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Telescopes for AQA A-level Physics

Telescopes

This page covers the following topics:

1. Types of telescopes
2. Ray diagrams
3. Angular magnification
4. Focal lengths
5. Angular resolution

An astronomical telescope is a refracting one made of two converging lenses: the objective lens, which collects the light being shone on it and creates a real image of it, and the eyepiece lens, which magnifies this image enough to be seen by producing a virtual image. The objective lens is large and has a long focal length, so as to maximise the amount of light it can collect. The most common reflecting telescope is the Cassegrain reflecting telescope. It is made of a concave primary mirror with a long focal length and a small convex secondary mirror, and the collected light is focused onto an eyepiece lens. Single dish radio telescopes create images of astronomical objects by using a parabolic dish which focuses radio waves onto a receiver. These can be placed on ground due to the fact that the atmosphere doesn't absorb radio waves, however they must be located far away from any other radio sources to avoid interference. I-R, U-V and X-ray telescopes create astronomical objects using infrared, ultraviolet and X-ray radiation respectively. I-R, UV and X-rays are all absorbed by the atmosphere, therefore they must be placed in space rather than on ground. I-R telescopes use large concave mirrors to focus radiation onto a detector and are shielded to avoid thermal contamination from any surrounding IR sources. UV telescopes use the photoeletcric effect to turn photons into electrons, which in turn go through a circuit. X-ray telescopes are made up of extremely smooth parabolic and hyperbolic mirrors. Large diameter telescopes are advantageous, since they can collect greater amounts of radiation and can produce separate images of objects which are close together more effectively.

Types of telescopes

A ray diagram shows the path that a light ray entering a lens follows to form the image. To draw a ray diagram, two steps must be followed. Firstly, a ray must be drawn from the object to the lens parallely to the principal axis, which when goes through the lens, must pass through the principal focus. Secondly, a second ray must be drawn from the object passing through the centre of the lens. A third additional ray can also be drawn into the ray diagram. The normal at the point which the ray enters the lens is the line perpendicular to the surface at that point. The angle of incidence is the angle between the incident ray and the normal, whereas the angle of refraction is the angle between the refracted ray and the normal.

Ray diagrams

The angular magnification of a telescope is its magnifying power. It is given by the following formula: M = ฮฑ/ฮฒ, where ฮฑ is the angle subtended by the image at the eye and ฮฒ is the angle subtended by the object at the unaided eye. This is equivalent to dividing the focal length of the objective lens by the focal length of the eyepiece lens.

Angular magnification

The focal length of a lens is the distance between its centre and the principal focus.

Focal lengths

A radian is the angle subtended at the centre of a circle by an arc who has length of the radius of the circle. 2ฯ€ radians gives a full cycle of 360ยบ. The resolving power of a lens is its ability to produce individual images for objects which are close together. For this to occur, Rayleigh's Criterion must hold. This states that the angle between the lines from Earth to each object must be of minimum angular resolution.

Angular resolution

1

What are the advantages of large diameter telescopes?

image

What are the advantages of large diameter telescopes?

2

Draw a diagram for a light ray entering a concave lens, labelling the normal, the angle of incidence and the angle of refraction.

image

Draw a diagram for a light ray entering a concave lens, labelling the normal, the angle of incidence and the angle of refraction.

3

Define what a single dish radio telescope is.

Single dish radio telescopes create images of astronomical objects by using a parabolic dish which focuses radio waves onto a receiver.

Define what a single dish radio telescope is.

4

Explain why I-R, UV and X-ray telescopes have to be placed in space.

The atmosphere absorbs I-R, UV and -ray radiation, therefore the telescopes cannot be placed on ground and must be in space.

Explain why I-R, UV and X-ray telescopes have to be placed in space.

5

Explain why single dish radio telescopes must be placed in isolated locations.

They must be located far away from any other radio sources to avoid interference.

Explain why single dish radio telescopes must be placed in isolated locations.

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