Ch. 6 HW

What is a charge-coupled device (CCD)? Why have CCDs replaced photographic film for recording astronomical images?

A charge-coupled device (CCD) is a semiconductor device that is sensitive to light. Data are recorded digitally for easy enhancement with a computer. A CCD is about 35 times more sensitive to light than typical photographic film is.

What is light pollution? What effects does it have on the operation of telescopes? What can be done to minimize these effects?

Any human-made light that interferes with astronomical observations is light pollution. It reduces the number of faint objects that can be seen. The use of special streetlights and locating observatories in remote areas are two solutions.

Why must astronomers use satellites and Earth-orbiting observatories to study the heavens at x-ray and gamma-ray wavelengths?

Because X rays and gamma rays are absorbed high in Earth's atmosphere, instruments to observe these radiations must be placed in orbit above the atmosphere.

Explain why the light rays that enter a telescope from an astronomical object are essentially parallel.

If a lens is located very far from the light source, only a few of the light rays will enter the lens, and these rays will essentially be parallel.

Explain why combining the light from two or more optical telescopes can give dramatically improved angular resolution.

The angular resolution for multiple-mirror telescopes is better because of the larger effective diameter of the system and the large separation of the mirrors.

Why are radio telescopes so large? Why does a single radio telescope have poorer angular resolution than a large optical telescope? How can the resolution be improved by making simultaneous observations with several radio telescopes?

The angular resolution is given by θ=2.5×105λ/D. Because the wavelength of radio emissions (e.g., 21 cm) is so much larger than visible light (e.g., 550 nm), the resolution is much worse. Using a large reflecting dish can improve the resolution. Interferometers like the VLA provide very high resolution.

What is a spectrograph? Why do many astronomers regard it as the most important device that can be attached to a telescope?

A spectrograph allows light from a celestial object to be divided into the individual wavelengths present. This permits a determination of the chemical composition of the object.

What is chromatic aberration? For what kinds of telescopes does it occur? How can it be corrected?

Chromatic aberration refers to the fact that refractors do not focus different wavelengths of light at the same focal point. It can be corrected by designing compound objective lenses that have two or more lenses made of different kinds of glass.

What is diffraction? Why does it limit the angular resolution of a telescope? What other physical phenomenon is often a more important restriction on angular resolution?

Diffraction in a telescope results in a spreading out of the light waves and a corresponding blurring of the image. A larger objective lens or mirror reduces the amount of diffraction and improves the angular resolution. Due to atmospheric turbulence and distortion, Earth-based telescopes never actually achieve their theoretical resolution.

Quite often advertisements appear for telescopes that extol their magnifying power. Is this a good criterion for evaluating telescopes? Explain answer.

Magnification is not the most important aspect of a telescope. The reason is that there is a limit to how sharp any astronomical image can be, due either to the blurring caused by Earth's atmosphere or to fundamental limitations imposed by the nature of light itself.

Do most professional astronomers actually look through their telescopes? Why or why not?

Professional astronomers do not look through their telescopes because film or CCDs are much more sensitive than the eye and produce "hard copies" of the image.

Explain some of the disadvantages of refracting telescopes compared to reflecting telescopes.

Refracting: -Chromatic abberation -defect-free glass expensive -opaque glass -heavy Reflecting: -

Describe refraction and reflection. Explain how these processes enable astronomers to build telescopes.

Refraction refers to the bending of light rays as they pass from one medium to another. Reflection is the bouncing light off of a reflecting surface. By curving the glass surfaces to form lenses and mirrors, the refracted or reflected light can be made to pass through a point of focus.

What are the optical window and the radio window? Why isn't there an x-ray window or an ultraviolet window?

The optical window and the radio window are wavelength regions where our atmosphere is transparent. There are no X-ray or ultraviolet windows because our atmosphere is not transparent to these wavelength regions, which of course is necessary for life to exist on Earth.