# Astrophysics - Telescopes

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• Created by: Lauren
• Created on: 01-05-14 18:52
Converging Lens
Makes parallel rays converge to a focus
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Principal Focus or Focal Point of a Lens
The point where parallel rays are focused
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Diverging Lens
Makes parallel rays diverge (i.e. spread out). The point where the rays appear to come from if the principal focus of this type of lens.
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Focal Length
The distance from the lens to the principal focus.
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Focal Plane
The plane on each side of the lens perpendicular to the principal axis containing the principal focus.
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Real Image
An image formed on a screen where the light rays meet. The object must be beyond the principal focus of the lens to form a real image.
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Virtual Image
An image formed where the light rays appear to come from. The object must be between the lens and the principal focus and the image is formed on the same side of the lens as the object.
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Principal Axis
Straight line through the centre of the lens perpendicular to the lens.
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3 Key Construction Lines for Ray Diagrams
1. Parallel to the principal axis before the lens so it is refracted by the lens through F. 2. Through the centre of the lens without change of direction. 3. Through F before the lens so is refracted parallel to the axis.
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Linear Magnification of the Image
Height of Image/Height of Object
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Magnified Image
Image height is greater than object height.
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Diminished Image
Image height is smaller than object height.
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Simple Refracting Telescope
2 converging lenses of different focal lengths.
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Objective Lens
The lens with the longer focal length which faces the object.
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Eyepiece
The lens with the shorter focal length which the viewer looks through.
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The telescope is adjusted so the virtual image is seen by the viewer at infinity. (The principal focus of the eyepiece is at the same position as the principal focus of the objective, the distance between two lenses is the sum of their focal lengths)
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Angular Magnification of a Telescope in Normal Adjustment
Angle subtended by the final image at infinity to the viewer/ angle subtended by the distant object to the unaided eye OR Objective focal length/ Eyepiece focal length
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Reflecting Telescope
Concave mirror is used as the objective. Referred to as primary mirror as a secondary smaller mirror reflects light from the concave reflector into the eyepiece.
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Principal Axis of a Mirror
The line normal to its reflecting surface through its centre.
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Principal Focus of a Mirror
If rays are parallel to the principal axis of the concave mirror then the point at which they converge.
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Focal Length of a Mirror
Distance from principal focus to the centre of the mirror
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Cassegrain Reflecting Telescope
The secondary mirror is a convex mirror positioned near the focal point of the primary mirror between this point and the primary mirror itself. (To focus light into or just behind a small hole in the concave reflector)
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Spherical Aberration
With a spherical reflecting surface the outer rays of a bean parallel to the principal axis are brought to focus nearer the mirror than the focal point. (Primary mirror must be parabolic to reduce this)
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Chromatic Aberration
White light is split when it is refracted so the image formed by the lens is tinged with colour, particularly noticeable near the edge of the lens.
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Angular Separation of 2 Stars
The angle between the straight lines from the Earth to each star.
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Rayleigh Criterion
Resolution of the images of two point objects is not possible if any part of the central spot of either image lies inside the first dark ring of the other image (Diffraction patterns)
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Resolution or Resolving Power
The quality of a telescope in terms of the minimum angular separation it can achieve.
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Charge-Coupled Devices (CCD)
An array of light-sensitive pixels which become charged when exposed to light. After being exposed for a set amount of time, the array is connected to an circuit which transfers the charge collected in sequence to a capacitor.
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Consist of a large parabolic dish with an aerial at the focal point of the dish. A steerable dish can be directed at any astronomical source of radio waves in the sky.
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Infrared Telescopes
A large concave reflector which focuses infrared radiation onto an infrared detector at the focal point of the reflector.
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UV Telescopes
Use mirrors to focus incoming UV radiation onto a UV detector.
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X-Ray Telescopes
Reflecting X-Rays off highly polished metal plates at 'grazing' incidence onto a suitable detector.
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Gamma Ray Telescopes
Detecting gamma photons as they pass through a detector containing layers of 'pixels', triggering a signal in each pixel it passes through it.
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## Other cards in this set

### Card 2

#### Front

The point where parallel rays are focused

#### Back

Principal Focus or Focal Point of a Lens

### Card 3

#### Front

Makes parallel rays diverge (i.e. spread out). The point where the rays appear to come from if the principal focus of this type of lens.

### Card 4

#### Front

The distance from the lens to the principal focus.

### Card 5

#### Front

The plane on each side of the lens perpendicular to the principal axis containing the principal focus.