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Astrophysics Telescopes Flashcards

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1
Q

How do you know if an image is virtual

A

When an object is on the same side of the focal point from the lens (this means the image is not inverted)

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2
Q

How do you know if an image is real

A

When the object is opposite to the focal point (this means the image is inverted)

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3
Q

What does a convex light do?

A

Focuses the incident light

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4
Q

What does a concave light do?

A

Spreads out the incident light

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5
Q

What is the principal axis

A

The straight line passing through the centre of the lens 90 degrees to its focus

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6
Q

What is the principal focus on convex and concave lens

A

Convex lens: the point where the incident light rays converge on the principal axis

Concave: the point where light rays should have appeared from behind the lens on the principal axis

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7
Q

What is the focal length?

A

The horizontal distance between the centre of the lens and the principal focus

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8
Q

What are real images

A

When light rays cross after refraction, real images can be formed on screen

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9
Q

What are virtual images

A

Formed on the same side of the lens, the light rays do not cross so a virtual image appears on the screen

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10
Q

What is the power of a lens>

A

A measure of how closely a lens can focus a beam that is parallel to the principal axis

  • converging lens: positive
  • diverging lens: negative
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11
Q

What is a refracting telescope? 1 mark

A

A telescope with two convex lenses
- objective lens and eyepiece lens

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12
Q

What is the role of the objective lens in a refracting telescope?

A

To collect light and create a real image of distant object.

It has a long focal length and is large to collect as much light as possible

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13
Q

What does the eye piece lens of a refracting telescope do?

A

Magnifies the image of the objective lens
lens produces a virtual image at infinity
reduces eye strain

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14
Q

What is normal adjustment for a refracting telescope

A

When the distance between the objective lens and eye piece lens is the sum of their focal length (fo + fe)

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15
Q

What is the formula for angle of magnification in a refracting telescope?

A

M = alpha / beta = fo/fe

Alpha is always bigger than beta,

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16
Q

What is the cassegrain reflecting telescope?

A

A telescope comprising of a small convex mirror and large concave mirror and an eye piece lens

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17
Q

Why are the mirrors in cassegrain reflecting scopes very thin and coated by silver atoms?

A

Allows mirrors to be as smooth as possible and minimise distortions in the image

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18
Q

What is chromatic aberration?

A

The focal length is of red light is greater than blue light which means they focus at different points (blue is refracted more than red)

  • this causes a white object to produce an image with coloured fringing
  • chromatic aberration is caused by refraction
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19
Q

What is spherical aberrration?

A

The curvature of the lens or mirror can cause light rays at the edge to be focused at different position, this leads to image blurring

  • occurs mostly in lenses with large diameters which can be avoided using parabolic objective mirrors.
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20
Q

What is Achromatic doublet

A

A method to reduce chromatic and spherical aberration, this is done by cementing together a convex lens and concave lens to allow all rays of light to focus in the same position

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21
Q

What are the disadvantages of refracting telescopes?

A
  • glass must be pure from defects
  • large lenses can bend under their own weight
  • chromatic and spherical aberrations
    -refracting telescopes are heavy
  • large magnifications require large diameters
  • lenses must be support only from the edges
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22
Q

Advantages of reflecting telescopes

A
  • mirrors are few nanometers thick and give excellent image quality
  • unaffected by by chromatic aberrations
  • mirrors not as heavy
  • easier to support from behind
23
Q

What is collecting power

A

A measure of the ability of a lens or mirror to collect incident EM radiation

24
Q

What is collecting power proportional to?

A

Pc = pi/4 * d^2

25
Q

What is normal adjustment?

