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Physics (Paper 2 + 3) > Telescopes > Flashcards

Telescopes Flashcards

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1
Q

Draw a ray diagram to show a converging lens producing a real inverted & diminished image

A
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2
Q

Draw a ray diagram to show a converging lens producing a magnified and virtual image

A
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3
Q

What are the main components of a refracting telescope

A

Objective & eyepiece lens

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4
Q

Explain what is meant by the term normal adjustment for a refracting telescope

A

The distance between the object lens and the eye piece lens is equal to the sum of their focal lengths

d = F(o) + F(e)

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5
Q

Draw a ray diagram for a refracting telescope in normal adjustment

A

1) F(o) > F(e) in a telescope

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6
Q

Describe the image produced my a refracting telescope

A

A Magnified and virtual image

image is seen at infinity

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7
Q

Describe the main components of a Cassegrain telescope

A

Concave Primary Mirror with a slit in the centre

Small Convex mirror

Eyepiece lens

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8
Q

Draw a ray diagram to show the path of two light rays,

. Parallel to the principle axis
. Through a Cassegrain telescope

A

primary concave mirror must be 1 mirror with a slit in the middle not two mirrors

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9
Q

State the Advantages of a reflecting telescope compared to refracting telescopes (4)

A

. Mirrors don’t suffer from Chromatic aberration as much
. Mirrors can be larger and therefore have greater collecting powers & smaller minimum angle resolution
.Parabolic mirrors are easier to manufacture then parabolic lens’
.Reflecting telescopes are more compact for the same effective focal length

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10
Q

State the disadvantages of a reflecting telescope in comparison to refracting telescopes

A

.Secondary Mirror blocks some light so less light hits the primary mirror

. Light is diffracted as it passes the secondary mirror affecting the image

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11
Q

Draw a ray diagram to show how spherical aberration can occur in a reflecting telescope

A
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12
Q

State how the problem of spherical aberration can be eliminated

A

Using perfectly parabolic mirrors for reflecting telescope and perfectly parabolic lenses from refracting telescopes

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13
Q

What is Chromatic aberration

A

Image distortion (image is tinged with colour) due to light of different wavelengths focussing at different points along an optical axis

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14
Q

Draw a diagram to show how a converging lens can produce chromatic aberration

A
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15
Q

Explain what is meant by the term Airy Disc

A

The Smallest point to which a beam of light can be focussed using a lens

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16
Q

What is meant by the Rayleigh Criterion

A
  • identifies the minimum subtended angle between objects whose images can be resolved
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17
Q

State 2 advantages of using telescopes with a larger diameter objective (lens or mirror)

A

Increased collecting power (light collected is proportional to the area of the objective)

Light Collected is proportional to 1/4 * Pi * Diameter^2

Increased Resolving power —> Can resolve smaller angles

Size of angle resolved is inversely proportional to diameter

18
Q

Describe the Structure of a CCD (3)

A

P type silicon doped in boron gas (more conductive)

Divided into an array of the picture elements/ pixels

Each pixel has 3 metal electrodes embedded in an insulating layer of silicon dioxide

19
Q

Describe how a CCD works to produce an image (5)

A

Photons are incident of the CCD

Electrons are liberated from the surfaces of the electrodes within the pixels

Number of liberated electrons is proportional to the number of incident photons (brightness)

The Liberated electrons are trapped in a Potential Well within the Silicon layer

After Exposure the Charge on each pixel is measured and an image is formed

20
Q

State Advantages of using CCD’s for Astronomical Observations

A

Higher Quantum Efficiency

Long exposure times to capture faint images

Device can be directly linked to a computer for capture and analysis

21
Q

What is meant by Quantum efficiency

A

% of incident phoned on a photoreceptors that are detected

Number of photons producing an electron / total number of incident photons * 100

22
Q

What is the Principle Focus (3)

A

A point on the axis
which is the same distance from the lens (optical Centre) as the focal length
Where light rays traveling parallel to the principle axis prior to refraction converge

23
Q

Define Focal Length

A

Distance Between the centre of the lens and the principle focus

24
Q

In the lens Equation

1/v + 1/u = 1/f

1) What does u represent?
2) What does v represent?
3) What does f represent?

