Telescopes

Question 1

A convex lens means…

Answer 1

Rays converge.

Question 2

A concave lens means…

Answer 2

Rays diverge.

Question 3

Define focal length.

Answer 3

The distance between the lens centre and the focal point.

Question 4

What does a more powerful lens do?

Answer 4

Bend the light more, therefore making the focal length shorter.

Question 5

How many focal points does every lens have?

Answer 5

2.

Question 6

If an object is more than 2 focal lengths from the lens, the image that’s produced will be…

Answer 6

Diminished, inverted and real.

Question 7

If the object is exactly 2 focal lengths from the lens, the image that’s produced will be…

Answer 7

The same size, inverted and real.

Question 8

If the object is between 1 and 2 focal lengths from the lens, the image that’s produced will be…

Answer 8

Magnified, inverted and real.

Question 9

If the object is less than 1 focal length from the lens, the image that’s produced will be…

Answer 9

Magnified, upright and virtual.

Question 10

What is normal adjustment?

Answer 10

When the distance between the eye piece and objective lens is the sum of their focal lengths.

Question 11

State 2 advantages of having a bigger objective lens/mirror.

Answer 11

1. It collects more light, so fainter objects can be observed.
2. It has a higher resolution, so it’s possible to distinguish two objects closer together.

Question 12

State 5 advantages of using mirrors in telescopes, rather than convex lenses.

Answer 12

1. No chromatic aberration, because mirrors don’t refract light.
2. No spherical aberration, as long as a parabolic mirror is used.
3. No distortion, because a mirror can be supported across its whole surface.
4. Better resolving power, as mirrors can be made larger.
5. Brighter image, because lenses absorb more light than mirrors.

Question 13

State 2 disadvantages of using mirrors in telescopes.

Answer 13

1. The secondary mirror blocks some light from hitting the primary mirror, which reduces light intensity.
2. The secondary mirror and its supports create a diffraction effect around the edges, which causes a slight blurring.

Question 14

What’s chromatic aberration?

Answer 14

Light with different wavelengths is focussed to different points, which produces an image with coloured edges around the outside. Shorter wavelength light is focussed closer to the lens, and vice versa.

Question 15

What’s important about the centre of curvature of the secondary mirror in a Cassegrain telescope?

Answer 15

It must be further away from the primary mirror than the primary focal point.

Question 16

What’s spherical aberration?

Answer 16

In a spherical mirror, light further away from the principal axis gets focussed to a point closer to the mirror. This effect is eliminated using a parabolic mirror.

Question 17

What’s the resolving power of a telescope?

Answer 17

It’s ability to observe objects with small angular separation.

Question 18

What’s the Airy disc?

Answer 18

The bright central maxima at the centre of the diffraction pattern produced when a point source is viewed through a circular aperture.

Question 19

State the Rayleigh criteria.

Answer 19

Two point objects can just be resolved when the central maximum of one object lies over the first minimum of the other.

Question 20

In order to resolve 2 objects…

Answer 20

Their angular separation must be equal to or greater than the minimum angular separation.

Question 21

The two most important attributes of a telescope are…

Answer 21

1. Its resolving power- the ability to see detail.

2. It’s collecting power- the ability to see dim objects.

Question 22

Define collecting power.

Answer 22

The rate at which energy is collected.

Question 23

What’s collecting power proportional to?

Answer 23

Diameter squared.

Question 24

What’s a charged coupled device?

Answer 24

A silicon chip divided into pixels.

Question 25

How does a CCD work?

Answer 25

1. Incident photons cause electrons to be released. 2. These electrons are trapped in potential wells within the CCD. 3. An electron pattern is built up, which is identical to the image formed on the CCD. 4. When exposure is complete, the charge is processed to give an image.

Question 26

What’s quantum efficiency?

Answer 26

The ratio of the number of electrons released to the number of photons absorbed.

Question 27

What’s the quantum efficiency of a CCD?

Answer 27

>80%

Question 28

State 2 advantages of space-based telescopes.

Answer 28

1. Images produced aren’t distorted by Earth’s atmosphere, which is especially important for capturing gamma rays, X-rays and infrared. 2. Negligible light pollution from artificial sources.

Question 29

State 2 disadvantages of space-based telescopes.

Answer 29

1. Much less convenient- it’s much more difficult and expensive to send a telescope into space. 2. Increased risk of damage due to space debris.

Question 30

What’s a segmented mirror telescope and why are they better than single mirror telescopes?

Answer 30

They consist of an array of smaller mirrors, designed to act as segments of a single large mirror. This is advantageous because it is possible to make very large aperture telescopes.

Question 31

What’s radio interferometry and why’s it better than a single radio dish?

Answer 31

Radio interferometry is an array of radio antennas that are used in astronomical observations simultaneously. This provides higher resolution images than a single radio dish, due to effectively having one huge aperture.