Telescope Basics

The fundamental function of any telescope is to act as a light bucket. With the exception of high energy cosmic rays and neutrinos, what we know of the Universe beyond the solar system (and the majority of what we learn within the solar system) comes to us in the form of light over the entire range of the electromagnetic spectrum. If we view the particles of light, photons, as elements of a message, perhaps the letters of the electromagnetic alphabet, our ability to read, to interpret, and to understand the message is enhanced the more photons we collect. The stronger the signal, the easier it is to read. The stronger the signal, the greater our ability to dissect it with greater resolution in time, in space, and in wavelength. Since the distances to most astronomical objects are fixed, our primary means of strengthening the signal is to collect a larger fraction of the light from the source that intercepts the Earth on its journey through the Universe. Without the aid of a telescope, we are left with the amount of light we can collect in a fraction of a second using only the area contained in the pupil of our eye. If the photon flux (number of photons per unit area) from a source at the distance of the Earth is fixed, we boost the signal by increasing the collecting area of our telescope, i.e., we build a bigger bucket. Moreover, we can enhance the signal even more by exchanging our eye for a detector such as a photograhic emulsion or a CCD imager that has the capability of collecting and counting the photons for an extended time period.

Though Galileo(1564-1642)did not invent the telescope and probably was not the first individual to look at the sky with one of the first, crude refracting systems, his improvements to the quality of the lenses used and the discoveries resulting from his application of the instrument to the celestial sphere triggered an observational revolution in astronomy that continues even today, as astronomers attempt to squeeze more photons, more information, and more insight from objects at greater and greater distances.

While the purpose of a telescope is to intercept a greater fraction of the light directed at the Earth, the simplest function of the telescope design is to redirect, to refocus the light to an area compatible with the size of the detector being used to count and/or analyze the signal. In the case of the human eye, this reduces to the opening area of the pupil; for a modern telescope, this usually refers to the area of the CCD chip within the detector. In either case, the telescope requires a design that makes use of some technique with the facility for changing the direction of the light rays from parallel beams entering the telescope to converging rays upon approach to the detector. The two dominant methodologies for refocussing the light make use of two basic wave properties of light, refraction and reflection.

Refraction refers to the fact that a wavefront, upon passage from a medium of one density to another, e.g., air to water, will change direction if the wave strikes the boundary at any alignment other than parallel to the boundary. The ultimate source of the deviation is the changing speed of the wave in different media; higher density implies a slower wave speed and a greater alteration of the light path. In going from less dense to more dense, the light ray bends so that it is closer to a line drawn orthogonal to the boundary on the dense side of the medium. In passing from more to less dense, the ray direction is altered so that it passes farther from the orthogonal line on the less dense side of the boundary. The triangular shape of a prism is designed so that light passing into and out of the prism has its direction altered in the same way at both boundaries, thereby doubling the effect of the refraction.

The bottom line is that one can refocus light from a large area to a point using a lens designed so that the rays striking the outer edges of the lens strike at a more oblique angle while those passing through the center of the lens strike it perpendicular to the surface, leading to the classic double convex shape associated with simple lenses.

The first and most basic flaw in refracting telescopes derives from the association most people have with prisms, the ability to separate light by color or wavelength. Light has wave-particle duality (as do all fundamental particles in the universe): it acts like both a wave and a particle. Refraction is a wave property that is wavelength dependent; the shorter the wavelength of the wave striking the transition boundary between two media, the greater the change in direction of the wave. Since we detect the wavelength of optical light as light of different colors, (red, orange, yellow, green, blue, indigo, and violet ranging from 7000 A to 3000 A, where 1 A = 10-8 cm), light of mixed wavelength, white light, is sorted by color through refraction. Among optical wavlengths, red light is bent the least while violet light is refracted the most. While this is wonderful if one wishes, as in spectroscopy, to sort the light into appropriate wavelength bins, it means that it is impossible to focus light of more than one color using a simple convex lens. If the lens is adjusted so that the red light is in focus, optical light of decreasing wavelength is increasingly out of focus. If one focuses on the violet light, the optical band is increasingly out of focus toward increasing wavelength. This problem, commonly called chromatic aberration, can be corrected at multiple wavelengths by adding corrective lenses with different indices of refraction, each additional optical path layer drives up the cost of making the lens, increases the weight of the lens, and reduces the fraction of light that will pass unhindered through the optical system, i.e., it lowers the number of photons the telescope will send to the detector.

