Telescopes

Question 1

What does a convex lens do?

Answer 1

Focuses incident light

Question 2

What does a concave lens do?

Answer 2

Spreads out incident light

Question 3

What is the principal axis?

Answer 3

The line passing through the centre of the lens at 90° to its surface

Question 4

What is the focal length?

Answer 4

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

Question 5

What’s the relationship between focal length and lens strength?

Answer 5

The shorter the focal length, the stronger the lens.

Question 6

What are u, v and f in the Lens formula?

Answer 6

u = distance of the object from the centre of the lens
v = distance of the image from the centre of the lens
f = focal length of the lens

Question 7

What is meant by the power of a lens?

Answer 7

How closely a lens can focus a beam that is parallel to the principal axis (how short the focal length is)

Question 8

What is the power of a lens measured in?

Answer 8

Dioptres (D)

Question 9

What types of lenses does a refracting telescope comprise of?

Answer 9

Two converging lenses; the objective and eyepiece lens.

Question 10

How are distortions minimised in a reflecting telescope?

Answer 10

By applying a very thin coating of aluminium or silver atoms onto a backing material, making the mirror as smooth as possible.

Question 11

What is chromatic aberration?

Answer 11

An effect caused by the different focal lengths of different wavelengths of light that leads to different colours being focused at different points.
It can cause a white object to appear as if it has coloured edges.

Question 12

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

Answer 12

It’s caused by refraction, so it only occurs in the eyepiece lens.

Question 13

What is spherical aberration?

Answer 13

When the curvature of a lens/mirror leads to light rays at the edges being focused in different places to those from the centre. This can cause an image to be blurry or distorted.

Question 14

Where is spherical aberration most noticeable?

Answer 14

In lenses with a large diameter.

Question 15

How can spherical aberration be avoided in reflecting telescopes?

Answer 15

By using parabolic objective mirrors.

Question 16

What are some of the disadvantages of refracting telescopes?

Answer 16

-Chromatic and spherical aberration both affect lenses.
-Large lenses can bend and distort under their own weight.
-The glass used must be pure and free from defects. Achieving this for a large diameter lens is very difficult.

Question 17

What are some advantages of reflecting telescopes?

Answer 17

-Mirrors are unaffected by chromatic aberration, and spherical aberration can be solved using parabolic mirrors.
-Mirrors aren’t as heavy as lenses, so they’re easier to handle and manoeuvre to follow astronomical objects/events.
-Large primary mirrors are easy to support from behind as you don’t need to see through them.

Question 18

Where can radio telescopes be built?

Answer 18

-They can be ground-based, as the atmosphere is transparent to a large range of radio wavelengths.
-They should be built in isolated locations to avoid interference from nearby radio sources.

Question 19

What are the similarities and and differences between radio and optical telescopes?

Answer 19

-Both function in the same way by intercepting and focusing incoming radiation to detect its intensity.
-Both can be moved to focus on different source, or to track a moving source.
-Both can be built on the ground as radio waves and optical light can pass easily through the atmosphere.

-Radio wavelengths are much larger than visible wavelengths, so radio telescopes have to be much larger in diameter to have the same resolving power. Therefore radio telescopes have a larger collecting power.
-Construction of radio telescopes is cheaper and simpler because a wire mesh is used instead of a mirror.
-Radio telescopes experience a large amount of man-made interference (eg. Radio transmissions, phones, microwave ovens), whereas optical telescopes experience interference from weather conditions, light pollution, stray radiation etc.

Question 20

What are the features of an infrared telescope?

Answer 20

-Large concave mirrors which focus radiation onto a detector.
-Must be cooled using cryogenic fuels (eg. N and H) to almost absolute zero, because all objects emit infrared radiation as heat.
-Must be well shielded to avoid thermal contamination from nearby objects as well as its own infrared emission.

Question 21

Why must infrared telescopes be launched into space and accessed remotely from the ground?

Answer 21

Because the atmosphere absorbs most infrared radiation, making it difficult to observe the cooler regions of space.

Question 22

Why do UV telescopes need to be positioned in space?

Answer 22

Because the ozone layer blocks all UV rays that have a wavelength of less than 300nm.

Question 23

How do UV telescopes detect UV rays?

Answer 23

-They utilise the Cassegrain configuration by focusing UV light onto a detector.
-This detector is made of a material that emits electrons when struck by UV photons, through the photoelectric effect.
-These emitted electrons generate an electric current proportional to the intensity of the incident UV rays.
-This current is then measured and converted into data that allows us to create images or spectra of the observed astronomical objects.

Question 24

What are the difficulties in constructing an X-ray telescope, and how do they work?

Answer 24

-Because X-rays have such high energy, using mirrors like an ordinary optical telescope would not work as they would just pass straight through.
-Therefore, X-ray telescopes must be made from a combination of parabolic and hyperbolic mirrors, all of which must be extremely smooth.
-The rays enter the telescope, skim off the mirrors, and are brought into focus on CCDs, which convert light into electrical impulses.

Question 25

What can X-ray telescopes be used to observe and why?

Answer 25

High-energy events and areas of space such as active galaxies, black holes and neutron stars, because X-rays are high energy.

Question 26

What is collecting power?

Answer 26

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

Question 27

What is resolving power?

Answer 27

The ability of a telescope to produce separate images of close-together objects

Question 28

What does the Rayleigh criterion state?

Answer 28

That two objects will just be resolved when the first maximum of the airy disc in the diffraction pattern of one image coincides with the central maximum of the airy disc of the other.

Question 29

What are Charge-couple devices (CCDs)?

Answer 29

An array of light-sensitive pixels, which become charged when they are exposed to light by the photoelectric effect.

Question 30

What features of a CCD can be compared with the human eye?

Answer 30

-Quantum efficiency: the % of incident photons which cause an electron to be released (~80% vs. 4-5%)
-Spectral range (Infrared, UV, Visible vs. Visible only)
-Pixel resolution (~50 megapixels vs. ~500 megapixels)
-Spatial resolution (10 micrometers vs. 100 micrometers)
-Convenience (Needs to be set up but images produced are digital vs. No need for extra equipment)

Question 31

What are CCDs more useful than eyes at?

Answer 31

Detecting finer details and producing images which can be shared and stored