(9.1) - Telescopes

Question 1

What is meant by “normal adjustment” for an astronomical telescope?

Answer 1

Normal adjustment is when the telescope is set up so that the final image is formed at infinity. This occurs when the distance between the objective and eyepiece lenses is equal to the sum of their focal lengths (fo + fe)

Question 2

State two relative merits of a reflecting telescope over a refracting telescope.

Answer 2

1. No chromatic aberration
2. Greater magnifications can be achieved due to much larger diameter of mirror

Question 3

State two relative merits of a refracting telescope over a reflecting telescope.

Answer 3

1. Not as sensitive to temperature change
2. Require less maintenance

Question 4

What is Chromatic Aberration?

Answer 4

Different wavelengths of light are refracted by different amounts, causing the edges of an image to appear coloured

Question 5

How do you reduce Chromatic Aberration?

Answer 5

Have a lens of opposite dispersive power (Converging then diverge again)

Question 6

What is Spherical Aberration?

Answer 6

Rays of light come to focus at different points due to the spherical curvature of a lens or mirror, causing the image to become very blurred

Question 7

How do you reduce Spherical Aberration?

Answer 7

Using a parabolic mirror

Question 8

What is Resolving Power?

Answer 8

The ability of a telescope to distinguish fine detail or separate two closely spaced objects. θ≈λ/D, so more wavelength, more angular resolution, poorer resolving power

Question 9

What is Collecting Power?

Answer 9

The ability of a telescope to gather light (radiation), which is directly proportional to diameter of objective lens/mirror^2

Question 10

State one similarity and one difference between a single-dish radio telescope and an optical telescope in terms of structure

Answer 10

+Both use parabolic surfaces to reflect waves
-Radio uses 1 primary reflector, optical uses 2

Question 11

State one similarity and one difference between a single-dish radio telescope and an optical telescope in terms of positioning

Answer 11

+Both can be ground-based as the atmosphere is transparent to most radio and optical wavelengths
-Both have to be remote to avoid distorting signals, but optical has to be high off the ground to avoid atmospheric distortion

Question 12

State one similarity and one difference between a single-dish radio telescope and an optical telescope in terms of uses

Answer 12

+Both are used to detect hydrogen emission lines
-Radio waves are not absorbed by dust, whereas optical waves are

Question 13

Compare the resolving power of a single radio dish to a single optical telescope of the same diameter.

Answer 13

θ≈λ/D
Radio waves has larger lamdba, so larger angular resolution and therefore a poorer resolving power

Question 14

Compare the collecting power of radio telescopes to optical telescopes.

Answer 14

• Collecting power is directly proportional to square of the diameter of the primary dish
• Since radio signals are really weak, radio telescopes need much larger diameter than optical to gather enough photons to form a detectable signal

Question 15

Give three reasons why CCDs have replaced the eye in professional astronomy.

Answer 15

1. Higher quantum efficiency (detects a lot more light, seeing fainter objects)
2. Smaller resolution, so CCD can seperate two objects that are really close together
3. Can be viewed remotely and digitally, and can detect a wider range of wavelengths