Chapter 5 Telescopes Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Learning Objectives Upon completing this chapter you should be able to: 1. Classify the common types of telescope designs. 2. Compare the advantages and disadvantages of reflectors and refractors. 3. Describe what causes refraction and how lenses focus light. 4. Identify the important aspects for determining a telescope's sensitivity. 5. Compare the lightgathering power of different telescopes. 6. Describe the factors affecting telescope resolution, and calculate the diffraction limit for a telescope. 7. Describe the idea of interferometry and how astronomers use it to improve resolution. 8. Describe the methods used for detecting visible light and other wavelengths of electromagnetic radiation. 9. Discuss the problems caused by observing through the Earth's atmosphere, and describe the methods astronomers use to overcome these problems. 10. Identify the wavelength ranges in which telescopes cannot operate from the ground and the reasons for this. 11. Describe the causes and remedies for light pollution.
Tools of the Trade: Telescopes Stars and other celestial objects are too far away to test directly Astronomers passively collect radiation emitted from distant objects Extremely faint objects make collection of radiation difficult Specialized Instruments Required Need to measure brightness, spectra, and positions with high precision Astronomers use mirrored telescopes and observatories Modern Astronomers are rarely at the eyepiece, more often they are at a computer terminal!
Collecting Power The Powers of a Telescope Bigger telescope, more light collected! Focusing Power Use mirrors or lenses to bend the path of light rays to create images Resolving Power Picking out the details in an image To double the resolving power of a telescope, you must increase the diameter by a factor of two
The two most important properties of any telescope are the light gathering power and the resolving power. Light Gathering Power Light collected proportional to collector area Pupil for the eye Mirror or lens for a telescope Telescope funnels light to our eyes for a brighter image Small changes in collector radius give large change in number of photons caught Telescopes described by lens or mirror diameter (inches)
Focusing Power Refraction Light moving at an angle from one material to another will bend due to a process called refraction Refraction occurs because the speed of light is different in different materials
Due to the refraction caused by atmospheric irregularities stars twinkle. Refraction The Sun looks flattened near the horizon because the larger refraction near the horizon lifts the lower edge of the Sun more than the upper edge and makes the Sun look flattened.
Refracting Telescopes A lens employs refraction to bend light Telescopes that employ lenses to collect and focus light are called refractors
Disadvantages to Refractors Lenses have many disadvantages in large telescopes! Large lenses are extremely expensive to fabricate A large lens will sag in the center since it can only be supported on the edges Dispersion causes images to have colored fringes Many lens materials absorb short-wavelength light
Reflecting Telescopes Reflectors Used almost exclusively by astronomers today Twin Keck telescopes, located on the 14,000 foot volcanic peak Mauna Kea in Hawaii, have 10-meter collector mirrors! Light is focused in front of the mirror
Reflecting Telescopes A secondary mirror may be used to deflect the light to the side or through a hole in the primary mirror Multi-mirror instruments and extremely thin mirrors are two modern approaches to dealing with large pieces of glass in a telescope system
Styles of Refractors
Infrared telescope Infrared telescope, instrument designed to detect and resolve infrared radiation from sources outside Earth s atmosphere such as nebulae, young stars, and gas and dust in other galaxies. Infrared telescopes do not differ significantly from reflecting telescopes designed to observe in the visible region of the electromagnetic spectrum. The main difference between the two types is in the physical location of the infrared telescope,
The resolving power of a telescope is affected by the property of light called diffraction. Resolving Power A telescope s ability to discern detail is referred to as its resolving power Resolving power is limited by the wave nature of light through a phenomenon called diffraction Waves are diffracted as they pass through narrow openings A diffracted point source of light appears as a point surrounded by rings of light
Resolving Power and Aperture Two points of light separated by an angle a (in arcsec) can be seen at a wavelength l (in nm) only if the telescope diameter D (in cm) satisfies: D > 0.02 l/a
Increasing Resolving Power: Interferometers For a given wavelength, resolution is increased for a larger telescope diameter An interferometer accomplishes this by simultaneously combining observations from two or more widely-spaced telescopes Using interferometry, scientists can use a few smaller telescopes to take images with the same resolution as a much larger telescope.
Interferometers The resolution is determined by the individual telescope separations and not the individual diameters of the telescopes themselves Key to the process is the wave nature of interference and the electronic processing of the waves from the various telescopes
The Moon appears bigger near the horizon due to an optical illusion. Detecting the Light The Human Eye Once used with a telescope to record observations or make sketches Not good at detecting faint light, even with the 10-meter Keck telescopes Photographic Film Chemically stores data to increase sensitivity to dim light Very inefficient: Only 4% of striking photons recorded on film Electronic Detectors Incoming photons strike an array of semiconductor pixels that are coupled to a computer Efficiencies of 75% possible CCD (Charged-coupled Device) for pictures
Nonvisible Wavelengths Many astronomical objects radiate in wavelengths other than visible Cold gas clouds radiate in the radio Dust clouds radiate in the infrared Hot gases around black holes emit x- rays
Radio Observatories Radio telescope, astronomical instrument consisting of a radio receiver and an antenna system that is used to detect radio-frequency radiation between wavelengths of about 10 meters (30 megahertz [MHz]) and 1 mm (300 gigahertz [GHz]) emitted by extraterrestrial sources, such as stars, galaxies, and quasars.
