Telescopes

Question 1

What is a convex lens?

Answer 1

Focuses incident light

Question 2

What is a concave lens?

Answer 2

Spreads incident light

Question 3

What is the principal axis?

Answer 3

Line passing through centre of lens at 90 degrees to its surface

Question 4

What is the principal focus?

Answer 4

Point where light converge
Point where light appears to come from

Question 5

What is the focal length?

Answer 5

Distance between centre of lens and principal focus

Question 6

What is a real image?

Answer 6

Formed when light rays cross after refraction, can be formed on a screen

Question 7

What is a virtual image?

Answer 7

Formed on same side of the lens, light rays do not cross so cannot be formed on a screen

Question 8

What is the lens formula?

Answer 8

1/u+1/v = 1/f

Question 9

What is the power of a lens?

Answer 9

How short the focal length is, how closely a lens can focus a beam

Question 10

What is a refracting telescope?

Answer 10

Two converging lenses
Objective - collects light (is large and long focal length to collect lots of light) to create a real image
Eyepiece - magnifies image, produces a virtual image at infinity reducing the eye strain for observer as no need to refocus every time they look.

Question 11

What does normal adjustment mean in terms of refracting telescope?

Answer 11

Distance between objective and eyepiece lens is the sum of their focal lengths

Question 12

Which is which angle in magnifying power equation?

Answer 12

a is angle between observer and virtual image
B is angle between observer and object

Question 13

What is an example of a reflecting telescope?

Answer 13

Cassegrain
Newtonian

Question 14

Draw a Cassegrain reflecting telescope

Answer 14

Google

Question 15

Draw a Newtonian reflecting telescope

Answer 15

Google

Question 16

Draw a converging telescope in normal adjustment

Answer 16

Google

Question 17

What is beneficial about using mirrors in reflecting telescoped?

Answer 17

Very thin
As smooth as possible as made from aluminium and silver atoms
Minimising distortion

Question 18

What causes chromatic aberration? Which telescope minimises this?

Answer 18

Focal length of red light is different to blue light so they focus at different points. This is mainly caused by refraction so reflecting telescopes are better

Question 19

What is spherical aberration? What can be done to prevent it?

Answer 19

Curvature causes light to be focussed differently at edges when compared to middle leading to blurring and distortion. Most notable in lenses with large diameters
Use parabolic objective mirrors in reflecting telescopes

Question 20

What can be used to prevent both aberration?

Answer 20

Achromatic doublet

Question 21

What is achromatic doublet?

Answer 21

Convex lens made of crown glass and a concave lens made of flint glass cemented together in order to bring all rays of light into focus at same position

Question 22

Disadvantages of refracting telescopes?

Answer 22

Glass must be pure and free from defects
Lenses can bend and distort under own weight
Chromatic and spherical aberration
incredibly heavy
Require very large diameter objective lenses
Only be supported from the edges

Question 23

Advantages of reflecting telescopes

Answer 23

Excellent image quality as thin mirrors
Parabolic mirrors prevent spherical aberration
Easier to handle
Achromatic doublet used to prevent chromatic aberration
Easy to support as don’t need to see through them
Large composite primary mirrors can be made

Question 24

How do radio telescopes work and what do they detect?

Answer 24

They detect radio waves to create images of astronomical objects, commonly through the use of a large parabolic dish

Question 25

Why can radio telescopes be ground based?

Answer 25

Atmosphere is transparent to large range of radio wavelengths (it does not absorb them). They do need to be isolated though to avoid interference from nearby radio sources

Question 26

Similarities between radio telescopes and optical telescopes

Answer 26

Intercept and focus incoming radiation to detect its intensity can be moved to focus on different sources or to track a moving source Parabolic dish is similar to that of reflecting optical telescope radiowaves and optical light can pass easily through the atmosphere

Question 27

Differences between radio and optical telescopes

Answer 27

radio telescopes have to be much larger due to larger wavelengths Radio telescopes are cheaper and simpler as wire mesh is used instead of wires Optical experience interference from weather conditions etc whereas radiowaves experience interference from phones etc

Question 28

How do infrared telescopes work? How can contamination and interference be prevented?

Answer 28

Large concave mirrors launched into space. However, all objects emit infrared heat so telescopes must be cooled using cyrogenic fluids to almost absolute zero as well as being well shieled to prevent thermal contamination from nearby objects.

Question 29

Why must infrared telescopes be launched into space?

Answer 29

Earths atmosphere absorbs most infrared radiation

Question 30

How do UV telescopes work?

Answer 30

Cassegrain configuration brings UV rays to focus which are detected by solid state devices.

Question 31

Why do UV telescopes need to be launched into space?

Answer 31

Ozone layer blocks all UV rays less than 300nm

Question 32

Why do X-rays need to launched into space?

Answer 32

All x-rays are absorbed by the atmosphere

Question 33

Why can't X-ray telescopes use standard mirrors? What do they use instead?

Answer 33

X-rays would pass straight them due to high energy. They are made from a combo of parabolic and hyperbolic mirrors

Question 34

How are rays brought into focus in x-ray telescopes?

Answer 34

Rays skim off mirrors and are brought into focus on CCD's convert light into electric pulses

Question 35

How does a gamma telescope work?

Answer 35

Gamma rays have so much energy that they would just pass straight through mirrors. Instead they use a detector made of pixels. Gamma photons cause a signal in each pixel they come into contact with

Question 36

What can gamma telescopes detect?

Answer 36

Gamma ray bursts, quasars, black holes and solar flares

Question 37

What are the two types of Gamma ray bursts?

Answer 37

Short lived - up to 1 second Long lived - 10-1000s (type 2 supernova)

Question 38

What is collecting power?

Answer 38

Measure of the ability of a lens or mirror to collect incident EM radiation. The greater the collecting power the brighter the images produced by the telescope

Question 39

What is the resolving power?

Answer 39

Ability of a telescope to produce separate images of close-together objects.

Question 40

What is the equation for minimum angular resolution?

Answer 40

0= wavelength/diameter of objective lens

Question 41

What is the Rayleigh Criterion?

Answer 41

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

Question 42

What does CCD stand for?

Answer 42

Charge-coupled device

Question 43

What is charge couple device?

Answer 43

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

Question 44

What features can be compared with the human eye?

Answer 44

Quantum efficiency Spectral range Pixel resolution Spatial resolution Convenience

Question 45

Why are CCD's advantageous over human eye?

Answer 45

Better quantum efficiency Wider spectral range (infared, UV and visible) Allows images to be shared Provides finer details

Question 46

Why is the human eye better than CCD's

Answer 46

Better pixel resolution Higher spatial resolution Simpler as no extra equipment