Telescopes

Question 1

Give four optical advantages which a reflecting telescope has over a refracting telescope?

Answer 1

Collects more light as mirrors can be larger than lens, better revolving power as diameter of mirrors larger, no chromatic aberration and no spherical aberration

Question 2

Explain chromatic aberration

Answer 2

Different wavelengths of light are refracted by different amounts meaning the image is blurred and has coloured edges

Question 3

Explain spherical aberration

Answer 3

When axial rays converge to different focal points at different distances from the axis

Question 4

Define quantum efficiency and state the value of it for a CCD and the eye

Answer 4

Ratio of the number of photons incident on a detector that are being detected to the total number of photons incident on the detector, >80% for CCD, 1% for eye

Question 5

Describe a CCD and how it produces an image

Answer 5

Silicon chip divided into pixels, incident photons causes electrons to be released (number released proportional to intensity), which are trapped I potential wells forming a pattern which is processed to give an image

Question 6

State the Rayleigh criterion

Answer 6

The resolution of two point objects is not possible if any part of the central spot of either image lies inside the first dark ring of the other image

Question 7

What is the maximum mesh spacing of the reflector of a radio telescope (which is made from wire mesh) for effective reflection?

Answer 7

Wavelength/20

Question 8

What is the angle alpha in a telescope in normal adjustment?

Answer 8

Angle sub tended by object at unaided eye

Question 9

What is the angle beta in a telescope in normal adjustment?

Answer 9

Angle subtended by image at eye

Question 10

Why can you see more stars when looking through a telescope than using the naked eye?

Answer 10

Telescopes have greater diameter objective lenses compared to pupil of the eye, meaning greater collecting power

Question 11

Why are the images of stars never magnified by telescopes?

Answer 11

They are point objects

Question 12

Explain why the image of a planet viewed using a telescope is not significantly brighter than the planet when it is viewed directly

Answer 12

This is because, although more light per second enters the eye when a telescope is used, the virtual image is magnified, so it is spread over a larger part of the field of view. As a result, the amount of light per second per unit area of the virtual image is unchanged

Question 13

Give two disadvantages of using CCDs in astronomy

Answer 13

Expensive as they need to have a large number of pixels in a small area and need to be cooled to low temperatures to prevent the random emission of electrons (which cause ‘dark’ currents) which does not depend on intensity of light

Question 14

Give two examples of objects in space which can be detected by infrared telescopes

Answer 14

Dust clouds and planets which are not hot enough to emit visible light

Question 15

Give two disadvantages of infrared telescopes

Answer 15

Need the be cooled to stop infrared radiation from its own surface affecting measurements and some infrared radiation absorbed by water vapour in the atmosphere

Question 16

Why must UV telescopes be on satellites and use reflectors?

Answer 16

UV radiation absorbed by atmosphere and glass (which lenses often made from)

Question 17

What range of radio waves can be transmitted through the Earth’s atmosphere?

Answer 17

0.001m - 10m