Radio Observations False color images are typically used to depict wavelength distributions in non-visible observations In a false color image, colors can represent photon energies or the intensity of electromagnetic radiation. A galaxy with almost no starlight but plenty of cool clouds of hydrogen gas would be best observed with a radio telescope.
Radio telescopes vary widely, but they all have two basic components: (1)a large radio antenna and (2) a sensitive radiometer, or radio receiver. The sensitivity of a radio telescope the ability to measure weak sources of radio emission depends both on the area and efficiency of the antenna and on the sensitivity of the radio receiver used to amplify and to detect the signals. For broadband continuum emission over a range of wavelengths, the sensitivity also depends on the bandwidth of the receiver.
The 305-metre (1,000-foot) radio telescope at the Arecibo Observatory, Puerto Rico.
Major Space Observatories Why put them in space?
Space vs. Ground-Based Observatories Space-Based Advantages Freedom from atmospheric blurring Freedom of atmospheric absorption Ground-Based Advantages Larger collecting power Equipment easily fixed Ground-Based Considerations Weather, humidity, and haze Light pollution
The best site for placing a ground-based optical telescope is A mountain top. Observatories The immense telescopes and their associated equipment require observatories to facilitate their use and protection from the elements Thousands of observatories are scattered throughout the world and are on every continent including Antarctica Some observatories: Twin 10-meter Keck telescopes are largest in U.S. The Hobby-Eberly Telescope uses 91 1-meter mirrors set in an 11-meter disk Largest optical telescope, VLT (Very Large Telescope) in Chile, is an array of four 8-meter mirrors
Going Observing To observe at a major observatory, an astronomer must: Submit a proposal to a committee that allocates telescope time If given observing time, assure all necessary equipment and materials will be available Be prepared to observe at various hours of the day Astronomers may also observe via the Internet Large data archives now exist for investigations covering certain wavelengths sometimes for the entire sky Archives help better prepare astronomers for onsite observations at an observatory
Computers and Astronomy For many astronomers, operating a computer and being able to program are more important than knowing how to use a telescope Computers accomplish several tasks: Solve equations Move telescopes and feed information to detectors Convert data into useful form Networks for communication and data exchange
Refraction is also responsible for seeing Twinkling of stars AKA Scintillation Temperature and density differences in pockets of air shift the image of the star Scintillation
Twinkling of stars in sky, called scintillation, is caused by moving atmospheric irregularities refracting star light into a blend of paths to the eye Atmospheric Blurring The condition of the sky for viewing is referred to as the seeing Distorted seeing can be improved by adaptive optics, which employs a powerful laser and correcting mirrors to offset scintillation The distortion of an image due to an atmospheric turbulence is seeing.
One of the biggest problems for ground based astronomy today is light pollution makes it difficult to observe faint objects. Light Pollution
Gamma-ray telescope Gamma-ray telescope, instrument designed to detect and resolve gamma rays from sources outside Earth s atmosphere. Gamma rays are the shortest waves (about 0.1 angstrom or less) and therefore have the highest energy in the electromagnetic spectrum. Since gamma rays have so much energy, they pass right through the mirror of a standard optical telescope. Instead, gamma rays are detected by the optical flashes they produce when interacting with the material in a specially designed instrument such as a scintillation detector. Earth s atmosphere blocks most gamma rays, so most gamma-ray telescopes are carried on satellites and balloons. However, some ground-based telescopes can observe the Cherenkov radiation produced when a gamma ray strikes Earth s upper atmosphere.
The Compton Gamma Ray Observatory as seen through the space shuttle window during deployment in NASA
X-ray telescope, instrument designed to detect and resolve X-rays from sources outside Earth s atmosphere. Because of atmospheric absorption, X-ray telescopes must be carried to high altitudes by rockets or balloons or placed in orbit outside the atmosphere. Balloon-borne telescopes can detect the more penetrating (harder) X-rays, whereas those carried aloft by rockets or in satellites are used to detect softer radiation. X-rays are blocked by ozone and oxygen present in the Earth's atmosphere.
Ultraviolet telescope, telescope used to examine the ultraviolet portion of the electromagnetic spectrum, between the portion seen as visible light and the portion occupied by X-rays. Ultraviolet radiation has wavelengths of about 400 nanometres (nm) on the visible-light side and about 10 nm on the X-ray side. Earth s stratospheric ozone layer blocks all wavelengths shorter than 300 nm from reaching ground-based telescopes. As this ozone layer lies at an altitude of 20 40 km (12 25 miles), astronomers have to resort to rockets and satellites to make observations from above it.