telescopes

Question 1

What are refracting telescopes made of?

Answer 1

Two converging lenses: the objective lens and the eyepiece lens

Question 2

What does the objective lens of a refracting telescope do?

Answer 2

Collect light and create a real image of a very distant object. The collecting power of a telescope is directly proportional to the square of the diameter of the objective lens.

Question 3

What does the eyepiece lens of a refracting telescope do?

Answer 3

Magnifies the image produced by the objective lens so that the observer can see it. This lens produces a virtual image at infinity since the light rays are parallel. This reduces eye strain for the observer as they do not have to refocus every time they look between the telescope image and the object in the sky.

Question 4

When is a refracting telescope in normal adjustment?

Answer 4

distance between lenses = f0 + fe
so the principal focus of the two lenses is in the same place
and when the final image is at infinity

Question 5

How do you calculate magnifying power / angular magnification, M?

Answer 5

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

When both angles are less than 10 degrees, you can say
M = alpha/beta = f0/fe

Question 6

What is chromatic aberration?

Answer 6

Light of different wavelengths refracted to different foci.
For a given lens, the focal length of red light is greater than that of blue light, which means they are focused at different point (since blue is refracted more than red). This can cause a white object to produce an image with coloured fringing, with the effect being most noticeable for light passing the edges of the lens. It only occurs in the eyepiece lens.

Question 7

What is spherical aberration?

Answer 7

Different focal points for rays at different distances from axis.
The curvature of a lens or mirror can cause rays of light at the edge to be focused in a different position to those near the centre, leading to image blurring and distortion. This effect is most pronounced in lenses with a large diameter, and can be avoided completely by using parabolic objective mirrors in reflecting telescopes.

Question 8

How is material a disadvantage for refracting telescopes and an advantage for reflecting telescopes?

Answer 8

Glass must be pure and free from defects. Achieving this for a large diameter lens is very difficult.
Mirrors that are just a few nanometres thick can be made and these give excellent image quality.

Question 9

How is aberration a disadvantage for refracting telescopes and an advantage for reflecting telescopes?

Answer 9

Chromatic and spherical aberration both affect lenses.
Mirrors are unaffected by chromatic aberration, and spherical aberration can be solved by using parabolic mirrors. Though chromatic aberration can affect the eyepiece lens, this can be solved by using an achromatic doublet.

Question 10

How is weight a disadvantage for refracting telescopes and an advantage for reflecting telescopes?

Answer 10

Large lenses can bend and distort under their own weight due to how heavy they are. They are incredibly heavy and therefore can be difficult to manoeuvre.
Mirrors are not as heavy as lenses, so they are easier to handle and manoeuvre to follow astronomical objects/events.

Question 11

How is support a disadvantage for refracting telescopes and an advantage for reflecting telescopes?

Answer 11

Lenses can only be supported from the edges, which can be an issue when they are large and heavy.
Large primary mirrors are easy to support from behind since you do not need to be able to see through them.

Question 12

What are the components of a Cassegrain reflecting telescope?

Answer 12

A concave primary mirror with a long focal length and a small convex secondary mirror in the centre.

Question 13

Why does chromatic aberration have very little impact on reflecting telescopes?

Answer 13

It is caused by refraction.

Question 14

Why do radio telescopes need to be in isolated locations?

Answer 14

To avoid interference from nearby radio sources.

Question 15

What is the similarity between radio and optical telescopes in how they function?

Answer 15

Both intercept and focus incoming radiation to detect its intensity.

Question 16

What is the similarity between radio and optical telescopes in how they can be moved?

Answer 16

Both can be moved to focus on different sources of radiation, or to track a moving source.

Question 17

What is the similarity between radio and optical telescopes in terms of where they can be built?

Answer 17

Both can be built on the ground since both radio waves and optical light can pass easily through the atmosphere.

Question 18

What is the difference in size between optical and radio telescopes? And which one has a larger collecting power?

