Telescopes

Question 1

Draw a ray diagram for an astronomical telescope in normal adjustment. Should show the path of three non-axial rays

Answer 1

-Both focal points labelled on the principle axis, and coincide, with fO>fe
- 3 non axial rays through objective lens
-3 rays through eyepiece lens, parallel to construction line

Question 2

Define the angular magnification of a telescope

Answer 2

Angular magnification is the ratio of the angle subtended by an object at the eye when viewed through an optical instrument to the angle subtended by the object when viewed directly. It is a measure of the magnifying power of an optical instrument.

Question 2

Define focal length

Answer 2

The distance between the centre of a lens and the principal focus
The ​shorter the focal length​, the ​stronger the lens.

Question 3

Draw a cassegrain reflecting telescope diagram

Answer 3

At least two light rays parallel to the principal axis (one above, one below)
Concave primary mirror with gap in it
Light is reflected by mirror onto secondary mirror then through eyepiece lens

Question 4

Draw a ray diagram for an object further than 2F and a converging lens, and describe the image formed

Answer 4

Real, inverted diminished

Question 5

Draw a ray diagram for an object closer than F and a converging lens, and describe the image formed

Answer 5

Virtual, upright magnified

Question 6

Draw a ray diagram for an object between F and 2F and a converging lens, and describe the image formed

Answer 6

Real, inverted, magnified

Question 7

Draw a ray diagram for an object at 2F and a converging lens, and describe the image formed

Answer 7

Real, inverted, but the same size

Question 8

How does an astronomical refracting telescope work?

Answer 8

• Two converging lens, the objective lens and the eyepiece lens
• Objective lens collects light and creates a real image of a distant object
• This image is than magnified by the eyepiece lens, which creates a virtual image

Question 9

How does a cassegrain telescope work?

Answer 9

• Concave primary mirror with a long focal length and a small convex secondary mirror in the centre
• Light is collected by the primary mirror and focused onto the secondary mirror
• Which then reflects it onto an eyepiece lens

Question 10

What is chromatic aberration? How can be resolved? Draw a diagram to show it

Answer 10

When a lens refracts different colours by different amounts as they have different wavelength. This causes an image to form with coloured fringes around the edges.
Resolved by filtering light so it is one wavelength
Diagram showing two focal lengths, one for blue, one for red eg.
Blue Bends Best

Question 11

What is spherical aberration? How can it be resolved? Draw a digram to illustrate it

Answer 11

When light is focused in different places due to the curvature of the mirror, causing the image to blur. Can be resolved in a reflecting telescope by used a parabolic mirror.
Digram showing different angles of refection of incident rays, preventing them from focusing

Question 12

Advantages of refracting telescopes

Answer 12

They are less sensitive to temperature changes than reflectors.
They require less maintenance than reflectors because mirrors have to be re-aluminised periodically.

Question 13

Advantages of reflecting telescopes

Answer 13

Mirrors are ​unaffected by ​chromatic aberration​, and ​spherical aberration​ can be solved​ by using parabolic mirrors (with axial rays)
Mirrors can be larger and therefore have greater collecting powers
Mirrors can be larger and therefore have a greater minimum angular resolution.