A

1) Distance between the object and the centre of the lens (always positive)
2)Distance between image and the centre of the lens ( +ve for real images , -ve for virtual images)
3) Focal Length of lens (+ve for converging lens, -ve for diverging lens)

25
Q

Give both formulas for angular magnification in Normal Adjustment

A

-> indication of widely an image takes up your field of vision

M = Angle subtended by image at eye / angle subtended by object at an unaided eye.

M = Focal length of objective / focal length of eyepiece
(using small angle approximations only)

26
Q

How does an Astronomical Refracting telescope work (4)

A
  • Two converging lens’ (eyepeice and objective)
  • Objective lens role is to collect light and create a real image of a distant object
    -Eye piece lens role is to magnify the image created by the objective lens
    -Eye piece lens produces virtual image formed at infinity for the user to observe,
27
Q

How does a Cassegrain telescope work (4)

A

-large concave primary mirror with a slit in the centre
Smaller convex secondary mirror placed in front of the slit
-Primary mirror collect light and focus it onto the secondary mirror
-Light then is reflected into an eye piece lens producing a virtual image forming at infinity

28
Q

What is Spherical aberration

A

-When light is focused at different points along the principle axis
-due to a non-perfectly parabolic curvature of a lens or mirror
-Causing a blurred image

29
Q

Draw a lens Diagram representing spherical aberration in a lens

A
30
Q

What happens when you increase the size of the objective mirror/lens

A

Increasing diameter means you can observe fainter objects, as resolving power increases. meaning a smaller angle between two images can be resolved

Collecting power us proportional to (objective Diameter)^2

produces brighter images

31
Q

Explain the Structure ,Positioning and uses of a single dish radio telescope.

A

Structure : Large parabolic dish that focus’ em radiation onto a receiver

Positioning: Can be ground based but must be in isolating locations with not a lot of interference

uses: observing things such as galaxies, stars and black holes.

32
Q

Why do radio telescopes need tho be larger then optical telescopes?

A

Radio waves have a larger wavelength then visible waves

in order to achieve the same resolving power the objective diameter must be larger in accordance to (angle) = wavelength/ Diameter

33
Q

Explain the Structure ,Positioning and uses of an infrared telescope

A

Structure: Large Concave mirror focusing light onto a detector, must be cooled with cryogenic fluids to avoid interference
positioning: must be in space as infrared light is blocked by the atmosphere
uses: observing coolers regions of space

34
Q

Explain the Structure ,Positioning and uses of an ultraviolet telescope

A

Structure: Cassegrain configuration that focus’ radiation onto solid state devices
Positioning: must be in space as uv is blocked by the ozone layer
uses: observing the interstellar mediums and star formation regions

35
Q

Explain the Structure ,Positioning and uses of an x-ray telescope

A

Structure: Combination of hyperbolic and parabolic mirrors to focus radiation onto a CCD
Positioning: must be in space as x-rays are blocked by the atmosphere
Uses: Observing high-energy events and areas such as active galaxies, black holes and neutron stars

36
Q

Explain the Structure ,Positioning and uses of a Gamma Telescope

A

Structure: No mirrors , radiation passes through a. detector made of layers of pixels
Positioning: must be in space as gamma rays are blocked by the atmosphere
uses: observing gamma ray bursts, quasars, black hole and solar flares

37
Q

Write the equation that is used in the “Rayleigh Criterion’

A

Smallest angle that can be resolved = wavelength/ diameter on objective lens

38
Q

What is Collecting power

A

Rate at which useful energy is collected by a telescope

proportional to the diameter^2 of the objective or aperture

proportional to the area of the aperture

39
Q

Compare the Resolution of a CCD to the human eye

A

CCD has a spacial resolution of 10 micrometers, human eye has 100 micrometers

CCD can resolve a smaller angle then the human eye

So the image has a higher resolution

40
Q

What is Spacial Resolution

A

Minimum distance two objects must be apart to distinguish them

Smaller the distance, smaller the angle the telescope can resolve

41
Q

Explain the advantages of a Parabolic Dish Shape over a Spherical dish shape

A

No Spherical Aberration

Physics (Paper 2 + 3) (13 decks)

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