawback of image distortion due to atmospheric turbulence [6]. Theory and Function Optical telescopes fall into three categories: reflecting, refracting and catadioptric. The latter uses a combination of techniques employed by reflecting and refracting telescopes. The refracting scope was the first telescope to be constructed, and operates by using lenses to refract light. As light propagates from one medium to another, it undergoes an energy change known as refraction. If the incident light is not orthogonal to the boundary of the new medium, the light will shift direction. By employing a series of two lenses, a convex objective and concave eye, light entering the objective may be narrowly focused to a point adequate for observation [1]. This is demonstrated in figure 2a. Figure 2: Select optical telescopes In 1670, Sir Isaac Newton successfully constructed a reflecting telescope in an attempt to solve the chromatic aberration associated with the refracting scope. The reflecting telescope uses a combination of flat and curved mirrors to focus light. This is demonstrated in figure 2b. The focal length is set by selecting a distance from the mirror to the focal plane during construction of the device [7]. Unlike the refracting telescope, there is no glass required in the manufacture of a reflecting scope, and thus imposes no limitations as to the size of the scope that can be constructed. Another reflecting scope, the Cassegrain (figure 2c), uses a parabolic primary mirror and hyperboidal secondary mirror. The purpose of this design is to reduce the diffraction effects due to support issues associated with the Newtonian design [3]. X-ray telescopes operate using charge-coupled device (CCD) detectors, microcalirometers or transition edge sensors. Using a CCD, an image is obtained over an exposure time by allowing single photons to charge individual CCD pixels. X-ray photons are capable of producing very large charges in a CCD sensor. Microcalorimters, however, are only capable of detecting photons one at a time. Despite this limitation, the lack of x-ray photons directed towards earth allows microcalorimeters to be suitable for astronomical observation. Transition edge sensors are essentially improved microcalorimeters where super cooled metals are kept near transition temperatures. An x-ray photon will cause a transition and allow the instrument to record the corresponding energy level of the incident x-ray [1]. Gamma ray telescopes are typically application or mission specific. As an example, NASA’s Compton Gamma Ray Observatory, a space telescope, contains no less than four instruments for the detection of gamma rays: a burst and transient source, an oriented scintillation spectrometer, an imaging Compton telescope, and an energetic gamma ray telescope [5]. Ultraviolet telescopes are considered to be a subset of optical astronomy, however require much greater optics, resolution and instrument tuning to detect [1]. Summary The understanding and formal study of the universe has been long facilitated and perhaps made entirely possible due to the creation of a single device. During this time the telescope has undergone a series of changes and improvements, which may be shown in the form of a heterogeneous progression as follows: refracting reflecting radio telescope telescope telescope lens x ray space gamma ray UV telescope telescope telescope telescope Although not completely linear, this progression demonstrates the continual feedback that each previous scope design had on the next. Additionally, it is worth noting that many of these scopes are used in a complementary fashion by astronomers and cosmologists when studying the universe. To conclude, the following table summarizes some of the differences between select telescope types. Table 1: Sample Telescope Comparison Telescope Type How it Works Refracting telescope Reflecting telescope Space telescope Multi-Mirror Telescope Light refraction through lenses Reflection and concentration of light using mirrors Various scope technologies in one package. Identical array of telescopes, allowing images to be focused in a central location. Benefits Low maintenance Inexpensive to produce Thickness and internal imperfections not as important Mirror can be supported uniformly No limits in reflector size Free from stray light Avoids atmospheric turbulence Reduced size in construction Reduced construction time Reduced cost Disadvantages Requires cool down time to prevent lens distortion Chromatic aberration in bright images Glass requirements in size, weight and cost introduce limits on size Maintenance required on mirror surfaces Requires cool down time to prevent mirror distortion Suffers from coma Distortion of field of view Extremely expensive Susceptible to temperature distortions Drive motors required to move the array are costly References [1] Blackwell, Basil & New Scientist (1984). Observing the Universe. Oxford: Basil Blackwell Ltd. [2] Binney, James & Merrifield, Michael (1998). Galactic Astronomy. New Jersey: Princeton University Press [3] Telescope. (n.d.). Wikipedia. Retrieved October 23, 2006, from http://en.wikipedia.org/wiki/Telescope [4] Cheng, Desmond & Woo, George C. (2000). Technical Note: The calibration of a 2.5x Galilean focusable telescope as an optometer for refraction. Ophthalmic and Physiological Optics 20-4, 342-347 [5] NASA (2006) Retrieved October 25, 2006, from http://www.nasa.gov/ [6] Lloyd-Hart, Michael (2003). Taking the Twinkle out of Starlight. IEEE Spectrum 40-12, 22-29 [7] Astronomy Magazine. (2006) Retrieved October 25, 2006, from http://www.astronomy.com