Answer 18

As radio wavelengths are much larger than visible wavelengths, radio telescopes have to be much larger in diameter. This means that they have a larger collecting power.

Question 19

What is the difference in price between a radio and an optical telescope?

Answer 19

Constructing radio telescopes is cheaper and simpler because a wire mesh is used instead of a mirror.

Question 20

What causes interference in radio telescopes?

Answer 20

Radio transmissions, phones, microwave ovens.

Question 21

What causes interference in optical telescopes?

Answer 21

Weather conditions, light pollution, stray radiation.

Question 22

What has to be done when making infrared telescopes?

Answer 22

As all objects emit infrared radiation as heat, infrared telescopes must be cooled using cryogenic fluids (such as liquid nitrogen or hydrogen), to almost absolute zero. They must be well shielded to avoid thermal contamination.

Question 23

Can infrared telescopes be used to observe cooler regions of space?

Answer 23

Yes. However, as the atmosphere absorbs most infrared radiation, these telescopes must be launched into space and accessed remotely from the ground.

Question 24

How do ultraviolet telescopes work?

Answer 24

They use the Cassegrain configuration to bring ultraviolet rays into focus, which are detected by solid state devices which use the photoelectric effect to convert UV photons into electrons, which then pass around a circuit. As the ozone layer blocks all ultraviolet rays that have a wavelength of less than 300nm, they have to be positioned in space.

Question 25

What are UV telescopes used to observe?

Answer 25

Interstellar medium and star formation regions.

Question 26

How do X-ray telescopes work?

Answer 26

A combination of parabolic and hyperbolic mirrors. They convert light into electrical pulses. Since all X-rays are absorbed by the atmosphere, they need to be in space to collect data.

Question 27

What are X-ray telescopes used to observe?

Answer 27

Active galaxies, black holes, and neutron stars.

Question 28

How do gamma telescopes work?

Answer 28

A detector made of layers of pixels. As the gamma photons pass through, they cause a signal in each pixel they come into contact with.

Question 29

What are gamma telescopes used to observe?

Answer 29

Gamma ray bursts (GRBs), quasars, black holes, and solar flares.

Question 30

What is a short-lived GRB?

Answer 30

Last between 0.01 and 1 second, associated with merging neutron stars (forming black holes), or a neutron star falling into a black hole.

Question 31

What is a long-lived GRB?

Answer 31

Last between 10 and 1000 seconds, associated with Type II supernova.

Question 32

What is the collecting power of a telescope?

Answer 32

A measure of the ability of a lens or mirror to collect incident EM radiation. Collecting power is directly proportional to the square of the diameter of the objective lens.

Question 33

What is resolving power?

Answer 33

The ability of a telescope to produce separate images of close-together objects. For an image to be resolved, the angle between the straight lines from Earth to each object must be at least the minimum angular resolution, measured in radians: theta = lambda / D Where lambda is the wavelength of radiation and D is the diameter of the objective lens or mirror.

Question 34

What is the Rayleigh Criterion?

Answer 34

The minimum angular resolution. Two objects will not be resolved if any part of the central maximum of either images falls within the first minimum diffraction ring of each other.

Question 35

What are charge-coupled devices (CCDs)?

Answer 35

A detector. An array of light-sensitive pixels, which become charged when they are exposed to light by the photoelectric effect. They are useful for detecting finer details and producing images which can be shared and stored.

Question 36

What is quantum efficiency (CCDs)?

Answer 36

The percentage of incident photons which cause an electron to be released.

Question 37

What is spectral range (CCDs)?

Answer 37

The detectable range of wavelengths of light.

Question 38

What is pixel resolution (CCDs)?

Answer 38

The total number of pixels used to form the image on a screen.

Question 39

What is spatial resolution (CCDs)?

Answer 39

The minimum distance two objects must be apart in order to be distinguishable.

Question 40

What is convenience (CCDs)?

Answer 40

How easy images are to form and use.