3.9 Astrophysics

Question 1

How do you calculate the magnification of a refracting telescope?

Answer 1

(Angle subtended by image at eye) / (angle subtended by object at unaided eye)

Question 2

What is normal adjustment?

Answer 2

Eye views parallel rays as if they’ve come from an object at infinity.

Question 3

What are the disadvantages of refracting telescopes?

Answer 3

Chromatic aberration - Different wavelengths are refracted by different amounts so, the edges of an image appear coloured.
Spherical aberration
Light, usually blue/UV, are absorbed by the lens
Tube length must be very long for a good magnification
Distortion of lens
Lenses are heavy

Question 4

What is spherical aberration?

Answer 4

When the shape of a lens isn’t fully parabolic, then light doesn’t converge at a single point.

Question 5

What are the advantages of a cassegarian telescope?

Answer 5

Lighter and more compact
The mirror has no chromatic aberration
The objective can be made much bigger- better resolution

Question 6

What are the disadvantages of a cassegarian telescope?

Answer 6

The secondary mirror obstructs light- there’s less light entering the telescope which decreases the brightness of the image.
Costly to maintain- recoating the mirror and cleaning
The eyepiece lens has spherical and chromatic aberration

Question 7

What is the relationship between collecting power and the diameter objective lens?

Answer 7

Collecting power ∝ diameter^2

Question 8

What property of the telescope does collecting power effect?

Answer 8

The higher the collecting power, the brighter the image.

Question 9

What is angular resolution?

Answer 9

The smallest angle of separation between 2 objects which can be distinguished.

Question 10

What happens to a wave when it passes through an aperture?

Answer 10

The wave is diffracted which can result in circular interference fringes. This leads to a loss of detail.

Question 11

What is an airy disc?

Answer 11

The large central maximum on a diffraction pattern when light is diffracted. It is twice as wide as the other maxima in the pattern.

Question 12

What is the Rayleigh criterion?

Answer 12

2 objects are resolvable when the first minimum of one airy disc coincides with the central maximum of the other.

Question 13

What is quantum efficiency?

Answer 13

A measure of the effectiveness of an imaging device to convert incident photons into electrons

Question 14

What is the quantum efficiency of a CCD?

Answer 14

> 70%

Question 15

What is the quantum efficiency of the eye?

Answer 15

1% - 10%

Question 16

What is the resolution of a CCD?

Answer 16

> 200 MP

Question 17

What is the resolution of the eye?

Answer 17

Approximately 100m light sensitive cells in the retina.

Question 18

What is the ‘convenience of use’ of a CCD?

Answer 18

It can be used in low temperatures of space
It can be used remotely
Detects a wider range of wavelengths
Exposure can last longer

Question 19

What is the ‘convenience of use’ of the eye?

Answer 19

You experience a loss of colour vision at low light levels

Question 20

Why are CCDs better than the naked eye?

Answer 20

Quantum efficiency is higher so, dimmer stars can be seen more easily.
CCDs can detect a wider range of wavelengths so, objects emitting non-visible light can be seen.
More convenient- image can be viewed remotely and recorded
Pixel size is smaller so, stars can be more easily seen as separate.

Question 21

How does a single dish radio telescope work?

Answer 21

It uses a big parabolic dish as a reflector and an aerial picks up the signal.
The larger the dish, the more energy per second it collects. Collecting power ∝ diameter^2

Question 22

What are the similarities between a radio telescope and an optical telescope?

Answer 22

The larger the diameter, the larger the collecting power, the larger the resolving power.

Question 23

What are the differences between a radio telescope and an optical telescope?

Answer 23

Wavelength used be a radio telescope are much longer so, is resolution is much lower than an optical telescope.

Question 24

What are the advantages of a radio telescope?

Answer 24

They are manoeuvrable, allowing the source of the wave to be tracked.

Question 25

How can radio telescopes be modified to increase resolution?

Answer 25

Multiple telescopes are linked together and their data can be combined to produce a single image. So, a resolution that is a thousand times better than an optical telescope can be produced.

Question 26

What are the disadvantages of radio telescopes?

Answer 26

They are affected by man-made interference from: Radio transmissions Mobile phones Radar systems Microwave ovens

Question 27

What is an alternative way in which radio telescopes can be made to make them cheaper?

Answer 27

Instead of a mirror, a wire mesh can be used where the precision of the dish must be about lambda/20 to avoid spherical aberration. This makes construction easier and cheaper than optical telescopes.

Question 28

What is the structure of an IR or UV telescope?

Answer 28

Similar to an optical reflecting telescope. But, CCDs are used as radiation detectors. The mirrors have to be more perfectly made as the light has a shorter wavelength and therefore more affected by imperfections.

Question 29

Where are IR telescopes located and why?

Answer 29

They must be positioned at high altitudes in dry places as most IR is absorbed by water vapour in the atmosphere. But, ideally they are sent to pace.

Question 30

Where are UV and x-ray telescopes located and why?

Answer 30

They can be strapped to high altitude weather balloons, aeroplanes or sent into space as the radiation is absorbed by most of the upper atmosphere.

Question 31

What are the disadvantages of IR telescopes?

Answer 31

Can only be built in specific locations They must be heat resistant and have its components cooled by liquid nitrogen to operate effectively.

Question 32

What are IR telescopes used for?

Answer 32

They’re useful for imaging relatively cool objects that don’t emit much visible light, like clouds of gas and dust where new stars are forming.

Question 33

What are UV telescopes used for?

Answer 33

Used to observe very hot stars with temperatures above 1000k that are dim and mostly emit UV.

Question 34

What are the disadvantages of x-ray telescopes?

Answer 34

They’re difficult to manufacture. X-rays are either fully absorbed by or fully penetrate most materials. So, you need special components. They must be outside the earths atmosphere.

Question 35

What are x-ray telescopes used for?

Answer 35

Observing extremely high energy emissions of matter entering a black hole.

Question 36

What is the structure of an x-ray telescope?

Answer 36

Grazing telescope. X-rays reflect if they graze the surface of a mirror. You can use multiple mirrors to gradually focus x-rays.

Question 37

What is the Hipparcos scale?

Answer 37

A first magnitude star is the brightest A sixth magnitude star is the dimmest

Question 38

What is the difference in intensity between a first and sixth magnitude star?

Answer 38

A magnitude 1 star has an intensity 100 times greater than a magnitude 6 star.

Question 39

What is the intensity of a star?

Answer 39

The effective brightness

Question 40

Which factors affect the brightness of a star?

Answer 40

Luminosity- total energy emitted per second Distance from earth

Question 41

How do you calculate the difference in brightness using the Hipparcos scale?

Answer 41

2.51^n

Question 42

What is a parsec?

Answer 42

The distance a star is from earth when the angle of parallax is 1 arc second.