Telescopes

Question 1

What is the principal axis of a lens?

Answer 1

The straight line through the lens perpendicular to the lens.

Question 2

What does a converging lens do and where is its focal point?

Answer 2

It makes parallel rays converge to a focus. The point where rays parallel to the principal axis are focussed to is the focal point.

Question 3

What does a diverging lens do and where is its focal point?

Answer 3

It makes parallel rays diverge (spread out). The point where the rays appear to come from is the focal point.

Question 5

What is the focal length of a lens?

Answer 5

The distance between the centre of the lens and the focal point.

Question 6

How is a real image formed from a converging lens?

Answer 6

An object is placed beyond the focal point of the lens and on the opposite side to a screen. The distance between the screen and lens is adjusted so a clear image is seen. The image is real because it is formed on the screen where the light rays meet.

Question 7

How is a virtual image formed by a converging lens?

Answer 7

The object is placed nearer than the focal point and on the opposite side to a screen. A magnified image is formed but can only be seen looking from the opposite side of the lens to the object. The image is virtual as it is formed where the light rays appear to come from.

Question 8

How to predict where the real image of an object will be the other side of a converging lens using a scale diagram.

Answer 8

Draw a vertical line up from the principal axis pointing to the height the object is above it. Line 1 is from point of arrow parallel to axis and is refracted through the focal point. Line 2 is from the point of the arrow through the centre of the lens and is not refracted. Line 3 is from point of arrow through focal point then refracted to parallel. Where the three lines meet is where the real image will form.

Question 9

How to predict where the virtual image of an object will through a converging lens using a scale diagram.

Answer 9

Vertical line from principal axis pointing to height the object is above it (closer than focal point). Line 1 from point of arrow parallel to axis refracted through focal point. Line 2 from point of arrow to centre of lens not refracted. Where the two lines meet on the same side as the object is where the virtual image is formed.

Question 10

How is a refracting telescope in normal adjustment arranged?

Answer 10

Has two converging lenses of different focal lengths. The lens with the longer focal length is the objective lens. The distance between the two lenses is the sum of their focal lengths.

Question 11

Why does a refracting telescope produce a magnified virtual image of a distant object?

Answer 11

The objective lens focuses the light rays from the distant object to form a real image on the same plane as the focal point. The eyepiece lens gives the viewer a magnified view of the real image. The image seen is virtual as it is formed where the light rays emerging from the eyepiece appear to have come from.

Question 12

What is the focal plane?

Answer 12

The plane one each side of the lens perpendicular to the principal axis and containing the focal point of the lens.

Question 13

Describe the ray diagram for a simple refracting telescope.

Answer 13

3 parallel rays enter the objective lens with the centre ray going through the centre of the lens. They converge at the focal point with the centre ray not being refracted. There is a constructing line from the point they converge through the centre of the eyepiece lens. The rays are refracted by the eyepiece lens and exit parallel to the construction line. They go back in the same direction as dotted lines to the top of the final image which in normal adjustment is seen as virtual at infinity.

Question 14

Why does a refracting telescope produce an inverted image?

Answer 14

The objective lens produces a real, inverted and diminished image. The eyepiece lens magnifies the real image. The viewer sees a magnified, virtual image the same way up as the real image so it is seen as inverted.

Question 15

What are the angles used when finding the magnification of a refracting telescope?

Answer 15

Angle subtended by object at unaided eye is angle between centre line on ray diagram and the principal axis. Angle subtended by the image at eye is the angle from construction line to principal axis.
Angles in radians.

Question 16

Why does a star appear brighter when viewed through a telescope?

Answer 16

The telescope objective is wider than the pupil of the eye, so more light can enter the eye using a telescope than when the eye is unaided.

Question 17

What is collecting power?

Answer 17

The amount of light a telescope collects. It is proportional to the square of the diameter of the objective lens

Question 18

Describe the arrangement of a cassegrain reflecting telescope and how it works.

Answer 18

A concave mirror (primary mirror) is the objective. This reflects parallel rays and focuses them to a point on the focal plane. A smaller convex mirror (secondary mirror) is placed near this point which focuses the light onto or just behind a small hole at the centre of the primary mirror. The light is then refracted to a parallel beam by the eyepiece lens placed behind the primary mirror.

Question 19

Where is the focal point of a concave mirror?

Answer 19

The point at which rays parallel to the principal axis converge after being reflected

Question 20

What type of image is seen using a reflecting telescope?

Answer 20

A virtual image at infinity.

Question 21

Describe spherical aberration

Answer 21

Where the outer rays entering a lens or being reflected by a concave mirror are brought to a focus closer to the mirror or lens than the focal point.

Question 22

Describe chromatic aberration

Answer 22

White light splits into colours when refracted because the blue end is refracted more than the red end of the spectrum. The image formed by the lens is tinged with colour, particularly noticeable near the edge of the lens.

