CHAPTER 5
Telescopes: Windows to the Universe
CHAPTER OUTLINE
5-1 Refraction and Image Formation
1. Light travels in a straight line as long as it remains in the same medium (i.e., the material through which light travels).
2. Refraction is the bending of light as it crosses the boundary (interface) between two materials in which it travels at different speeds.
3. The amount of refraction is determined by two factors: (i) Relative speeds of light in the two materials (e.g., air and glass). (ii) Angle between a light ray and the interface; the smaller the angle between a light ray and the interface, the more the light bends on passing through the interface.
4. Image is the visual counterpart of an object, formed by refraction or reflection of light from the object.
5. Focal point (of a converging lens or mirror) is the point at which light from a very distant object converges after being refracted or reflected.
6. Focal length is the distance from the center of a lens or a mirror to its focal point.

7. When light passes from the (almost perfect) vacuum of space into Earth’s atmosphere, it continuously refracts as it moves through the air. We must take this into account when measuring positions of celestial objects.

5-2 The Refracting Telescope
1. A refracting telescope uses lenses to bring an image into focus.

2. The objective is the main light-gathering element—lens or mirror—of a telescope. It is also called the primary.
3. An eyepiece (which may be a combination of lenses) added just beyond the focal point of the telescope’s objective acts as a magnifier to enlarge the image for direct viewing.
Chromatic Aberration
1. Chromatic aberration is a defect of optical systems that results in light of different colors being focused at different places. The resulting image will be fuzzy at the edges.
3. An achromatic lens (or achromat) is an optical element that has been corrected so that it is free of chromatic aberration. This is done by combining two or more lenses made of different kinds of glass.
5-3 The Powers of a Telescope
Angular Size and Magnifying Power
1. Angular size of an object is the angle between two lines drawn from the viewer to opposite sides of the object.
2. Magnifying power (or magnification) is 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 powerdepends on the focal lengths of the objective and the eyepiece: M = fobjective/feyepiece
3. The greatest magnification can be achieved by having a long-focal length objective and a short-focal length eyepiece. However the greatest magnification is not always desired.
4. Field of view is the actual angular width of the scene viewed by an optical instrument. As magnification increases the field of view decreases.
Light-Gathering Power
1. Light-gathering power is a measure of the amount of light collected by an optical instrument.
2. Light-gathering power is related to the size of the objective, which is usually given as a diameter. Remember that the area of a circle is proportional to the (diameter)2.
Resolving Power
1. Diffraction is the spreading of light upon passing the edge of an object.

2. The amount of diffraction that occurs when light passes through an opening depends on the wavelength of the light and the size of the opening.

3. Resolving power (or resolution) is the smallest angular separation detectable with an instrument. It is a measure of an instrument’s ability to see detail. A telescope’s resolving power can be calculated by using the expression(in arcseconds)= 2.5 × 105×(wavelength in m)/D(objective diameter in m).
4. The resolving power of a human eye is about 0.5 arcminute (1/120 of a degree).

5. Astronomical seeing is the blurring and twinkling of the image of an astronomical light source caused by the Earth’s atmosphere.
6. Seeing is the best possible angular resolution that can be achieved.
7. Because of atmospheric turbulence (which causes the stars to twinkle), even the largest Earth-based telescopes have a practical resolving power of between 0.5 and 0.25 arcsecond.
7. Operating above the atmosphere, the Hubble Space Telescope has a resolving power of 0.1 arcsecond or better.
5-4 The Reflecting Telescope
1. An inwardly curved—or concave—mirror can bring incoming light rays to a focus and is used to construct reflecting telescopes.
2. In the late 1660s, Isaac Newton invented a special focal arrangement for a reflecting telescope by placing a small flat mirror in front of the objective mirror to deflect light rays out to the eyepiece.
3. Reflectors can be made larger (and less expensively) than refractors because:
(a) There are fewer surfaces to grind, polish, and configure correctly.
(b) Reflecting mirrors do not exhibit chromatic aberration as do lenses.
(c) Light doesn’t transmit through a mirror so imperfections in the glass are not critical.
(d) Mirrors can be supported on their backs, thus minimizing shape deformations due to gravity; lenses must be supported along their rims.

4. A large reflector is much less expensive and more practical than a large refractor. As a result, all really large telescopes are reflectors.

Large Optical Telescopes
1. A Newtonian focus reflecting telescope has a plane mirror mounted along the axis of the telescope so that the mirror intercepts the light from the objective mirror and reflects it to the side.
2. A Cassegrain focus reflecting telescope has a secondary convex mirror that reflects the light back through a hole in the center of the primary mirror.
3. Prime focus is the point in a telescope where the light from the objective is focused (i.e., the focal point of the objective).
4. The Coude design allows for large and heavy equipment to be set at the focal point, outside the main telescope tube; here light reflects off three mirrors before it exits.
5. For best viewing conditions large telescopes are located on top of mountains in dry, clear climates.
Active and Adaptive Optics
1. Active optics is a technology that relies on a system that monitors and changes the shape of a telescope’s objective to produce the best image. The telescope’s mirror is actively kept at its optimal shape against environmental factors such as gravity and wind.
2. Adaptive optics is a technique that improves image quality by reducing the effects of astronomical seeing. It relies on an active optics system that rapidly deforms the mirror.
Telescope Accessories
1. Cameras can be attached to a telescope to take photos, either with photographic film or electronic light detectors.
2. Charge-coupled device (CCD) is an electronic “film” that serves as a light detector. It works by collecting electrons excited into higher energy states when the detector is struck by incident photons. The data collected is formed into images by a computer.
3. Photometry is the measurement of light intensity from a source, either the total intensity or the intensity at each of various wavelengths. Early photometers were like a camera’s light meter; modern photometers use a CCD for greater speed and accuracy.
4. Spectral analysis uses a spectrometer—an instrument that separates electromagnetic radiation according to wavelength. A spectrographis a spectrometer that produces a photograph of the spectrum.
5. A spectrometer commonly uses a diffraction grating—a device that uses the wave properties of EM radiation to separate the radiation into its various wavelengths.
5-5 Radio Telescopes
1. Compared to visible light, radio waves from a star have less intensity; also, their longer wavelengths leadto images of smaller resolution.
2. For better resolution in detecting radio waves, very large dishes are required. However, the dish surfaces do not have to be as smooth as glass mirrors; even though longer wavelengths diffract more when going through an opening, they don’t require as smooth a surface for reflection.

