Telescopes

Question 1

What is a convex/converging lens?

Answer 1

Lens which focuses incident light.

Question 2

What is a concave/diverging lens?

Answer 2

Lens which spreads out the incident light.

Question 3

Define Principle focus/ Focal point.

Answer 3

In a ​convex lens: The point where incident beams ​parallel to the principal axis​ will ​converge​.
In a concave lens: The point from which light rays appear to come from.

Question 4

Define focal length.

Answer 4

The distance between the centre of a lens and the principal focus.

Question 5

What does focal length tell you about the lens used?

Answer 5

Shorter focal length = stronger the lens.

Question 6

Draw a ray diagran showing the action of a converging lens on a beam of (parallel) light.

Answer 6

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Question 7

What kinds of image does a converging lens produce?

Answer 7

Produces both a real image and virtual image.

Question 8

What is a real image?

Answer 8

Image formed when light rays cross after refraction (on the lens-air boundary).

Question 9

What is a virtual image?

Answer 9

Image that appears to have come from behind the lens. Light rays do not cross.

Question 10

What is one difference between real and virtual images?

Answer 10

Real images can be formed on a screen, a virtual image cannot.

Question 11

When is a real image formed?

Answer 11

When an object is further away than the focal length a real image is formed, otherwisea virtual image is formed.

Question 12

Draw a ray diagram showinf how a real and virtual image is formed when light is passed through a convex lens.

Answer 12

• Show three rays coming from object: one axial ray (ray parallel to the principal axis), one passing through the centre of the lens (this one does not get refracted at all), and a third one that travels to the end of the convex lens to be refracted.
• If object sits on principal axis, axial ray and ray through principles axis will be the same.
• Label ‘object’ and ‘convex lens’, the ‘focal length’ and ‘virtual or real image’ - depends which one you are drawing (DRAW BOTH).

When drawing the virtua image diagram you can draw just two of the rays - axial ray and ray that passes through principal axis.

Question 13

What will happens to NON-AXIAL parallel light incident on a conve lens

Answer 13

They will be incident on the focal place. Check cgp book to see a picture.

Question 14

You dont need to know this equation but it is helpful.

The distance between the object and the lens axis (u) and the distance between image and the lens (v) are related by the equation?

Answer 14

1/f = 1/u + 1/v

where:
f = focal length
u = distance between the object and the lens axis
v = distance between image and the lens

Question 15

How do you know if the image is virtual or real from the equation: 1/f = 1/u + 1/v

Answer 15

If v is positive = real image. If v is negative = virtual image

Question 16

What are the two main types of optical telescopes?

Answer 16

• Refracting telescopes

- Reflecting telescopes

Question 17

What does a refracting telescope consist of?

Answer 17

It consists of two convex lenses that focus the incident light by refracting it.

Question 18

What are the two convex lenses that it contains?

Answer 18

The objective lens which produces a real inverted image of the very distant object and the eyepiece lens magnifies and reinverts the image this image correcting it to produce a magnified inverted virtual image.

Question 19

In a refracting telescope, how are the eyepiece and objective lens arranged?

Answer 19

The refracting telescope is put in normal adjustment.

Question 20

What is a refracting telescope in normal adjustment?

Answer 20

A telescope in normal adjustment is set up so that the principal focus of the objective lens is in the same position as the principal focus of the eyepiece lens.

Question 21

Why is the eyepiece lens adjusted so it hs the same principal focus as the objective lens? i.e. why is a refractive telescope put in normal adjustment?

Answer 21

This is so that the light rays from each point of the real image leave the eyepiece parallel to to one another (the reverse of what has happened with the objective lens). To the viewer these rays appear to come from a virtual image at infinity.

Question 22

Draw a ray diagram showing light rays from a istant object passing through a refracting telescope in normal adjustment.

Answer 22

• Draw a straight non-axial ray that passes through the CENTRE of the object lens and ends at the eye lens axis.
• Draw two non-axial parallel rays parallel to the first one drawn on either side and end at objctive lens axis. Draw a straight line from where each of these rays meet the objective lens axis, so that all of the rays cross at the same point on the focal plane and reach the eye lens axis. A real imgage is formed where they cross.
• Draw a dotted line that passes through the point the rays cross and the centre of the eyepiece lens. Label construction line. Continue the three rays drawn so they are refracted at the eye lens axis and leave the lens parallel with the construction line. You can show the virtual image formed at infinity by extending these lines backwards using these dotted line. Label objective lens, eyepiece lens, objective lens focal length, eyepiece lens focal length

Question 23

What is angular magnification? Equation?

