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Unformatted text preview: Telescopes Lowell 24inch Telescope Keck 10m Telescope Manipulate Light
In order for a telescope to be advantageous, it must manipulate light in a
way that is beneficial to the observer. Optimal designs give the observer
(1) a magnified view (so that it appears closer),
(2) a brighter view (of the dim object), and
(3) a highresolution view (so fine details can be seen).
There are two ways to manipulate light to our advantage:
Refraction and Reflection Refraction Uses a lens to manipulate light.
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g
Focal Length is the distance from
the lens to the focus.
focus Refraction Focal Length is the same for either
side of the lens. Refracting Telescope 1. Magnification Magnification occurs because the angular size of the image on the eye is
much larger than the angular size of the object. Note that the image is
upside down.
Magnification = Focal Length of Objective / Focal Length of Eyepiece
M = F / f PRS Question
1. Calculate the magnification for this telescope:
Objective lens:
diameter = 1 m; focal length = 2 m.
Eyepiece:
diameter = 5 cm; focal length = 20 mm.
a. 10 X
b. 20 X c. 100 X
d. 200 X Reflection Uses a mirror to manipulate light.
light
Angle of Incidence = Angle of Reflection
But flat mirrors do not have a focal length. Reflection
Must use a curved (spherical)
mirror to bring the light to a focus.
The focal length is half the radius
of curvature.
Where d
Wh do you put the eyepiece so
t th
i
that the object is not obscured by
your head? Reflecting Telescopes Reflecting Telescopes 1. Magnification
Magnification = Focal Length of Primary Mirror / Focal Length of Eyepiece M = F / f
(Same equation as for Refractors) 2. Light Gathering Ability
Proportional to the Area
(The Bigger, the Better)
Refractors: Objective Lens
Reflectors: Primary Mirror
y
Area = R2
LGA R2
LGA D2 PRS Question
2. Compare the light gathering capability of a 50inch to a 10inch telescope.
a. 0.2 X
b. 5 X c. 25 X
d. 2500 X 3. Resolving an Image Resolution Resolution is a how small an angle the telescope can detect.
It is a function of the wavelength observed and the diameter of the telescope. = (1.22) (206265) / D = 2.5 x 105 / D is in arcseconds Resolution = 2.5 x 105 / D
Example: Georgia Tech 16inch telescope
16 inch = 2.5 x 105 (500 x 109 m) / (0.4 m) = 0.31 arcsec
The atmosphere limits all telescopes to a resolution of ~1.0 arcseconds. Formulae
Magnification M=F/f Light Gathering Ability LGA R2 (or D2 ) Resolution = 2.5 x 105 / D
25 Chromatic Aberration Different Wavelengths are brought to a Different Focal Point.
The solution is to add different types of material and different shapes together.
(Only affects Refracting Telescopes.) Spherical Aberration Different Reflections are brought to a Different Focal Point.
The solution is to use a Parabolic Mirror.
(Only affects Reflecting Telescopes.) Comparison
Lens
Mirror Two sides to grind and polish
One side to grind and polish
g
p Lens
Mirror Light must travel through the glass
Light only interacts with the surface
g
y Lens
Mirror Only supported around the edge
Supported on back and sizes; therefore, larger Formulae
Magnification M=F/f Light Gathering Ability LGA R2 (or D2 ) Resolution = 2.5 x 105 / D
25 ...
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This note was uploaded on 03/04/2012 for the course PHYS 2022 taught by Professor Jarrio during the Spring '12 term at Central GA Tech.
 Spring '12
 Jarrio

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