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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 that
transmits light).
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 counterpoint 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.
5-2 The Refracting Telescope
1. Objective is the main light-gathering element—lens or mirror—of a telescope. It is also called
the primary.
2. 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.
Chromatic Aberration
1. Dispersion is the separation of light into its various wavelengths upon refraction.
2. 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 power:
M = fobjective/feyepiece
3. Long focal length eyepieces produce less magnification; short focal length eyepieces produce
more magnification.
4. Field of view is the actual angular width of the scene viewed by an optical instrument.
5. 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. 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.
3. The resolving power of a human eye is about 1 arcminute (1/60 of a degree). A 15-cm (6-inch)
telescope has a maximum resolving power of 1 arcsecond (1/3600 of a degree).
4. Astronomical seeing is the blurring and twinkling of the image of an astronomical light source
caused by the Earth’s atmosphere.
5. Seeing is the best possible angular resolution that can be achieved.
6. Because of atmospheric turbulence (which causes the stars to twinkle), even the largest Earthbased telescopes have a practical resolving power of between 1 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.
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.
2. Adaptive optics is a technique that improves image quality by reducing the effects of
astronomical seeing.
Telescope Accessories
1. Camera with photographic plates.
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 spectrograph is a visual record of the spectrum taken by a
spectrometer.
5. A spectrometer 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 lead to 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.
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. Near infrared—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.
2. Far infrared—greater than 40,000 nm—can be detected from aircraft (e.g., the SOFIA
project, NASA’s Airborne Observatory).
3. 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.
4. Ozone is the chief absorber of wavelengths shorter than about 400 nm. Ultraviolet (e.g., the
GALEX telescope), X-ray (the Chandra telescope), and gamma-ray telescopes must be located
in space.
 Tools of Astronomy: 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. Its successor is the James Webb Space Telescope, schedule
to launch in 2011.