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Physics A-Level > Astro A2 - Telescopes > Flashcards

Astro A2 - Telescopes Flashcards

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1
Q

What is the principal focus

A

Where the light rays which were refracted by a lens cross

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2
Q

What is the principal axis

A

Line parallel to light rays before entering the lens

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3
Q

What is the focal length

A

Length between the lens and the principal focus

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4
Q

Why is a lens depicted as a very thin lens

A

So that a light ray travelling through the middle of the lens can be depicted as a straight line

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5
Q

What occurs when an object is further than one focal length away from a lens

A

Real image, inverted on the other side of the object, magnified

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6
Q

What occurs when an object is one focal length away from a lens

A

An image is not formed

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7
Q

What occurs when an object is less than one focal length away from a lens

A

Virtual image, upright behind the object, magnified

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8
Q

What is in a refracting telescope

A

Two lenses, an objective lens and an eyepiece lens

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9
Q

What is the objective lens

A

The lens that gathers all the light from the object at infinity

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10
Q

What is the eyepiece lens

A

The lens which forms a magnified virtual image at infinity

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11
Q

What does it mean for a telescope to be in “normal adjustment”

A

Distance between both lenses = objective focal length + eyepiece focal length

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12
Q

How to find the angle subtended by an object

A

Theta = h/d, where h is object height and d is distance away

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13
Q

What will occur when a circular mirror reflects light

A

There are many focal points on the principal axis

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14
Q

What is the shape of a mirror to have a single focal point on the principal axis from parallel light

A

Parabolic mirror

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15
Q

Describe the Cassegrain telescope

A

Light is first reflected off a parabolic mirror. This travels to a second convex reflector at the focal point. This directs the light out of a small hole in the parabolic mirror, and into an eyepiece

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16
Q

What is chromatic aberration

A

Light with longer wavelength refract less, so causes different focal points for different wavelengths. Only an issue for diffractive telescopes. Causes blurred edges

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17
Q

What is spherical aberration

A

Aberration caused by spherical lenses

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18
Q

What are the main advantages of reflective telescopes

A

Reflectors can be made much larger than refractors because a mirror can be supported from behind, whereas a lens must be supported at the edge. A large lens is likely to break under its own weight

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19
Q

What are the similarities of an optical telescope and a radio telescope

A

Parabolic surfaces to reflect

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20
Q

What are the differences of an optical telescope and a radio telescope

A

No need for a secondary reflector as the aerial can be placed at the focal point

21
Q

What are some issues with the radio telescope

A

Poor resolving power, man-made interference is a big issue. Interference can be minimised by building telescopes away from centres of population

22
Q

What are advantages of a radio telescope

A

The reflective dish can be made of a mesh with size smaller than λ/20, due to radio’s large wavelength

23
Q

Where and why are most telescopes placed

A

Into space due to absorption of waves by the atmosphere, light pollution, interference at ground level, atmosphere changes direction of light

24
Q

Where are infrared telescopes used

A

4200m above sea level for finding exoplanets

25
Q

Where are ultraviolet telescopes used

A

Low earth orbit for analysing quasars

26
Q

Where are x-ray telescopes used

A

Earth orbit to discover supermassive black holes

27
Q

What are the advantages of large diameter telescopes

A

Greater collecting power so images are brighter, better resolving power so images are clearer

28
Q

What is collecting power

A

The rate at which useful energy is received by a telescope. Proportional to area

29
Q

How to increase collecting power

A

Larger diameter, increase exposure time, use sensitive detectors

30
Q

What is resolving power

A

Same as the minimum angular resolution

31
Q

Describe the diffraction pattern by a circular aperture

A

A large central maximum called airy disc which is twice as wide as the further maxima. Propagates outwards with maxima and minima

32
Q

What is resolution

A

How much detail the telescope can show

33
Q

What is the Rayleigh Criterion

A

‘Two objects will be just resolved if the centre of the diffraction pattern of one image coincides with the first minimum of the other’

34
Q

What is the equation for minimum angular separation by the Rayleigh Criterion

A

θ = λ/D, where D is the diameter of the objective, θ in radians

35
Q

The lower the minimum angular separation…

A

The better the resolution

36
Q

What affects the real value of the resolving power

A

Refraction of light, the spider holding the secondary mirror in a Cassegrain mirror

37
Q

What is 1 arcminute

A

1/60 of a degree

38
Q

What is 1 arcsecond

A

1/3600 of a degree

39
Q

What is a CCD

A

A charge-coupled device is a semiconductor which light is converted directly into digital information

40
Q

Describe the structure of a CCD

A

A silicon chip made of millions of pixels, connected to electrodes.

41
Q

How does a CCD work

A

Photons striking the silicon liberate electrons which are then trapped in potential wells of the pixels. When sufficient electrons have been liberated for a full exposure, the electrodes are used to shuffle the electrons to be measured and create the image

42
Q

What is the response of a CCD

A

Linear response, so that very faint parts of a relatively bright image can be obtained

43
Q

What is quantum efficiency

A

Number of photons detected/ Number of photons incident

44
Q

What is the quantum efficiency of a CCD

A

70-80%

45
Q

What is the quantum efficiency of an eye

A

1% at 550nm

46
Q

What is the resolution, compared between CCD and the eye

A

CCDs and the human eye have roughly similar resolution

47
Q

Why are CCDs more convenient than using a telescope or the eye

A

Allows for remote viewing, direct analysis, long exposure times and detection of wavelength beyond the visible spectrum

48
Q

What is an airy disc

A

The central maximum of spherical diffraction of light

49
Q

What is the response of an eye

A

Logarithmic

Physics A-Level (19 decks)

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