Telescopes

Question 1

Draw a ray diagram to show a converging lens producing a real inverted & diminished image

Answer 1

Question 2

Draw a ray diagram to show a converging lens producing a magnified and virtual image

Answer 2

Question 3

What are the main components of a refracting telescope

Answer 3

Objective & eyepiece lens

Question 4

Explain what is meant by the term normal adjustment for a refracting telescope

Answer 4

The distance between the object lens and the eye piece lens is equal to the sum of their focal lengths

d = F(o) + F(e)

Question 5

Draw a ray diagram for a refracting telescope in normal adjustment

Answer 5

1) F(o) > F(e) in a telescope

Question 6

Describe the image produced my a refracting telescope

Answer 6

A Magnified and virtual image

image is seen at infinity

Question 7

Describe the main components of a Cassegrain telescope

Answer 7

Concave Primary Mirror with a slit in the centre

Small Convex mirror

Eyepiece lens

Question 8

Draw a ray diagram to show the path of two light rays,

. Parallel to the principle axis
. Through a Cassegrain telescope

Answer 8

primary concave mirror must be 1 mirror with a slit in the middle not two mirrors

Question 9

State the Advantages of a reflecting telescope compared to refracting telescopes (4)

Answer 9

. Mirrors don’t suffer from Chromatic aberration as much
. Mirrors can be larger and therefore have greater collecting powers & smaller minimum angle resolution
.Parabolic mirrors are easier to manufacture then parabolic lens’
.Reflecting telescopes are more compact for the same effective focal length

Question 10

State the disadvantages of a reflecting telescope in comparison to refracting telescopes

Answer 10

.Secondary Mirror blocks some light so less light hits the primary mirror

. Light is diffracted as it passes the secondary mirror affecting the image

Question 11

Draw a ray diagram to show how spherical aberration can occur in a reflecting telescope

Answer 11

Question 12

State how the problem of spherical aberration can be eliminated

Answer 12

Using perfectly parabolic mirrors for reflecting telescope and perfectly parabolic lenses from refracting telescopes

Question 13

What is Chromatic aberration

Answer 13

Image distortion (image is tinged with colour) due to light of different wavelengths focussing at different points along an optical axis

Question 14

Draw a diagram to show how a converging lens can produce chromatic aberration

Answer 14

Question 15

Explain what is meant by the term Airy Disc

Answer 15

The Smallest point to which a beam of light can be focussed using a lens

Question 16

What is meant by the Rayleigh Criterion

Answer 16

• identifies the minimum subtended angle between objects whose images can be resolved

Question 17

State 2 advantages of using telescopes with a larger diameter objective (lens or mirror)

Answer 17

Increased collecting power (light collected is proportional to the area of the objective)

Light Collected is proportional to 1/4 * Pi * Diameter^2

Increased Resolving power —> Can resolve smaller angles

Size of angle resolved is inversely proportional to diameter

Question 18

Describe the Structure of a CCD (3)

Answer 18

P type silicon doped in boron gas (more conductive)

Divided into an array of the picture elements/ pixels

Each pixel has 3 metal electrodes embedded in an insulating layer of silicon dioxide

Question 19

Describe how a CCD works to produce an image (5)

Answer 19

Photons are incident of the CCD

Electrons are liberated from the surfaces of the electrodes within the pixels

Number of liberated electrons is proportional to the number of incident photons (brightness)

The Liberated electrons are trapped in a Potential Well within the Silicon layer

After Exposure the Charge on each pixel is measured and an image is formed

Question 20

State Advantages of using CCD’s for Astronomical Observations

Answer 20

Higher Quantum Efficiency

Long exposure times to capture faint images

Device can be directly linked to a computer for capture and analysis

Question 21

What is meant by Quantum efficiency

Answer 21

% of incident phoned on a photoreceptors that are detected

Number of photons producing an electron / total number of incident photons * 100

Question 22

What is the Principle Focus (3)

Answer 22

A point on the axis
which is the same distance from the lens (optical Centre) as the focal length
Where light rays traveling parallel to the principle axis prior to refraction converge

Question 23

Define Focal Length

Answer 23

Distance Between the centre of the lens and the principle focus

Question 24

In the lens Equation

1/v + 1/u = 1/f

1) What does u represent?
2) What does v represent?
3) What does f represent?

