3.9.1 - Telescopes

Question 1

What do lenses do?

Answer 1

Change the direction of light rays by refraction.

Question 2

What are rays parallel to the principle axis called?

Answer 2

Axial rays.

Question 3

What are rays not parallel to the principle axis called?

Answer 3

Non-axial rays.

Question 4

When is an image magnified?

Answer 4

If the image is further from the lens axis than the object.

Question 5

When is an image diminished?

Answer 5

When the image is closer to the lens axis than the object.

Question 6

When is a real image formed?

Answer 6

When light rays from a point on an object are made to pass through another point in space.

Question 7

When is a virtual image formed?

Answer 7

When light rays from a point on an object appear to have come from another point in space.

Question 8

If the image is formed on the same side of the lens what is it?

Answer 8

Virtual.

Question 9

Describe an astronomical refracting telescope.

Answer 9

Usually made up of two converging lenses. The objective lens converges the rays from the object to form a real image. The eye lens acts as a magnifying glass on this real image to form a magnified virtual image, which the observer can then view.

Question 10

How does a reflecting telescope work?

Answer 10

A parabolic concave primary mirror convergeds axial rays from an object at its principle focus, forming a real image. An eye lens magnifies this image in the same way as in a refracting telescope.

Question 11

Why is the Cassegrain arrangement used?

Answer 11

So that you can observe light without blocking the incoming rays.

Question 12

What is a charged coupled device?

Answer 12

Used to capture images digitally.

Question 13

In terms of quantum efficiency compare CCDs and the human eye.

Answer 13

Not every photon that hits the silicon in a CCD causes an electron to be released. The quantum efficiency of a CCD is 80% or more. In comparison on average 1 in 100 photons is detected by the human eye, so CCDs detect far more light than the eye.

Question 14

In terms of spectrum compare CCDs and the human eye.

Answer 14

CCDs can detect a wider spectrum of light than the human eye. The eye can only detect visible light, whereas CCDs can detect infrared, visible and UV light.

Question 15

In terms of spatial resolution compare CCDs and the human eye.

Answer 15

The minimum resolvable distance of the human eye is around 100um, whereas CCDs can have a spatial resolution of around 10um, so CCDs are better for capturing fine detail.

Question 16

In terms of convenience compare CCDs and the human eye.

Answer 16

The human eye doesn’t need any extra equipment so is more convenient, however CCDs produce digital images which can be stored, copied and shared globally.

Question 17

What is the resolving power of a telescope?

Answer 17

A measure of how much detail you can see.

Question 18

What is the resolving power dependant on?

Answer 18

The minimum angular resolution, which is the smallest angular separation at which the instrument can distinguish two points.

Question 19

What is chromatic aberration and in what does it occur?

Answer 19

Refracting - glass refracts colours of light by different amounts and so the image for each colour is in slightly different positions. This blurs the image.

Question 20

What is an issue with the size of refracting telescopes?

Answer 20

Building large lenses that are good quality is difficult and expensive. Moreover large lenses are heavy and can only be supported from their edges so their shape becomes distorted.

Question 21

What is the issue with the glass in refracting telescopes?

Answer 21

Any bubbles or impurities in the glass absorb and scatter some of the light, which means that very faint objects aren’t seen.

Question 22

What is the issue with large magnification for refracting telescopes?

Answer 22

For large magnification, the objective lens needs to have a very long focal length. This means that refracting telescopes have to be very long, leading to very large, expensive buildings needed to house them.

Question 23

Benefits of reflecting telescopes?

Answer 23

Large mirrors of good quality are much cheaper to build than large lenses. They can also be supported from underneath so they don’t distort as much as lenses.

Question 24

Why do mirrors suffer spherical aberration?

Answer 24

If the shape of the mirror isn’t quite parabolic, parallel rays reflecting off different parts of the mirror do not converge onto the same point.

Question 25

Why can the secondary mirror of the Cassegrain telescope cause problems?

