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AQA Physics A ­ Option A

Astrophysics Revision notes.

Lenses and Optical Telescopes


Principal focus
Rays parallel to the principle axis of the lens converge onto a point called the principle
focus parallel rays that aren't parallel to the principle axis converge somewhere else on
the focal plane.

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O = object

I = image

U = distance between object and lens axis

V = distance between image and lens axis

F = focal length.

A virtual image is formed when light rays from an object appear to have come from another point
in space. The light rays aren't…

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Ray diagrams

Lens equation

Astronomical telescope consisting of two converging lenses

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A telescope, in normal adjustment, is set up so that the principal focus of the
objective lens is in the same position as the principal focus on the eye lens, so the
final magnified image appears to be infinity.

Angular magnification in normal adjustment

The objective lens converges the rays…

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M = f0

If you assume the object is an infinity, then the rays from it are parallel, and the real image is formed
on the focal plane magnification in terms of focal length.
Telescopes magnifying power is a ratio of apparent size of an object seen through a…

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Schmidt-cassegrain telescope combines an extremely short focus primary mirror at the back end
of a sealed tube with a thin lens at the front.
Small convex mirror, a secondary mirror, to reflect light back through a hole cut in the primary
mirror at the bottom end of the tube

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Requires longer amount of time to cool to ambient temp

Some chromatic aberration in bright images

Commonly manifests itself as faint fringes of colour around objects like mars or Jupiter.

Expense of producing a large achromatic lens

A chromatic triplet lens has 6 surfaces which must be figured
Lost ratio…

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To counter defect an achromatic lens is used
o Combination of two or more lenses made of different glass
A properly curved mirror reflects all the starlight at the same point
o Image formed by a reflector had no blurred colours.

Resolving power

Appreciation of diffraction pattern produced by circular…

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Diffraction limits resolving power.

Resolving power of a telescope, Rayleigh criterion

A telescopes resolving power is its ability to produce sharp, detailed images under ideal
observing conditions.
Rayleigh Criterion
Depends directly on the size of the aperture and inversely on the wavelength of the
incoming light
Determines the smallest angle…

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Charge coupled device.

Use of CCD to capture images.

Monochrome CCD to obtain hi-res images
Red, green and blue filters
Data merged electronically when chips are dumped
Pixels of CCD are dumped, or unloaded, to a computer with software able to turn images into a
picture or into a raw…


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