Exercises: Telescopes and Detectors

Telescopes and Detectors

1. Compare the light-gathering power of the five meter mirror of the Palomar telescope to that of the human eye. (Assume a pupil size of five millimeters.)

2. Most of Galileo's telescopes were only about 2 centimeters in diameter. What is the resolving power of such a telescope? How does this compare to the human eye? (The eye's resolution is about 60 arc sec.)

3. Use the Fourmilab Earth Viewer to view the Earth from the COBE satellite:

http://www.fourmilab.to/cgi-bin/uncgi/Earth?xbird=COBE
Use the information on this viewer to plot the orbit of COBE over the surface for several successive revolutions (the period is 103 minutes). Update the browser window every 5 minutes, and plot a point on a latitude and longitude map of the Earth and write beside the point the altitude. On the same map, plot simultaneously the position of the Sun (the viewer at

http://www.fourmilab.to/cgi-bin/uncgi/Earth?&opt=-s

will give you the view of the Earth from the Sun and the latitude-longitude point that the Sun is over for that view). What is the nature of the orbit. Is it circular? What does the fact that the terminator (line between day and night) is always in view tell you qualitatively about the relative position of the satellite, Earth, and Sun? From the COBE orbit and Sun orbit plotted on your map, what is the angle between COBE and the Sun? Why do you think COBE is in this orbit (hint: it carries detectors that must be shielded from any heat coming from the Earth or Sun for them to work, and it must be able to survey the entire sky over a period of time)?

4. Repeat exercise 3, but for the Hubble Space Telescope:

http://www.fourmilab.to/cgi-bin/uncgi/Earth?xbird=HST

5. What is the corresponding frequency in megaherz for the 21 cm hydrogen line?

6. Why is it important for an observatory to be located in a high and dry place if it is to be used for IR observations?

7. Telescopes used to observe the Sun from the Earth are seldom larger than about 50 cm (mirror diameter). Why not build larger ones?

SOLUTIONS