Question 23

Advantages of reflecting telescopes over refracting telescopes

Answer 23

High quality concave mirrors can be made much wider than convex lenses can so they can have a greater collecting power. Image distortion due to spherical aberration can be reduced by using a parabolic mirror. No chromatic aberration. Have a greater angular magnification than refractors of same length so produce greater magnification of distant objects.

Question 24

Advantages of refracting telescopes over reflecting telescopes

Answer 24

Don’t contain secondary mirrors or supporting frames which block out some of the light from the object. Have a wider field of view than reflectors of the same length as their angular magnification is less. Therefore astronomical objects are easier to locate.

Question 25

What is the angular separation of two stars?

Answer 25

The angle between the two straight lines from the stars to the earth.

Question 26

Why does diffraction happen when observing stars through a telescope and what affects this diffraction?

Answer 26

The objective lens or mirror is an aperture (gap) which light from the object must pass through and diffraction of light always occurs when light passes through an aperture. This causes the image to spread out slightly. The narrower the objective, the greater the amount of diffraction that occurs.

Question 27

What does it mean to say that two stars are resolved?

Answer 27

They can be seen as separate stars.

Question 28

What does the Rayleigh criterion state?

Answer 28

Resolution of the images of two point objects is not possible if any part of the central maximum of either image lies inside the first dark fringe of the other image.

Question 29

What does theta mean in the Rayleigh criterion equation?

Answer 29

The minimum angular resolution (minimum resolving power). Used to describe quality of the telescope where lower values are better.

Question 30

Why don’t telescopes on earth achieve their theoretical resolution?

Answer 30

The movement of air in the atmosphere means the light from the object is refracted and the image is smudged slightly.

Question 31

How does a CCD (charge coupled device) work?

Answer 31

A CCD is an array of light sensitive pixels that become charged when exposed to light. After being exposed for a pre-set time, the array is connected to an electronic circuit which transfers the charge collected by each pixel in sequence to an output electrode connected to a capacitor. Voltage of output electrode is read electronically. Capacitor is discharged before next pulse of charge received. So output electrode produces a stream of voltage pulses with amplitude proportional to light energy received by each pixel. Pulses stored and used to create visual image on flat panel screen.

Question 32

What is quantum efficiency?

Answer 32

The percentage of incident photons that liberate an electron.

Question 33

CCD vs eye

Answer 33

Quantum efficiency is 70% to 1-2%. Can record changes of an image. Wavelength sensitivity is 100-1100nm to 350-650nm so can see infrared using suitable filters. CCDs used in astronomy have to cooled by liquid nitrogen otherwise random emission of electrons occurs causing a dark current which doesn’t depend on light intensity.

Question 34

Describe the resolution of a CCD and eye

Answer 34

For CCD it is typically 10micrometres depending on size of pixels. Two images on a CCD need to be separated by at least one un-illuminated pixel to be seen separately. The light sensitive receptor cells in eye are about 5micrometres in size. Although, they are not distributed evenly on the retina and away from centre there are several receptor cells per nerve fibre so effective size is more than 5micrometres.

Question 35

Describe a radio telescope and give typical uses and advantages and disadvantages

Answer 35

It has a large parabolic dish with an aerial at the focal point of the dish. Used for mapping Milky Way. +ve are it can be made to turn to compensate for the earth’s rotation, radio waves aren’t absorbed by dust clouds, don’t need a clear sky. -ve are it need to be big and you need a large steerable structure for a steerable dish.

Question 36

Describe an infrared telescope and give typical uses and advantages and disadvantages

Answer 36

Have a large concave reflector which focuses IR onto an infrared detector at the focal point of the reflector. Can be on ground or on satellites. Used to see objects that aren’t hot enough to emit visible light and dust clouds. +ve is it can provide images of objects that can’t be seen using optical telescopes. -ve are water vapour in atmosphere absorbs IR so ground ones need to be high and in dry place, both need to be cooled to stop IR from its own surface swamping IR from space.

Question 37

Describe a UV telescope and give typical uses and advantages and disadvantages

Answer 37

Mirrors reflect UV radiation onto a UV detector. Used to map hot gas clouds near stars and glowing comets, supernovae and quasars. +ve is images can be compared with those from optical or IR telescopes to find hot spots in the object. -ve are that it must be on satellite as atmosphere absorbs UV, also absorbed by glass so have to use mirrors.

Question 38

Describe X-ray and gamma telescopes and give typical uses and advantages and disadvantages

Answer 38

X-ray works by reflecting X-rays off highly polished metal plates onto a detector. Gamma one has a detector with pixels and a signal is triggered when gamma photons pass through them. Used to detect supernovae, gamma and X-ray bursters, X-ray pulsars. +ve is wavelength is so small that diffraction is insignificant. -ve is atmosphere absorbs it all.

Question 39

Describe an optical telescope and give typical uses and advantages and disadvantages

Answer 39

Can be refracting or reflecting. Used to observe stars and galaxies. +ve are they get detailed images and can detect distant galaxies. -ve are they can’t see through clouds and atmospheric refraction.