9.1 Telescopes

Question 1

What is the axis of symmetry called?

Answer 1

The principle axis.

Question 2

What is the principle of focus?

Answer 2

A point on the axis which is the same distance from the optical centre as the focal length. This is where light rays travelling parallel to the principle axis prior to refraction converge.

Question 3

Define focal length.

Answer 3

The distance between the centre of the lens and the principal focus.

Question 4

What does ‘u’ represent in lens diagrams and equations?

Answer 4

The distance between the object and the centre of the lens, u, is always positive.

Question 5

What does ‘v’ represent in lens diagrams and equations?

Answer 5

The distance between the image and the centre of the lens, v, is positive for real images and negative for virtual images.

Question 6

Give the formula for angular magnification in normal adjustment.

Answer 6

M = angle subtended by image at eye / angle subtended by object at unaided eye
Can also be written as M = α/β

Question 7

State the equation that relates M to the focal length for objective and eyepiece lenses.

Answer 7

M = fo/fe
This can only be used if both angles from M = α/β are less than 10 degrees.

Question 8

How does an astronomical refracting telescope work?

Answer 8

There are two converging lenses, the objective lens and the eyepiece lens. The role of the objective lens is to collect light and create a real image of a distant object. This image is then magnified by the eyepiece lens, which produces a virtual image (formed at infinity so as to reduce eye strain when looking between the object and the telescope image).

Question 9

How does a cassegrain telescope work?

Answer 9

There is a concave primary mirror with a long focal length and a small convex secondary mirror in the centre. The light is collected by the primary mirror and focused onto the secondary mirror, which then reflects it onto an eyepiece lens.

Question 10

What is chromatic aberration?

Answer 10

When a lens refracts different colours of light by different amounts as they have different wavelengths. This causes the image for each colour to form in a slightly different position, causing coloured fringes around the image.

Question 11

What is spherical aberration?

Answer 11

When light is focused in different places due to the curvature of a lens or mirror, causing image blurring. This can be resolved in reflecting telescopes by using a parabolic mirror.

Question 12

Describe a solution to chromatic and spherical aberration in lenses.

Answer 12

Using an achromatic doublet brings all rays of light into focus in the same position by using a convex lens and a concave lens of different types of glass cemented together.

Question 13

State 3 advantaged of reflecting telescopes.

Answer 13

1. There is very little chromatic aberration (only in eyepiece lens)
2. Simpler to increase the size of the objective since mirrors can be supported from behind and are lighted than lenses.
3. Using parabolic mirrors stops spherical aberration.

Question 14

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

Answer 14

Increasing the diameter of the objective means you can observe fainter objects. This is because collecting power is proportional to (objective diameter)^2

Question 15

Define the Rayleigh Criterion

Answer 15

Two objects will be resolved if the centre of the diffraction pattern of one image coincides with the first minimum of the other.
θ=λ/D

Question 16

Explain the structure, positioning and uses of a single dish radio telescope.

Answer 16

Structure: Large parabolic dish that focuses radiation onto a receiver.
Positioning: Can be ground-based but must be in isolated locations
Uses: Observing things such as galaxies, stars and black holes

Question 17

Why do radio telescopes need to be larger than optical telescopes?

Answer 17

Since radio waves have much larger wavelengths than visible light, in order to achieve the same resolving power as an optical telescope, the objective diameter must be much larger in accordance with θ=λ/D

Question 18

Explain the structure, positioning and uses of an infrared telescope.

Answer 18

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 cooler regions in space (from a few tens to 100K)

Question 19

Explain the structure, positioning and uses of an ultraviolet telescope.

Answer 19

Structure: Cassegrain configuration that focuses radiation onto solid state devices
Positioning: Must be in space as ultraviolet light is blocked by the ozone layer
Uses: Observing the interstellar medium and star formation regions

Question 20

Explain the structure, positioning and uses of an x-ray telescope

Answer 20

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 21

Explain the structure, positioning and uses of a gamma telescope

Answer 21

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 holes and solar flares

Question 22

What is a CCD and how does it work?

Answer 22

CCDs are silicon chips divided into pixels, when a photon is incident on a pixel, electrons are released from the silicon atoms via the photoelectric effect, the electrons are confined to the pixel so it accumulates charge, the position and magnitude of which is used to create a digital image.

Question 23

Compare the quantum efficiency of a CCD to the eye.

Answer 23

Quantum efficiency: the percentage of incident photons that liberate an electron in the photoelectric effect. This can be upwards of 80% for a CCD, compared to 4-5% for the human eye.

Question 24

Compare the resolution of a CCD to the eye

Answer 24

CCDs have a spatial resolution of 10μm, the minimum resolvable distance for the human eye is around 100μm so CCDs are better for capturing fine detail.
Spatial resolution is how far apart two objects need to be to distinguish them.

Question 25

Compare the convenience of a CCD to the eye

Answer 25

The CCD is more convenient for accessing data remotely. It is easier to analyse CCD data on computers and CCDs have a wider spectral range, allowing them to perceive wavelengths that cannot be detected by the human eye. That being said, looking down a telescope is an inconvenient task.