Answer 23

Angular magnification is the magnifying power of a refracting telescope. It can be calculated in terms of angles.

M = θₑ/θₒ

M = angular magnification
θₑ angle subtended by image at eye.
θₒ = angle subtended by object at unaided eye.

Look at cgp book for a really good image on what these terms^^ mean.

*Units do not matter as long as they are the same for both angles.

Question 24

If the refracting telescope is in normal adjustement you can also calculate the angular magnification in terms of? Equation?

Answer 24

If the refracting telescope is in normal adjustement you can also calculate the angular magnification in terms of focal lengths.

M = fₒ/fₑ

M = angular magnification
fₒ = objective lens focal length
fₑ = eyepiece lens focal length

*Units do not matter as long as they are the same for both lengths.

Question 25

What are some of the fundamental flaws of refracting telescopes?

Answer 25

Refracting telescopes suffer from chromatic and spherical abberations.

Question 26

What is chromatic abberation?

Answer 26

This is when refraction by the objective convex lens causes white light to seperate into its component colours (wavelengths) so that the different colours/wavelengths are focused by the objective lens over a range of focal lengths.

Question 27

Why does chromatic abberation occur?

Answer 27

This is bcause the refractive index of the objective lens' material is different for different wavelengths of light.

Question 28

What kind of image does chromatic abberation produce?

Answer 28

This produces coloured edges/fringes around the image.

Question 29

Draw chromatic abberation showing two different wavelengths of light e.g. red and violet

Answer 29

On the Collins Astrophysics book.

Question 30

What is spherical abberation?

Answer 30

This is when light rays in a parallel beam are refracted and focused at slightly different positions. Light rays near the edge of the lens are deviated more than those near the principal axis.

Question 31

What is spherical abberation caused by.

Answer 31

It is caused by the curvature of the lens being too spherical.

Question 32

How can the effect of spherical abberation be reduced in a refracting telescope?

Answer 32

The effect can be minimised by making both surfaces of the lens contribte equally (but oppositely) to the ray deviation. ASK SIR THIS QUESTION AND CHECK WITH HIM .

Question 33

Which refracting telescopes is the effect of spherical abberation greater?

Answer 33

The greater the diameter of the lens the greater the effect of spherical abberation.

Question 34

What type of image does spherical abberation produce?

Answer 34

Spherical abberation causes blurring and distortion of the image.

Question 35

Draw a ray diagram to show spherical abberation.

Answer 35

On the Collins Astrophysics book.

Question 36

What does a reflecting telescope consist of?

Answer 36

There are many different types of reflecting telescopes. The Cassegrain reflecting telescope arrangement uses a curved parabolic concave primary mirror which collects light from a distant object and directs it onto a curved hyperbolic secondary convex mirror which redirects the rays down an oppening in the primary mirror, where the image is magnified and focused using an eyepiece lens.

Question 37

Draw a ray diagram for a Cassegrain reflecting telescope arrangement. Show two incident waves.

Answer 37

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Question 38

Are reflecting telescopes affected by chromatic abberation and explain why?

Answer 38

Refelecting telescopes are unaffected by chromatic abberation. This is because chromatic abberation is caused by refraction, and mirrors use reflection, not refraction. However if an eyepiece lens, there may be some chromatic abberation due to it.

Question 39

Are reflecting telescopes affected by spherical abberation?

Answer 39

If the primary concave mirror has a spherical curvature, it is affected by spherical abberation.

Question 40

How can spherical abberation be reduced in reflecting telescopes?

Answer 40

The primary concave mirror should be made parabolic in shape, so that it focuses parallel light rays onto a single focal point.

Question 41

What other problems are there with reflecting telescopes?

Answer 41

The secondary convex mirror and the mirrors' supports will block some incoming light. Some of the light reflected from the primary mirror will diffract arround the secondary mirror. These both lead to a decrease in image clarity.

Question 42

Other disadvantages of refracting telescopes apart from abberations?

Answer 42

- Large diameter lenses are heavy and tend to distort their shape under their own weight - Heavy and difficult to manouvre quickly - Large magnifications require large objective lenses and very long focal lengths. - Any impurities in the glass absorb and scatter the light so faint objects cannot be seen - expensive to make good quality, large lenses.