Additional issues with refracting telescopes include:

a) The light passes through the lens, so any ideal lens must be free of imperfections, not only only the surface but within the lens itself;

b) The lens must be ground/polished on two sides to guarantee that the light of a given wavelength crossing the lens at all points comes to focus at the same point at the desired distance behind the lens. The order-of-magnitude estimate for the precision in the shape of the surface is that the deviations from the desired shape must be smaller than the wavelength of light the lens will be used to study. In fact, the more common standard is for the deviations to be smaller than 1/10th of a wavelength. At optical wavlengths, this implies that the defects must be smaller than 500 A or 0.005 micons; on a lens 1 meter in diameter, this is an extraordinary challenge.

c) In order to have a reasonable focal length, the curvature of the lens ensures that the thickness of the center of the lens is a non-negligible fraction of the diameter of the lens, exacerbating (a) and generating, for large-aperture (diameter) refractors a significant weight problem with lenses weighing hundreds of pounds. Since the lens is normally held along its edges at the top end of a large metallic tube due to the focal length, maintaining the optical path/focal plane over years of extended use can be difficult, especially if the telescope tube begins to sag due to metal fatigue.

d) Because the light must pass through the lens, the transparency of the lens as a function of wavelength can seriously impact what one can observe, all else being equal. Plain glass is increasingly opaque at shorter wavelengths, in the violet and ultraviolet, making shorter wavelength observations with older refractors an impossibility.

The alternative to refraction is reflection, a property of light that allows one to avoid or correct virtually every disadvantage noted above for refractors. The principle of reflection simply states that a wave that strikes a surface at a given angle to the normal to that surface reflects back at exactly the same angle to the normal. Better yet, the rule applies independent of wavelength, so chromatic aberration disappears as an issue. Next, the light doesn't pass through the mirror, it merely reflects off the surface. Thus, any imperfections in the structure of the mirror below the surface are irrelevant. The reflective surface of the mirror will ideally be the mathematical shape of a parabola, meeting the same accuracy specifications as a lens, but only on one side. Because light doesn't pass through the mirror, it can be supported along the entire base, not just the edges, and be held at the bottom of the telescope tube, rather than the top. The thickness of the mirror is set by stiffness requirements rather than optics. The wavelength response of the mirror is set by the material used to create the reflective surface, not the mirror itself. The material used to construct the mirror does matter in defining the thermal response of the mirror and thus in defining the shape of the mirror over time as temperature varies. Finally, larger mirrors can be constructed in segments controlled by actuators from below that adjust the position of each mirror to collectively ensure that the composite surface has the desired shape.

Given all the advantages noted above, the obvious question is why refracting telescopes dominated astronomy until the early part of the century despite the development of the first reflecting telescope by Newton (1642-1727) in the 17th century. Here again, Newton gets credit for promoting this design, but admitted that he got the idea from a book on optics by James Gregory (Park 1997). The answer is a complex mixture of rational choice and irrational bias. Lens/glass technology developed at a faster pace than mirrors. Attempts to construct reflective surfaces for telescopes were initially based upon the use metallic surfaces that were difficult to shape and, unlike glass, suffered decay of the reflective surface over time. In simple terms, reflecting telescopes acquired a reputation for being less effective and less reliable for astronomical research, a view retained by the observational community long after it had outlived its relevance. (It is interesting to note that this derogatory view of reflectors began with the first reflector developed by Newton. When presented to the Royal Society, it was met with ridicule by Robert Hooke (1635-1703), Curator of Experiments for the Royal Society, as being inferior to the little lens he carried around in his pocket. This slight was just one of the many sources of personal and professional antagonism between these two brilliant personalities that lasted a lifetime and beyond. (Hirshfeld 2001)) Fortunately, once the effectiveness of improved reflector technology was demonstrated, the switch from refractors to reflectors as the primary telescope technology was rapid and complete. No large-aperture refracting telescopes have been built for research in over a century

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