A

When focal planes of objective lens and eye piece lens overlap

26
Q

Why are reflector telescopes better due to being the same size as refractor telescopes

A

For the same size you get a high magnification due to the mirror creating a larger length

27
Q

Why do you want a small angular resolution

A

This allows you to resolve objects that are very close to each other

Diffraction of dark fringe (degree/radians) = wavelength / Diameter of aperture

28
Q

How do we know if an image is fully resolved or just resolved

A

Fully resolved: when fixed dark fringes touch
Just resolved: when central brightness is touching

29
Q

When drawing spherical aberration which light ray creates a focal point closest to the lens

A

the furthest ray from the centre

30
Q

explain the structure of CCD

A
  • HIGH QE
  • incident photons help release electrons
  • electrons released dependent on intensity
31
Q

What in the atmosphere is responsible for absorbing infrared radiation

A

water vapour

32
Q

Why does water vapour affect the reading of stars temperatures

A

As water vapour absorb longer wavelengths (infrared)

Peak wavelength would be shifted to the left

This makes stars appear hotter

33
Q

Lens formula

A

1/u + 1/v = 1/f
u = distance of object from centre of lens
v = distance of image from centre of lens
f = focal length of lens

34
Q

What are radio telescopes 1 mark

A

Telescopes that use radio waves to create images of astronomical images

35
Q

Characteristics of radio telescopes

A

Atmosphere is transparent enough to absorb radio waves
Mostly ground based
Must be isolated to avoid interference from radio sources
Uses a parabolic dish

36
Q

Why do radio telescopes require a large diameter

A

As radio waves are large, larger diameter is needed to achieve the same resolving power as normal telescopes

37
Q

What are infrared telescopes?

A

Use infrared radiation to create images of astronomical images

38
Q

Characteristics of infrared telescopes

A

Large concave mirrors
Must be cooled to almost absolute zero due to all object emitting infrared radiation (do not pick up radiation on the dish itself)
Must be well shielded to avoid thermal contamination
Must be accessed from space

39
Q

What are ultraviolet telescopes

A

Telescopes that use ultraviolet radiation to create images of astronomical objects

40
Q

Why can’t UV telescopes be used on the ground

A

Ozone layer absorbs UV radiation

41
Q

Characteristics of UV telescopes

A

Must be in space
Uses cassegrain configuration
rays are detected by solid state devices which use photoelectric effect to convert UV photons to electrons to pass around a circuit

42
Q

X-ray telescopes?

A

Telescopes that use x-rays to create images of astronomical images

43
Q

Characteristics of X-ray telescopes

A

Must be used in space
- must use the combination of parabolic and hyperbolic mirrors which are extremely smooth due to high energy received
- brought into focus by CCDs which convert light into electrical pulses

44
Q

What are X-ray telescopes used to observe

A

High energy events like active galaxies, black holes and neutron stars

45
Q

What are gamma ray telescopes

A

Telescopes that use gamma radiation to create images of astronomical objects

46
Q

Characteristics of gamma telescopes

A

Gamma rays pass through them so they’re detected via a detector made of layers of pixels

47
Q

What do gamma rays observe

A

gamma ray bursts, quasars, black holes and solar flares

48
Q

What are the two types of gamma ray bursts (GRB)

A

Short lived - (0.01-1sec) merging neutron stars, formation of black holes, neutron stars falling into black holes

Long lived (10-1000sec) - type 2 supernova (death of massive stars)

49
Q

The greater the collecting power the…

A

…brighter the image

50
Q

According to Rayleigh Criterion, when is two objects not resolved?

A

If any part of the central maximum is either of the images fall within the first minimum diffraction ring

51
Q

What are charged-coupled devices?

A

An array of light sensitive pixels which become charged when they’re exposed to light by the photoelectric effect

52
Q

What is quantum efficiency

A

Percentage of incident photons which can an electron to be released

53
Q

What features of CCD can be compared to the human eye?

A

Quantum efficiency
Spectral range - detectable range of wavelengths
Pixel resolution - total no. of pixels
Spatial resolution ( min distance to resolve two images)
Convenience

54
Q

What is CCD quantum efficiency compared to the human eye?

A

around 80 percent
eye is around 4-5

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