Telescopes

Question 1

What is the focal length of a lens?

Answer 1

Distance between focal plane and lens

(Parallel rays converge on the focal plane)

Question 2

How will this be different for a stronger lens?

Answer 2

Focal length shorter

Question 3

Why is a real image formed here?

Answer 3

The light rays pass through the point where the image forms

Question 4

Why is a virtual image formed here?

Answer 4

The light rays do not pass through the point where the image forms

Question 5

Why do these two object appear the same size?

Answer 5

The angle subtended at the eye is the same

Question 6

Generally what does a telescope need to do to magnify an object?

Answer 6

Increase the angle subtended at the eye

Question 7

Copy the diagram and label:

• Angles α and β
• Focal lengths
• Focal points
• Two lens names

Answer 7

Question 8

Copy the diagram and label:

• Where a real image forms
• Where a virtual image forms

Answer 8

Question 9

Draw a labelled diagram of the refracting telescope in normal adjustment

Answer 9

Don’t forget the dotted construction line!

Question 10

What is wrong here?

Answer 10

Rays from eyepiece lens must be parallel to dotted construction line

Question 11

How is the length of a refracting telescope calculated?

Answer 11

Add the two focal lengths

Question 12

How is angular magnification of a telescope calculated?

Answer 12

Question 13

1. What is spherical aberration?
2. Sketch a diagram to show it

Answer 13

Refraction increases towards edge of lens

Focal point different for rays passing through different parts of lens

Question 14

What is the resolution of a telescope?

Answer 14

Ability to distinguish between two points separated by angle

The smaller the resolution the better

Question 15

How many stars are seen?

Answer 15

2 because the stars are resolved

Question 16

How many stars are seen?

Answer 16

1 because the stars aren’t resolved

Question 17

How many times greater is this telescope’s collecting power?

Answer 17

4x

P proportional to SA

Question 18

What does a reflecting telescope in Cassegrain Arrangement look like?

Answer 18

Question 19

What is Spherical Aberration?

Answer 19

Focal length for lens or mirror is affected by distance from centre
(Forms distorted images)

Question 20

How is Spherical Aberration reduced?

Answer 20

Using reflecting telescopes with parabolic mirrors

Question 21

What are the Advantages of using Reflecting over Rafracting Telescopes?

Answer 21

Question 22

Why are Space Based Telescopes used?

Answer 22

1. No atmospheric or light pollution
2. Can image across entire EM spectrum

Question 23

What are the only EM regions visible to Ground Based Telescopes?

Answer 23

1. Visible
2. Radio
3. Some IR (dry mountainous regions)

Question 24

What are the disadvantages of Space Based Telescopes?

Answer 24

1. Expensive to build and launch into orbit
2. Difficult to repair ad maintain
3. Weight limits to size of mirrors and power
4. A lot more difficult to control remotely (communicatiuon delays)

Question 25

1. What is Quantum Efficiency?
2. What is the Q.E. for the eye?
3. What is the Q.E. for a CCD?

Answer 25

Q.E. eye ≈ 5%

Q.E. CCD > 70%

Question 26

How does a CCD Work?

Answer 26

1. Semiconductor chip with millions of pixels
2. CCD placed on focal plane of objective lens
3. Each pixel made of potential wells
4. Potential wells correspond to different photons
5. Photons incident on CCD release electrons
6. Electrons trapped in potential wells
7. Relative amounts of electrons in each well measured to produce image

Question 27

What is the Rayleigh Criterion Critical Limit (point where 2 objects are only just resolved)

Answer 27

Central Maxima of one diffraction pattern meets the First Minima of the other

Question 28

1. What is chromatic aberration?
2. Sketch a diagram to show it

Answer 28

Different wavelength’s refract different amounts

Focal point different for different colours