Question 43

Advantages of reflecting telescopes?

Answer 43

Large mirrors of good quality are much cheaper to build than large lenses.

Question 44

What are CCDs?

Answer 44

Sensitive light detectors to capture images digitally.

Question 45

What is quantum efficiency? Give equation.

Answer 45

Quantum efficiency = number of photons arriving at detector and being detected / total number of photons arrivign at detector.

Question 46

What is the quantum efficiency for a CCD?

Answer 46

The quantum efficiency of a CCD is > 80%

Question 47

The response of an eye to light is fairly complicated but we can say what it is at low light levels (the value below): What is the quantum efficiency of an eye?

Answer 47

The quantum efficiency of an eye is = 1%

Question 48

Resolution of a CCD?

Answer 48

Resolution of a CCD is related to the size of each pixel - smaller pixels allow more detail to be observed.

Question 49

What is the resolution of the eye?

Answer 49

The size of light sensitive cells in the eye (the equivalent of a pixel) is similar in size to that of a CCD pixel (2-4μm) so the resolution for an eye is SIMILAR to a CCD.

Question 50

How are CCDs different to the eye in terms of the wavelengths that they can detect?

Answer 50

The eye can only detect visible light, whereas CCDs can detect wavelength beyond the visible spectrum (visible, IR and UV)

Question 51

How convenient are CCDs in comparison to the eye?

Answer 51

CCDs are useful in telescopes because they allow for remote viewing, direct computer analysis and long exposure times (as well as being able to detect a wide range of wavelengths). CCDs also produce digital images that can be copied and shared. However, they need extra equipment and its easier to look down a telescope than setting up a CCD.

Question 52

What is the resolving power of an object?

Answer 52

This is the ability of a telescope to distinguish between two seperate/distant objects.

Question 53

What is the resolving power dependant on?

Answer 53

The resolving power of a telescope is dependant on the minimum angular resolution.

Question 54

What is the minimum angular resolution?

Answer 54

This is the smallest angular seperation at which the instrument can distinguish between two points. Image of CGP book

Question 55

The smaller the minimum angular resolution....

Answer 55

...the better the resolving power of the telescope.

Question 56

What is the resolving power limited by?

Answer 56

The resolving power/resolution is limited by diffraction..

Question 57

When does diffraction occur in a telescope?

Answer 57

When light enters the telescope through its aperture/opening it is slightly diffracted.

Question 58

What happens when light from an object diffracts at the telescope?

Answer 58

The light waves will interfere constructively and destrictvely to produce a diffraction pattern of bright and dark fringes are formed.

Question 59

What is the broad central maxima formed due to diffraction at the aperture of a telescope called?

Answer 59

The Airy disc

Question 60

What does the size of the Airy disc determine?

Answer 60

Determines the amount of detail that can be seen in an image or how much blurring effect there is.

Question 61

What does a smaller width of an Airy disc tell you?

Answer 61

Smaller width = higher resolution.

Question 62

How can we reduce the width of the Airy disc formed be reduced?

Answer 62

By increasing te aperture of the telescope as big as possible.

Question 63

Consider two stars, L distance away from each other. What will happen if they are too close?

Answer 63

Their Airy discs will overlap.

Question 64

Why is the degree of overlap important to us?

Answer 64

The degree of overlap determines whether the two stars can be resolved or not.

Question 65

Two light sources are just distinguished/resolved when..

Answer 65

...the centre of the Airy disc (central maximum of the diffraction pattern) from one source coincides with the first minimum of the second source. This is the Rayleigh criterion. This is the same as the minimum distance between the two light sources when the centre of one Airy disc falls outside the Airy disc of the other source.

Question 66

Rayleigh criterion equation?

Answer 66

θ = λ/D ``` θ = Minimum angular resolution (rad) - when it asks to calculate resolving power this is what you work out. λ = Wavelength (m) D = Diameter of the aperture (m) ``` SMALLER ANGULAR RESOLUTION = HIGHER RESOLVING POWER. * For telescopes, D is the diameter of hte objective lens or the objective mirror.

Question 67

What is collecting power proportional to? Collecting power is the energy collected per second.

Answer 67

Collecting power is proportional to the (dish) diameter² So the greater the area of the collecting dish, the greater the collecting power.

Question 68

What is the collecting area for a radio, optical, UV or IR telescope?

