Telescopes

Question 1

What is an astronomical refracting telescope made up of?

Answer 1

Two converging lenses.

Question 2

What are the two lenses in an astronomical refracting telescope called?

Answer 2

The objective lens and the eye lens.

Question 3

What does it mean if a telescope is in normal adjustment?

Answer 3

The principle focus of the objective lens is in the same position as the principle focus of the eye lens.

Question 4

How do you draw a ray diagram for an astronomical refracting telescope?

Answer 4

• Draw non-axial ray passing through centre of objective lens and ends at eye lens axis
• Draw non-axial rays either side of this ray parallel to it and ending at the eye lens
• Draw dotted line from point where rays cross through centre of eye lens
• Rays refracted through eye lens parallel to this dotted line

Question 5

What is angular magnification?

Answer 5

The angle subtended by the image at the eye, divided by the angle subtended by the object at the unaided eye.

Question 6

What is the length of the telescope equal to?

Answer 6

The sum of the focal lengths of the lenses.

Question 7

What is the Cassegrain arrangement in a reflecting telescope?

Answer 7

Parabolic concave mirror reflects light onto convex secondary mirror which goes through hold in middle of concave mirror and then passes through an eye lens like a refracting telescope.

Question 8

What is a CCD?

Answer 8

Charge-coupled Devices. Sensitive light detectors used to capture images digitally.

Question 9

What are 4 comparison points between the human eye and CCDs?

Answer 9

• CCDs detect much more light (80% quantum efficiency compared with 1% for eye)
• CCDs detect a wider spectrum of light
• CCDs have a smaller spatial resolution and can therefore capture fine detail
• CCDs less convenient to use but produce digital images which can be shared

Question 10

What does the resolving power of an instrument depend on?

Answer 10

The minimum angular resolution which is the smallest angular separation at which the instrument can distinguish two points.

Question 11

What happens if a beam of light passes through a circular aperture? What is the central circle called?

Answer 11

A diffraction pattern of bright maxima and dark minima is formed. Central circle is called the Airy disc.

Question 12

What is the Rayleigh Criterion?

Answer 12

Two light sources can be distinguished if the centre of the Airy disc from one source is at least as far away as the first minimum of the other source.

Question 13

What do the symbols and letters mean in the Rayleigh Criterion equation?

Answer 13

Theta = minimum angular resolution in radians
Lambda = wavelength in metres
D = diameter of objective.

Question 14

What are 2 advantages and 2 disadvantages of refracting telescopes?

Answer 14

Advantages
-Objective does not tarnish and does not need to be re-silvered
-Does not suffer from spherical aberration
Disadvantages
-Secondary can reduce total incident light on the primary
-Secondary mount can cause diffraction patterns

Question 15

What are 4 advantages of reflecting telescopes?

Answer 15

• Light does not pass through the glass and therefore not affected by defects within glass
• Objective can be supported over entire surface area - does not sag and gives more stability
• Does not suffer from chromatic aberration
• Active optics (movement of objective to maintain perfect shape), and adaptive optics (movement of secondary to correct for atmospheric distortions) possible

Question 16

What is chromatic aberration?

Answer 16

Refractive index of glass for blue light is slightly different to red light so blue refracts more than red causing distortion.

Question 17

How can chromatic aberration be solved?

Answer 17

Can be (partially) corrected with coated optics or an achromatic lens

Question 18

What is spherical aberration?

Answer 18

Occurs in mirrors which spherical cross section - rays that strike close to centre of mirror have larger focal length than rays striking edge of mirror so there is no focal point.

Question 19

How can you solve spherical aberration?

Answer 19

Objective made in the shape of a parabola.

Question 20

How does a radio telescope work?

Answer 20

• Parabolic dish reflects EM radiation and focuses it at antenna (antenna is the principle focus)
• Signals amplified twice and then put through tuner to select wavelengths you want
• Final signals go to computer to create a false-colour image

Question 21

What is the main problem with a radio telescope and why?

Answer 21

It has a very low resolving power as the wavelength of radio waves is much larger than the wavelengths of light.

Question 22

How do radio astronomers get around the problem with radio telescopes?

Answer 22

They link lots of telescopes together and using some computer programs combine their data to form an image - equivalent of one huge dish.

Question 23

What are the main benefits of radio telescopes?

Answer 23

• Construction much easier and cheaper as dish is made of wire mesh
• The longer the wavelength of radiation, the less it’s affected by imperfections in the shape of the dish

Question 24

How do I-R and U-V telescopes work?

Answer 24

• Use same parabolic mirror set up to focus radiation onto detector
• CCDs used as radiation detectors

Question 25

What is the main problem with I-R telescopes?

Answer 25

Produce own I-R waves if they get hot so need to be kept cold.

Question 26

What is the main problem with U-V telescopes?

Answer 26

U-V waves have much shorter wavelength than light so mirrors have to be very precisely made.

Question 27

How do X-Ray telescopes work?

Answer 27

Either a wire mesh absorbs the X-Rays or X-Rays reflected by grazing off mirrors until brought to a focus on a detector.

Question 28

Where can we use optical and radio telescopes and why?

Answer 28

We can use these two on the surface of the Earth because the atmosphere is transparent to these wavelengths.

Question 29

Where can we use I-R telescopes and why?

Answer 29

Best to set up telescope up high in dry places as I-R radiation absorbed by water vapour in the atmosphere.

Question 30

Where can we use U-V and X-Ray telescopes?

Answer 30

In outer space as these wavelengths cannot enter the atmosphere.

Question 31

Compare the resolving power of a radio telescope with a U-V telescope.

Answer 31

U-V telescope has much larger resolving power due to shorter wavelength.

Question 32

What is collecting power proportional to?

Answer 32

The collecting area (so dish diameter ^ 2)

Question 33

What is quantum efficiency?

Answer 33

The proportion of the incident photons that are detected by a light detector and release electrons for a CCD.