telescope - optical images
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ASTR 103: Telescopes - Optical Images
Telescope - Optical Images
Latest Modification: February 17, 1997
ormation of an Image
q In optical astronomy, astronomers work with an image of light source formed by principal lig
collecting, image-forming part of telescope, called an objective
q An objective is either a lens or a mirror; light rays from the light source:
r Are refracted in passing through a lens
r Are reflected from a mirror surface
q Diameter of objective is known as its aperture
q Focus is where light rays converge to form an image
q Focal length of objective is:r Distance behind a lens to the focus
r Distance in front of a mirror to the focus
q Telescopic image of a star is a point of light, while that of an extended object, such as the Mo
is extended but inverted
q Another small lens, called an eyepiece, is used to magnify images, much as a magnifying glas
enlarges small print
q Instead of an eyepiece, a photographic plate may be inserted into focal plane, transforming
telescope into a camera, where objective serves as camera lens
r Advantage of photography over observing with the eye is that time exposures can recofainter objects than those the eye sees
r In addition, the photograph is available for later study
roperties of an Image
q Several important properties of an image are as follows
q Size - physical size as formed in focal plane
r Image size is directly proportional to focal length
r Longer focal length means larger images
r Shorter focal length means smaller images
q Brightness - amount of radiant energy per unit area
r Image brightness is directly proportional to square of ratio of aperture to focal length
r Larger aperture means image is brighter
r Shorter focal length means image is smaller, but brighter
q Resolution - angular separation that can be observed in an image
r Image resolution is directly proportional to ratio of wavelength of light to aperture
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ASTR 103: Telescopes - Optical Images
r Larger aperture means small angles can be seen
r Shorter wavelength of radiation means better resolution
Mathematics of Images
q Let
r l = focal length of objective
r a = aperture of objective
r lambda = wavelength of radiation collected by objective
q Size
r Equation: s proportional to l , where s = size
q Brightness
r Equation: b proportional to (a/l)2, where b = brightness
q Resolution
r Equation: d proportional to (lambda/a), where d = angular resolution
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1995, J. C. Evans
hysics & Astronomy Department, George Mason University
aintained by J. C. Evans; [email protected]
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