Post on 18-Jan-2016
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TELESCOPESTELESCOPESINCREASE THE ANGLE
THAT AN OBJECT SUBTENDS AT THE EYE
REFRACTORS REFLECTORSTHE OBJECTIVE IS A LENS
THE OBJECTIVE IS A MIRROR
• NEWTONIAN• CASSEGRAIN
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MAGNIFYING POWERMAGNIFYING POWERMMDEFINITION
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eyethetoobjectfarthebysubtendedanglethe
eyethetoatformedimagefinalthebysubtendedangleTheM
The telescope is said to be in normal adjustment when the final image is formed at infinity
This allows a fair comparison of performance to be made between the telescope and the naked eye - which has to look at an object, which itself is at infinity.
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EXAMPLE
A telescope with a magnifying power of 40 is used to view the Moon which subtends an angle of 0.50 to the eye.What will the angle subtended to the eye by the final image, if the telescope is in normal adjustment?
The angle will be
40 x 0.50
= 200
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FORMATION OF THE FINAL IMAGE
Objective Eye Piece
Rays from bottom of object
Rays from top of object
Intermediate Image where focal points coincide
Fo & Fe
Rays emerge parallel because final image is at infinity
To top of final image at
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Objective Lens Eye Piece lens
MAGNIFYING POWER FORMULA
fo fe
h = height of intermediate image
For small angles in radiansofh
Guide ray
/
/
efh/
oe f
h
f
hM
/
h
f
f
hM o
e
/
e
o
ff
M When the telescope is in normal adjustment
h
and
TELESCOPE LENGTH = fTELESCOPE LENGTH = foo + f + fee
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The Eye Ring or Exit The Eye Ring or Exit PupilPupilThis is the image of the Objective Aperture in the Eyepiece Lens
Objective lens Eyepiece lens
object
Fe
EYE RING
RAY DIAGRAM CONSTRUCTION
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Objective lens Eyepiece lens
Guide line
Eye Ring
FORMATION OF THE EYE RINGFORMATION OF THE EYE RING
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Objective lens Eyepiece lens
Fe
u = fo + fe v
vffvuf eoe
11111
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eoe
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Do
Do = Diameter of Objective Lens
DER
DER = Diameter of the Eye Ring
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fff
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u
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heightobjectheightimage
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M BUT
Magnifying power MRingEyeofDiameter
ObjectiveofDiameterM
EYE RING EYE RING FORMULAFORMULA
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EXAMPLE 1EXAMPLE 1
A telescope consists of two convex lenses, one of focal length 500mm, the other of focal length 50mm. The telescope is in normal adjustment.
(a) What is the focal length of the eyepiece?
(b) What is the separation of the lenses?
(c) Where is the final image located?
(d) Is this image erect or inverted?
(e) What is the magnifying power?
(f) Where should the pupil of the eye be placed to obtain the best view through the telescope?
fe = 50 mm; fo = 500 mm
fe + fo = 50 + 500 = 550 mm
At infinity in normal adjustment
It is inverted
M = fo / fe = 500 50 = 10
u = 550 mm, fe = 50 mm , find v
55010
550111
5501
501111
ufv v = 55 mm
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EXAMPLE 2EXAMPLE 2
A small telescope, of length 330 mm, is rated as 10 x 50
(a) What does this mean?
(b) What is the focal length of the eyepiece?
(c) What is the focal length of the objective?
(d) What is diameter of its eye ring?
(e) What is the instrument’s relative light gathering potential?
M = 10 & the Diameter of the Objective is 50 mm
So fo= 11 fe. Then 11 fe = 330; fe = 30 mm33010 eo
e
o ffandff
fo = 300 mm
M = Do / DER ; so 10 = 50 DER ; Eye Ring Diameter = 10 mm
Diameter of Objective Diameter of Eye Ring = 50 10 = 5 times
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CHROMATIC CHROMATIC ABERRATIONABERRATIONLENSES ACT LIKE PRISMS AND DISPERSE LIGHT
A lens has a shorter focal length for blue light than it does for red
This causes image to be blurred and to have coloured edges
FBFB
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SPHERICAL SPHERICAL ABERRATIONABERRATIONOnly rays of light parallel and close to the principal axis pass through the focal point, F, of the lens.
Principal axis
F
Rays farther from the axis strike the lens with a greater angle of incidence and consequently have a greater angle of refraction.
USING A LENS WITH A WIDE APERTURE BLURS THE IMAGE SPHERICAL ABERRATION
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SPHERICAL MIRRORS ALSO SUFFER FROM SPHERICICAL ABERRATION
Ff
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PARABOLIC MIRRORS ELIMINATE SPHERICICAL ABERRATION
Ff
IN ADDITION MIRRORS DO NOT SUFFER FROM
CHROMATIC ABERRATION
ALL LARGE TELESCOPES ARE PARABOLIC REFLECTORS
i.e. There objective is a parabolic mirror
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NEWTONIAN NEWTONIAN REFLECTORREFLECTOR
Convex Eyepiece
Parabolic Concave Mirror
Plane Mirror at 450 to the axis
F
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CASSEGRAIN CASSEGRAIN REFLECTORREFLECTOR
Fsite of 1st intermediate image
Auxiliary convex mirror - in practice very much smaller than the objective
Parabolic Concave Objective
2nd intermediate image
Convex lens eyepiece, acts like a magnifying glass
The Final Image is at Infinity
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Advantages of ReflectorsAdvantages of Reflectors
• Less loss of light in reflection compared with refraction
• Do not suffer from chromatic aberration
• Parabolic reflectors do not suffer from spherical aberration
• Diameter can be much greater because a mirror - can it be supported
from behind - “glass sags” *• There is only one surface to be ground• No worry about trapped air bubbles in the glass
*Telescopes have large apertures
1. To gather more light
2. To increase their Resolving Power