Answer 24

1) Distance between the object and the centre of the lens (always positive)
2)Distance between image and the centre of the lens ( +ve for real images , -ve for virtual images)
3) Focal Length of lens (+ve for converging lens, -ve for diverging lens)

Question 25

Give both formulas for angular magnification in Normal Adjustment

Answer 25

-> indication of widely an image takes up your field of vision M = Angle subtended by image at eye / angle subtended by object at an unaided eye. M = Focal length of objective / focal length of eyepiece (using small angle approximations only)

Question 26

How does an Astronomical Refracting telescope work (4)

Answer 26

- Two converging lens’ (eyepeice and objective) - Objective lens role is to collect light and create a real image of a distant object -Eye piece lens role is to magnify the image created by the objective lens -Eye piece lens produces virtual image formed at infinity for the user to observe,

Question 27

How does a Cassegrain telescope work (4)

Answer 27

-large concave primary mirror with a slit in the centre Smaller convex secondary mirror placed in front of the slit -Primary mirror collect light and focus it onto the secondary mirror -Light then is reflected into an eye piece lens producing a virtual image forming at infinity

Question 28

What is Spherical aberration

Answer 28

-When light is focused at different points along the principle axis -due to a non-perfectly parabolic curvature of a lens or mirror -Causing a blurred image

Question 29

Draw a lens Diagram representing spherical aberration in a lens

Answer 29

Question 30

What happens when you increase the size of the objective mirror/lens

Answer 30

Increasing diameter means you can observe fainter objects, as resolving power increases. meaning a smaller angle between two images can be resolved Collecting power us proportional to (objective Diameter)^2 produces brighter images

Question 31

Explain the Structure ,Positioning and uses of a single dish radio telescope.

Answer 31

Structure : Large parabolic dish that focus’ em radiation onto a receiver Positioning: Can be ground based but must be in isolating locations with not a lot of interference uses: observing things such as galaxies, stars and black holes.

Question 32

Why do radio telescopes need tho be larger then optical telescopes?

Answer 32

Radio waves have a larger wavelength then visible waves in order to achieve the same resolving power the objective diameter must be larger in accordance to (angle) = wavelength/ Diameter

Question 33

Explain the Structure ,Positioning and uses of an infrared telescope

Answer 33

Structure: Large Concave mirror focusing light onto a detector, must be cooled with cryogenic fluids to avoid interference positioning: must be in space as infrared light is blocked by the atmosphere uses: observing coolers regions of space

Question 34

Explain the Structure ,Positioning and uses of an ultraviolet telescope

Answer 34

Structure: Cassegrain configuration that focus’ radiation onto solid state devices Positioning: must be in space as uv is blocked by the ozone layer uses: observing the interstellar mediums and star formation regions

Question 35

Explain the Structure ,Positioning and uses of an x-ray telescope

Answer 35

Structure: Combination of hyperbolic and parabolic mirrors to focus radiation onto a CCD Positioning: must be in space as x-rays are blocked by the atmosphere Uses: Observing high-energy events and areas such as active galaxies, black holes and neutron stars

Question 36

Explain the Structure ,Positioning and uses of a Gamma Telescope

Answer 36

Structure: No mirrors , radiation passes through a. detector made of layers of pixels Positioning: must be in space as gamma rays are blocked by the atmosphere uses: observing gamma ray bursts, quasars, black hole and solar flares

Question 37

Write the equation that is used in the “Rayleigh Criterion’

Answer 37

Smallest angle that can be resolved = wavelength/ diameter on objective lens

Question 38

What is Collecting power

Answer 38

Rate at which useful energy is collected by a telescope proportional to the diameter^2 of the objective or aperture proportional to the area of the aperture

Question 39

Compare the Resolution of a CCD to the human eye

Answer 39

CCD has a spacial resolution of 10 micrometers, human eye has 100 micrometers CCD can resolve a smaller angle then the human eye So the image has a higher resolution

Question 40

What is Spacial Resolution

Answer 40

Minimum distance two objects must be apart to distinguish them Smaller the distance, smaller the angle the telescope can resolve

Question 41

Explain the advantages of a Parabolic Dish Shape over a Spherical dish shape

Answer 41

No Spherical Aberration