Windows to the Universe Chapter 5 Telescopes: Windows to the Universe

5-1 Refraction and Image Formation Refraction: The bending of light as it crosses the boundary between two materials in which it travels at different speeds. Factors determining refraction: The relative speeds of light in the two materials. The angle the light strikes the interface.

Image: The visual counterpart of an object, formed by refraction or reflection of light from the object. Focal point (of a converging lens or mirror): The point at which light from a very distant object converges after being reflected or refracted. Focal length: The distance from the center of the lens or a mirror to its focal point.

Objects appear higher in the sky than they really are. As light passes from the vacuum of space into Earth's atmosphere it refracts. Objects appear higher in the sky than they really are. The Sun looks flattened when it is near the horizon. © Photodisc.

5-2 The Refracting Telescope Refracting telescopes use lenses. Objective lens (or objective): The main light gathering element of a telescope. It is also called the primary lens. Eyepiece: The magnifying lens (or combination of lenses) used to view the image formed by the objective of a telescope. A camera could be placed to capture the image instead of the eyepiece.

Chromatic Aberration Chromatic aberration: The defect of optical systems that results in light of different colors being focused at different places. Achromatic lens (or achromat): An optical element that has been corrected so that it is free of chromatic aberration.

5-3 The Powers of a Telescope Magnifying power Light-gathering power Resolving power Light-gathering and resolving powers are most important. The reasons for using large telescopes are related to these powers.

Angular Size and Magnifying Power Angular size (of an object): The angle between two lines drawn from the viewer to opposite sides of the object.

Magnifying power or magnification (of an instrument): The ratio of the angular size of an object when it is seen through the instrument to its angular size when seen with the naked eye. magnifying power = objective focal length / eyepiece focal length

As magnification increases, the field of view decreases. Field of view: The actual angular width of the scene viewed by an optical instrument. As magnification increases, the field of view decreases. Courtesy of NASA.

Light-Gathering Power Light-gathering power: A measure of the amount of light collected by an optical instrument. The main way to gather more light is with a larger objective. The amount of light gathered depends on the area of the objective. For a circular objective, the area is proportional to the diameter squared.

Resolving Power Diffraction: The spreading of light upon passing the edge of an object. The amount of diffraction through an opening depends on the wavelength () and the diameter (D) of the opening. In a telescope the objective is this opening. Courtesy of Karl Kuhn.

Resolving power (or resolution): The smallest angular separation detectable with an instrument. It is thus a measure of the instrument's ability to see detail. Courtesy of Don Pettit, ISS Expedition 6 Science Officer/NASA. Courtesy of David Richards

Turbulence in the Earth's atmosphere limits the resolution of large telescopes. Astronomical seeing: The blurring and twinkling of the image of an astronomical source caused by Earth's atmosphere. Seeing: The best possible angular resolution that can be achieved. This resolution limit is typically between 0.5 and 0.25 arcseconds.

To improve the resolution limit: The telescope can be placed in space. Use techniques such as adaptive optics or interferometry. Top right image uses adaptive optics. Lower right image does not. Courtesy of Gemini Observatory and Canada-France-Hawaii Telescope/Coelum/Jean-Charles Cuillandre.

5-4 The Reflecting Telescope An inwardly curved mirror is said to be concave and will bring rays of light to a focus, just like a lens.

Isaac Newton devised this arrangement to use a small mirror to deflect the rays out to an eyepiece.

Reasons to prefer reflecting over refracting telescopes: Fewer surfaces. An achromatic refractor requires two lenses, each with two surfaces. A mirror only has one. Reflectors automatically eliminate chromatic aberration. Lenses are never completely do. Mirrors only need a reflective surface. A lens is a thick piece of glass, may have imperfections, and absorbs some light. Lens can only be supported from the edges, but mirrors from their back. This provides better support and allows deformations to be corrected.

Large Optical Telescopes Newtonian focus: A reflecting telescope in which a plane secondary mirror is mounted along the axis of the telescope to intercept the light reflected from the objective mirror and reflect it to the side. Cassigrain focus: A reflecting telescope in which a convex secondary mirror is mounted so as to intercept light reflected from the object mirror and reflect the light back through a hole in the center of the primary.

The Newtonian focus is often used in small telescopes. The Cassegrain focus if often used in large telescopes. Large telescopes can also use: Prime focus: The point in a telescope where the light from the objective is focused. This is the focal point of the objective.

Conde focus: A reflecting telescope in which two mirrors are used to reflect the light coming from the objective to a remote focal point.

Active and Adaptive Optics Active optics: A technology that works by “actively” keeping a telescope's mirror at its optimal shape against environmental factors such as gravity and wind. It works on timescales of a second or more. Adaptive optics: A technique that improves image quality by reducing the effects of astronomical seeing. It relies on an active optics system and works on timescales of less than 0.01 seconds.

Telescope Accessories Charge-coupled device (CCD): A small semi-conductor chip that serves as a light detector by emitting electrons when it is struck by light. A computer uses the pattern of electron emission to form images. These can be more sensitive than photographic film. Photometry: The measurement of light intensity from a source. Courtesy of NOAO/AURA/NSF.

Spectrometer: An instrument that measures the wavelengths present in electromagnetic radiation. (A spectrograph is a spectrometer that produces a photograph of a spectrum.) This device can use a prism or a diffraction grating, to use the wave properties of electromagnetic radiation to separate the radiation into its various wavelengths.

5-5 Radio Telescopes Compared to visible light: The intensity of radio waves from stars is much less. Radio waves have longer wavelengths, corresponding to a decrease in resolution in radio images (because diffraction is greater with longer wavelengths). Courtesy David Parker, 1997, Science Photo Library and Cornell University. Courtesy of NRAO/AUI/NSF.

Courtesy of NOAO/AURA/NSF. Courtesy of Max-Planck-Institut fur Radioastronomie, Bonn (R. Beck, E.M. Berkhuijsen and P. Hoernes). The Andromedra galaxy shown in visible (top), infrared (middle) , and radio (bottom) light.

5-6 Interferometry Interferometry: A procedure that allows several telescopes to be used as one by taking into account the time at which the individual waves from an object strike each telescope. The farther apart the telescopes, the better the resolution that can be obtained. This technique is used to improve resolution with radio telescopes but is also being employed at some new optical telescopes.

Two small radio dishes (bottom) can be made to have the same resolution as a large radio telescope (top).

All portions of the incoming wave must reach a detector at the same time, or at least the time difference must be accounted for. High-speed computers are needed to obtain images from interferometric data.

5-7 Detecting Other Electromagnetic Radiation Radiation at wavelengths other than visible and radio is largely absorbed by the Earth's atmosphere. Near-infrared: Wavelengths from 1,200 to 14,000 nm are absorbed by water vapor but can be observed from the tops of mountains where the air is dry. Far-infrared: Longer infrared wavelengths are all absorbed by the atmosphere, so we must place telescopes to observe them even higher.

Wavelengths shorter than 400 nm (ultraviolet, x-rays, gamma-rays) are absorbed by ozone and telescopes to detect this radiation must be located in space. X-rays are very energetic and only reflect from mirrors if they strike at grazing angles. Gamma-rays are too energetic to reflect from mirrors, so solid-state detectors are used instead. Courtesy of CXC/S. Lee.