Answer 25

Some of the incoming light will be blocked by the secondary mirror and mirror supports. Moreover some of the light reflected from the primary mirror will diffract round the secondary mirror - both leading to a decrease in image clarity.

Question 26

How are radio telescopes similar to optical telescopes?

Answer 26

Parabolic dish. Works in the same way as the objective mirror of an optical reflecting telescope. Instead of a mirror, a wire mesh can be used since the long wavelength radio waves don't notice the gaps.

Question 27

How do radio telescopes work?

Answer 27

A preamplifier amplifies the weak radio signals without adding too much noise to the signal. The signal is them amplified further by a second amplifier before being passed through a tuner to filter out any unwanted wavelengths.

Question 28

How is a computer involved with radio telescopes?

Answer 28

A computer creates a false-colour image of the detected radio signals. Different colours are assigned to different wavelengths or intensities to produce false-colour images of non-visible EM radiation.

Question 29

What is the issue and how is it solved with resolving power and radio telescopes?

Answer 29

The wavelength of radio waves are about a million times longer than light, so for a telescope to have the same resolving power of an optical telescope, the dish would have to be a million times bigger. To get around this lots of telescopes are linked together. Resolutions thousands of times better than optical telescopes can be achieved this way.

Question 30

Benefits of radio telescopes?

Answer 30

- As they have a wire mesh, their construction is easier and cheaper than optical telescopes. - Dish doesn't need to be as perfect to avoid problems such as spherical aberration as the wavelength being detected is much longer.

Question 31

How is the wavelength of UV and IR affecting the telescopes used to detect them?

Answer 31

The longer the wavelength the less it is affected by the imperfections in the mirror. So IR telescopes don't need to be as perfectly shaped, however UV has a shorter wavelength so the mirrors in UV telescopes have to be even more precisely made.

Question 32

What is the added problem with IR telescopes?

Answer 32

They produce their own IR radiation due to their temperature, so they need to be cooled to very low temperatures using liquid helium or refrigeration units.

Question 33

How is the structure of X-ray telescopes different?

Answer 33

X-rays are reflected as they graze the mirror's surface. There is a series of nested mirrors gradually altering the direction of x-rays enough to bring them to a focus on a detector. This is called a grazing telescope.

Question 34

Where are radio and optical telescopes located?

Answer 34

On earth as the atmosphere is transparent these wavelengths.

Question 35

Where are IR telescopes located?

Answer 35

In high dry places such as Mauna Kea volcano and few wavelengths of IR can reach the earth's surface.

Question 36

Where are UV and X-ray telescopes located?

Answer 36

In space as the wavelengths are absorbed by the higher atmosphere.

Question 37

How does the Rayleigh criterion effect the resolving power?

Answer 37

Resolving power depends on the wavlength of the radiation and the diameter of the objective mirror or dish. So, for the same size dish, a UV telescope has a much better resolving power than a radio telescope, as the radiation it detects has a much shorter wavelength.

Question 38

How does the quality of the telescope affect the resolving power?

Answer 38

The resolving power of a telescope is limited by the resolving power of the detector. That can be how many pixels there are on a CCD, or for a wire mesh X-ray detector, how fine the wire mesh is.

Question 39

Collecting power is proportional to?

Answer 39

The diameter squared.

Question 40

What is a CCD?

Answer 40

A silicon chip made up of pixels and potential wells.

Question 41

What happens when photons of light hit the CCD?

Answer 41

They liberate electrons which are then trapped in the potential wells.

Question 42

What is the number of electrons trapped in each well proportional to?

Answer 42

The number of photons hitting the pixel, so the pattern of electrons in the array is the same as the image pattern.

Question 43

What does more incident light on the CCD mean?

Answer 43

More electrons trapped.

Question 44

On a CCD what is the amount of charge proportional to?

Answer 44

The brightness at a particular pixel location.