Read Chapter 6
1.) What is the function of a lens or a curved mirror in a telescope? Answer: Both change the angle of the incoming light rays (example, the lens bends the light rays) so that the rays of light will focus.
2.) How are telescope observations better than "naked eye" observations?
a.) telescopes can collect more light than a human eye can
b.) telescopes can have better angular resolution than a human eye
c.) both of the above
d.) none of the above
Answer: c
3.)
a.) In order to see the features on a planet clearly, do you want
your telescope's
diffraction limit to be smaller than the angular separation between
the features on the planet, or do you want your telescope's diffraction
limit to be larger than the angular separation between features on
the planet
Answer: You want the telescope's diffraction limit to be smaller than the
angular separation between the features on the planet.
b.) Explain why
Answer: The diffraction limit is the smallest angular separation
that you can see with your telescope. If the features on the planet
are spaced at even smaller angles than the diffraction limit, then
your telescope will blur their light together and you won't be
able to see the features clearly.
4.) Compare a 5 meter telescope with a 10 meter telescope:
a.) Which has the better diffraction limit and by what factor
is it better than the diffraction limit
of the other telescope?
(The wavelength of the light to be observed is 656 nm, but
it is possible for
you to do this problem without knowing the wavelength.)
b.) Which has the larger light collecting area and by what
factor?
Answer: The 10 meter telescope has a better diffraction limit.
It is half as large as that of the 5 meter telescope.
b.) Which has the better light collecting area and by what
factor?
Answer: The 10 meter telescope has 4 times as much light
collecting area as the 5 meter telescope.
5.) Briefly describe the advantages of putting telescopes in space.
Answer:
Atmospheric absorption: Some wavelengths of light are absorbed
by the Earth's atmosphere.
So, for those wavelengths, the only
practical way to detect the light is to have a
telescope in space
(i.e. above the Earth's atmosphere).
Atmospheric turbulence: Regions where the Earth's atmosphere
varies in temperature or density act like lenses. They
alter the light's path and thus distort the image.
Unfortunately, the atmosphere
moves turbulently, which means that these "lenses" are always on the
move, thus constantly shifting
the light's path in different ways.
If the telescope is on the ground, the ever-changing
image will be hard to accurately record,
but if the telescope is in space, atmospheric
turbulence will not be a problem. Less importantly:
Light pollution: Telescopes on Earth see some
of the photons
made in streetlights, carlights, etc.. These photons
pollute the observations. By
putting telescopes in space, we avoid much of this problem.
Less important: Infrared telescopes
are sensitive to the
photons made by the Earth's blackbody radiation. So,
putting infrared telescopes
in space avoids this problem.
6.) Which one of the following could be a true statement
(taken from textbook):
a.) The image was blurry because the photographic film was not placed
at the focal plane
b.) Thanks to adaptive optics, the telescope on Mount Wilson
can now make ultraviolet images of the cosmos
c.) New technologies will soon allow astronomers to use X-ray telescopes
on the Earth's surface
d.) Thanks to interferometry, a properly spaced set of 10-meter radio
telescopes can achieve the
light-collecting area of a single, 100-kilometer radio telescope
e.) All of the above are true
Answer: "a" is true. The others are false.
"b" is false because few ultraviolet photons are able to pass through
the Earth's atmosphere and
adaptive optics have nothing to do
with improving ultraviolet detections on Earth.
"c" is false because X-rays cannot travel far through the Earth's
atmosphere and new technologies
cannot solve that essential problem.
"d" is false in concept -- interferometry allows us to
achieve the angular resolution
(measured in terms of the
diffraction limit) of a larger telescope by spacing the smaller
telescopes out. But, if you want to
have the same light collecting area as a 100 km radio telescope,
you need to have area= pi*(50km)2. Each
5 meter telescope has pi*(2.5m)2 of area. So, in order
to have the same collecting area as a 100 km telescope, the number of
5 meter telescopes you
would need to connect is
(50 km x 1000 m/km / 2.5 m)2 = i.e. 4 x 108.
That isn't practical.
No, the advantage of using interferometry
is a big improvement in angular resolution,
not a big improvement in collecting area.
7.) If our eyes don't see radio frequency light, how do astronomers make images of radio emission? Answer: The detectors can detect the brightness of objects in radio-frequency light. They can also detect the shape of the object. This information is stored. So that humans can work with the information, the stored information is translated into a visible-wavelength image. This can be done by color-coding visible colors to radio wavelengths (that makes a "false color" image).
8.) Which one of the following would be an example of a spectroscopic
observation?
a.) Astrid, the astronomer wanted to study a particular optically thin gas cloud.
Unlike the relatively bright Orion Nebula, her cloud was very dim.
So, Astrid studied it by recording the light from a star
located beyond the gas cloud and measuring how much 410nm, 434nm,
486nm, and 656nm light the gas cloud had absorbed.
b.) Astrid, the astronomer wanted to study a particular binary star system.
Her binary star system was very far away and so she was not able to
resolve the two stars in her photographs. So, Astrid studied it by
recording the system's flux every minute over several weeks. She
could see that the flux varied in a regular, periodic fashion.
c.) Astrid, the astronomer wanted to study the bubbles made by exploding
stars. Since her favorite bubble was so faint, she had to
put a filter on her telescope. She used a filter that only allowed
656 nm light to come through to the detector. From her observations,
she was able to see the circular shape of the bubble.
d.) None of the above
Answer: a. In option a,
Astrid is measuring and analyzing aspects of the spectrum
(the emission lines) from the gas cloud. She is
using that information to learn about the cloud.
Option b is not right because it is a timing observation.
Option c is really an imaging observation. The use of the filter
to select a single color of light is really being done in order
to make a better image, not to learn about the spectrum. In order
to have a spectrum, you must have more than one color of light. But,
Astrid has thrown away all the other colors of light in option c.
9.)
When we look at the night sky, we see that the stars "twinkle".
Why is that?
Answer: The Earth's atmosphere distorts the light coming from
the star. It bends the light's path slightly. Because the air
in the atmosphere moves constantly,
the amount and direction of the bending changes constantly.
You could say that the atmosphere behaves like a constantly changing lens
which causes stars' light-rays to shift back and forth constantly.
10.) Which types of light (i.e. which wavelengths) cannot pass easily
through the Earth's atmosphere.
Answer: Gamma rays, X-rays, most ultraviolet wavelengths (all but the
longest ultraviolet wavelengths), most infrared wavelengths (all but the
shortest infrared wavelengths cannot reach even the Earth's mountaintops).