3. Images of a radio-emitting object can be obtained by scanning the radio telescope back and forth across the object and feeding the data into a computer that represents the various intensities of the radio waves as different colors.

Tools of Astronomy: Spinning a Giant Mirror

1. In the past, mirrors were made by melting glass to form large pieces with flat surfaces. After the glass hardened, the working surface of the glass was carefully ground into a curve to form a concave mirror surface.

2. Spin-casting is a modern technique where the furnace is spun which shapes a natural curve in the surface of the molten glass in the same way that swirling a liquid in a glass causes a curved surface. These mirrors are lighter and made more efficiently than the older technique.

5-6 Interferometry
1. Interferometry is a procedure that allows a number of telescopes to be used as one by taking into account the time at which individual waves from an object strike each telescope.
2. Interferometry is possible because extremely accurate atomic clocks allow for precise timing of the signals received by radio telescopes from a distant object.
3. The farther apart the telescopes, the better the resolution. The VLBA has a resolution of a fraction of a milliarcsecond, 10,000 times better than earth-bound optical telescopes.
4. Interferometry can also be employed with the newest optical telescopes, such as the VLT in Chile or CHARA on Mount Wilson.
5-7 Detecting Other Electromagnetic Radiation
1.Celestial objects emit radiation over the entire range from radio waves to gamma rays, and modern astronomy has tools that study each region of the spectrum.

2. Nearinfrared—1200 nm to 40,000 nm—can be detected from high, dry mountain tops such as Mauna Kea in Hawaii; water vapor is the main absorber of infrared light.
3. Far infrared—greater than 40,000 nm—can be detected from aircraft (e.g., the SOFIA project, NASA’s Airborne Observatory).
4. Infrared telescopes (such as the Spitzer Space Telescope) must be cooled so heat (IR radiation) from the surroundings does not mask the signals received from space.
5. Ozone is the chief absorber of wavelengths shorter than about 400 nm. For ultraviolet, X-rays, and gamma rays, telescopes must be located in space.

6. X-ray telescopes use mirrors at grazing angles, because X-ray light is very energetic and will be absorbed by the mirror at other angles.

7. Gamma rays are too energetic to be reflected by mirrors. Detectors using advanced solid-state technology must be used instead.

Ultraviolet(e.g., the GALEX telescope), X-ray (the Chandra telescope),and gamma-ray telescopes must be located in space.
Tools of Astronomy: Space Telescopes

Infrared Telescopes
1. The SOFIA project involves a modified Boeing 747-SP aircraft carrying a 2.7-meter reflecting telescope at altitudes as high as 12,000 meters (45,000 feet), above 99% of the atmosphere’s water vapor. The telescope’s wavelength range is between 0.3 m and 1600 m.

2. The Spitzer Space Telescope is a 0.85-m infrared telescope that observes the universe in the range of 3 to 180 m. The telescope has a shield to protect it from the Sun and the Earth’s infrared radiation, and it is cooled to 5.2 K by liquid helium, so that it can observe infrared signals from space without interference from its own heat.

3. The WISE missionwas a satellite that scanned the entire sky in infrared light one-and-a-half times, generating 3 million pictures at infrared wavelengths from 3.4 - 4.6 m and 12 - 22 m);

4. The Herschel observatoryhas a primary mirror of 3.5 m in diameter and a range from the far-infrared to submillimeter.

The GALEX Ultraviolet Telescope

1. TheGALEX mission is an orbiting 0.5-m space telescope that observes galaxies in ultraviolet light (130–300 nm).

X-Ray Telescopes

1.The Chandra X-ray Telescope is in an unusual elliptical orbit about the Earth and is one of the most sophisticated observatories ever built.

2. XMM-Newton is another X-ray telescope that has lower resolution than Chandra but collects more light. It also has a 0.3-m optical ultraviolet telescope.

3. The Suzaku X-ray Telescope is another Earth orbiting observatory.

4. The first high-energy X-ray telescope in space is NuSTAR. It has a deployable mast in order to achieve the long focal lengths required for focusing X-ray telescopes.

Gamma-Ray Telescopes

1. The Compton Gamma Ray Telescopeorbited from 1991 to 2000 and was designed to map the gamma ray sky.

2. INTEGRAL is a satellite that carries a gamma-ray imager and spectrometer, plus an X-ray monitor and an optical camera to allow for clear identification of gamma-ray sources.

3. TheFermi Gamma-Ray Telescope is another observatory currently in orbit.

The Hubble Space Telescope

1. HST is mainly an optical telescope, with a 2.4-m primary mirror, but is designed to observe across the spectrum from near infrared to near ultraviolet (115 – 2500 nm).

2. The HST, after a number of servicing missions including a successful repair in 1993, is now functioning at design specifications.

3. Its successor is the James Webb Space Telescope, scheduled to launch in 2018, which will have a 6.5-m primary mirror and observe in the mid-infrared (0.6–28 m).