Telescopes

Question 1

Astronomical telescope

Answer 1

Refracting

Question 2

Lenses in a refracting telescope

Answer 2

• Objective lens
• Eyepiece lens

Question 3

Which telescope lens has the longer focal length?

Answer 3

The objective lens

Question 4

Normal adjustment

Answer 4

• Final image is at infinity
• Didtance between lenses is fe + fo
• Rays emerge from the telescope parallel to each other

Question 5

Spherical aberration

Answer 5

Parallel rays reflected in a spherical mirror don’t meet at a single focal point

Question 6

Rays from the edges of a spherical mirror or lens meet at a point __________ the mirror/lens than rays reflected near the center of the mirror/lens.

Answer 6

closer to

Question 7

What shape are telescope mirrors?

Answer 7

Parabolic (reflect all rays traveling parallel to the principle axis through the focal point)

Question 8

Cassegrain arrangement telescope

Answer 8

• Concave primary mirror
• Convex secondary mirror
• Eyepiece lens

Question 9

Why do rays in a Cassegrain telescope appear to come from infinity?

Answer 9

Rays reflected from the convex mirror meet at the focal point of the eyepiece lens so they emerge from the lens parallel

Question 10

Advantages of reflecting telescopes

Answer 10

• Can be made larger (supported from behind to prevent distortion due to weight)
• No chromatic aberration
• Easier to build without spherical aberration
• Glass in lenses blocks some wavelengths and impurities reduce light intensity

Question 11

Chromatic abberation

Answer 11

• Blue light refracts more than red light
• Blue light has a shorter focal length than red light
• Causes the image to have coloured edges

Question 12

Why are bigger telescopes better/?

Answer 12

• Greater area - greater collecting power so brighter images
• Greater diameter - higher resolving power

Question 13

Collecting power of a telescope

Answer 13

The energy collected per second

Question 14

Disadvantages of reflecting telescopes

Answer 14

• Secondary mirror blocks some light lowering the brightness of image
• Diffraction around secondary mirror decreases sharpness of the image

Question 15

Airy disc

Answer 15

The central maximum of a diffraction pattern formed when light passes through a circular aperture

Question 16

Rayleigh criterion

Answer 16

Two objects can just be resolved when the central maximum of one diffraction pattern coincides with the first minimum of the other

Question 17

Resolving power of a telescope

Answer 17

The minimum angle in radians that can be resolved. (The smallest angle between objects where you can still tell them apart)

Question 18

1 arcsec

Answer 18

1/3600 degrees

Question 19

Charge couple device (CCD)

Answer 19

Detects photons to produces an image

Question 20

Quantum efficiency

Answer 20

% of photons falling on a devise that produce a signal

Question 21

Quantum efficiency of a CCD

Answer 21

Over 80%

Question 22

Quantum efficiency of the eye

Answer 22

Around 1%

Question 23

Benefits of using CCDs instead of the eye

Answer 23

• Higher quantum efficiency so brighter images
• Can detect UV and IR
• Better resolution
• Produces an image which can be analysed on a computer
• Can produce video

Question 24

Why can radio telescopes detect objects that light telescopes can’t?

Answer 24

Light is scattered by dust and gases, but this does not prevent radio waves reaching Earth

Question 25

Structure of a radio telescope

Answer 25

- Parabolic disk - Antenna at the principle focus with a receiver - Receiver is tuned to the correct frequency

Question 26

Why do radio telescopes have a reflecting disk?

Answer 26

Radio waves are very weak, this increases their intensity. Having a brighter image allows more measurements to be taken.

Question 27

Collecting power of a radio telescope

Answer 27

Proportional to the area of the telescope

Question 28

Equation for resolving power of a telescope

Answer 28

θ = λ/d

Question 29

Resolving power of a radio telescope is always ______ than that of light telescope because...

Answer 29

worse ....radio waves have a much larger wavelength

Question 30

Maximum size of imperfection / gap in a radio telescope reflector

Answer 30

1/20 of the wavelength

Question 31

What is the benefit of building radio telescope reflectors out of a wire mesh?

Answer 31

Lighter (prevents dish from being distorted by its own weight)

Question 32

To which wavelengths is the atmosphere transparent?

Answer 32

Visible light, radio and some UV and IR (opaque to all other wavelengths)

Question 33

UV is absorbed by....

Answer 33

Ozone

Question 34

IR is absorbed by...

Answer 34

Water vapour

Question 35

How does the Earth's atmosphere effect radiation?

Answer 35

- Changing densities causes refraction and distorts images - Pollution and humidity reduce intensity

Question 36

Where are telescopes built?

Answer 36

- High altitude (Reduces refraction by atmosphere) - Low humidity - Away from cities (light pollution and interference) - Away from sources of chemical pollution

Question 37

Where are UV, IR and X-ray telescopes located?

Answer 37

In space on satellites. The atmosphere is opaque to these wavelengths. (IR can alco be built on the top of high mountains)

Question 38

Similarity between optical and radio telescopes

Answer 38

Both use parabolic reflectors to focus waves

Question 39

Differences between optical and radio telescopes

Answer 39

- Optical telescopes have two reflectors - Radio telescopes are much larger - Radio has worse resolving power - Radio signal are weaker than optical signals - radio telescopes cannot produce a whole image

Question 40

How can an image be produced from a radio telescope?

Answer 40

- Radio telescope measures intensity of a narrow beam - Rotated to scan the sky in lines - An image is built up from this

Question 41

Benefits of IR telescopes

Answer 41

- Can detect objects not hot enough to emit visible or UV - Passes through gasses and dust more easily than light - Shorter wavelength than radio so better resolving power (and can be built smaller)

Question 42

How do IR telescopes avoid detecting radiation from themselves?

Answer 42

They are cooled close to absolute zero

Question 43

Resolving power of UV telescopes

Answer 43

Better than light telescopes but require a smoother surface

Question 44

What are X-ray telescopes used to study?

Answer 44

Violent and energetic events (like gasses swirling into a black hole)