Answer 68

This is the area of the objective mirror or dish.

Question 69

What is the collecting area for X-ray telescopes?

Answer 69

The size of the opening through which X-rays enter the telescope.

Question 70

SHOULD HAVE PUT THIS EQUATION EARLIER. How do you calculate the anfular size of an object - Equation?

Answer 70

θ = s/r ``` θ = angular size (in rad) s = distance to the object (m) r = diameter of the object (m) ```

Question 71

Non-optical telescopes: How do radio telescopes work?

Answer 71

Parabolic dish works in the same way as the primary concave lens of an optical reflecting telescope. However, instead of a mirror, a wire mesh is used, since the long wavelengths of radio waves do not pass through. It is reflected onto the antenna that is used as a detector at the principle focus so thee is only one reflectin surface rather than two. The signal is then amplified, filtered and analysed to produce a digital image.

Question 72

Compare the collecting power of a radio telescope and an optical one.

Answer 72

Collecting power is proportional to the diameter². As radio telescopes are much larger than optical telescopes, they'll have better collecting power and therefore be able to view more distant objects. Why does higher collecting power mean that they will be able to view more distant objects?

Question 73

Compare the resolving power of a radio telescope and an optical telescope.

Answer 73

Radio telescopes have a much lower resolving power than an optical microscope. This is because of the large wavelength of radio waves comapred to visible light (about a million time greater). Therefore, according to the equation θ = λ/D, to have a high enough resolving power, it must have a very very large diameter that's not possible. This is why radio telescopes tend to have a large diameter apertures - still not big enough to have a resolving power greater than an optical one.

Question 74

Where are radio telescopes positioned?

Answer 74

They are ground-based because the atmosphere is transparent to a large range of radiowavelengths. They are positioned away from radio transmitters which may drown radio signals from astronomical object. They are also located in isolated areas because suffer from interferences from other devices.

Question 75

Where are optical telescopes positioned?

Answer 75

Optical telescopes are both ground-based as well as in space. This is because even though visible light can pass through the atmosphere it is more affected by the atmosphere than radio waves.

Question 76

Benefits of radio telescopes?

Answer 76

Dish is made of wire messh than mirror - much cheaper and easier to make than optical reflectors. Longer wavelength being detected, so the less affected it is by imperfections in the shape of the dish or mirror collecting it. Does not have to be as perfect as optical mirrors or lenses to avoid problems like spherical abberation.

Question 77

Problems with ground based telescopes?

Answer 77

- Waves scattered/absorbed by Earth's atmosphere, affects amount of light detected and the resolution. - Light pollution and other interferences at ground level.

Question 78

Problems with telescopes in orbit?

Answer 78

- Need their own power source. - If something goes wrong, difficult to service or fix. - Signals have to be sent down back to Earth to be analysed.

Question 79

How do UV and IR telescopes work?

Answer 79

Similar setup to optical reflecting telescopes. Uses parabolic mirror to focus light onto a detectors. Uses CCDs to act as radiation detectors just like in optical telescopes.

Question 80

Compare optical telescope mirror shapes with that of UV and IR telescopes?

Answer 80

Longer wavelength = the less affected it is by imperfections in the mirror. As a results, mirrors in IR telescopes don't need to be as perfectly shaped as optical telescopes. However, UV telescopes need more precisely shaped mirrors than optical telescopes.

Question 81

Structure of an X-ray telescope?

Answer 81

Different structure to optical telescopes. They don't really reflect, rather absorbed or passes through. However if they just graze a mirror surface, they reflect. This can be done using nested mirrors, allowing it to be focused onto a detector. Known as a grazing telescope.

Question 82

Problem with IR telescopes?

Answer 82

They produce their own IR radiation due to their temperature, so need to be cooled to low temperatures.

Question 83

Ground telescopes?

Answer 83

Optical telescopes | Radio telescopes

Question 84

High altitude telescopes?

Answer 84

IR telescopes

Question 85

Space telescopes?

Answer 85

UV telescopes X-ray telescopes Gamma telescopes (They can also be taken into the atmosphere, rather than into space)

Question 86

What is the diameter of the holes in the wire mesh?

Answer 86

20mm

Question 87

How do we overcome the low resolving power of telescopes?

Answer 87

Banks of radio telescopes are linked together. Computer software combines data from the dishes and forms a single image. This is equivalent to one huge dish.