Telescope often used on object closer than infinity.

Telescope an effective way of producing magnification without changing the working distance

1. Gaurav Bhardwaj
Telescopes

2. Gaurav Bhardwaj
Telescopes represent an effective way of producing
magnification without changing the working distance.
Disadvantages
They have a restricted field of view
Often used to focus on objects closer than infinity
Can be modified to correct for the Px’s refractive error.

3. Gaurav Bhardwaj
There are two basic kinds of Telescope.
Keplerian or Astronomical

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Galilean Telescopes

5. Gaurav Bhardwaj
f
o
’ f
e
t
F
e
F
o
A
B
C
D
D
C
B
A
Keplerian or Astronomical Telescopes

6. Gaurav Bhardwaj
+ve Objective lens (F
o
) and +ve Eyelens (F
E
)
Focal points of these lenses are coincident
The image produced by a Keplerian Telescope is inverted
Prism are required to re-invert the image
The exit pupil is formed outside the instrument
Keplerian or Astronomical Telescopes
Specified in terms of the magnification and the diameter of the
objective lens (e.g. 4 x 12)

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The Exit Pupil in the Keplerian Telescope

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Galilean Telescopes
f
o
’
t
f
e
A
B
A
B
F
o
F
e

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Galilean Telescopes
+ve Objective lens (F
o
) and -ve Eyelens (F
E
)
The image produced by a Galilean Telescope is erect
The exit pupil is formed inside the instrument
The length of the Galilean Telescope is shorter than the Keplerian
Specified by the magnification 2x 2.5x etc.

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M = angle subtended at the eye by instrument image
angle subtended at the eye by object
Magnification in Telescopes
M = Fe
Fo
M = size of entrance pupil
size of exit pupil

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Property Galilean Keplerian

Objective (Fo)

+ve

+ve

Eyepiece (Fe)

-ve

+ve

Tube length

short

long

Magnification

up to 4X

up to 10X+

Exit Pupil

internal

external

Field Size

smaller

larger

Comparison of Keplerian & Galilean Telescopes

12. Gaurav Bhardwaj
So far we have dealt with afocal telescopes considering objects
placed at optical infinity and viewed by emmetropic eyes.
Afocal – the tube length of the system is fixed and cannot be
adjusted for different viewing distances and refractive
errors.
Afocal and Focal Telescopes

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In practice telescopes are used by ametropic eyes to
view objects that are not at optical infinity.
There are 3 ways by which a telescope can be adapted to
compensate for spherical ametropia:
1. Add full Rx to eyepiece
2. Partial Rx correction of the objective
3. Changing the length of the telescope
Telescopes which can be adjusted in these ways are known as
focal

14. Gaurav Bhardwaj
Field of View with Telescopes
A telescope gives maximum field of view if the objective
lens is as large as possible.
To gain maximum benefit, the Px’s pupil should be as close as
possible to the exit pupil of the telescope.
This is much easier for a Keplerian telescope than for a
Galilean.

15. Gaurav Bhardwaj
Telemicroscopes
The accommodation required to view an object at a particular
working distance through a telescope is given by:
Accom
n
= Working Dist. (in Dioptres) x (M)
2

e.g. to view an object @ 40cm using a telescope with M = 4x
Accom
n
= 2.50 x (4)
2
= 2.5 x 16.00
= 40 Diopters!
Vergence Amplification (Freid 1977)

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Even modest working distances can make high
demands on the accommodative effort.
In order to be useful for near work telescopes have to be modified
This can be achieved in two ways:
1.Increase the tube length
2.Introduce a reading cap into the device.
- this is a +ve lens of power equivalent to the required
working distance

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F
o
F
e
RC
F
RC
Telemicroscope
Telescope
Telemicroscopes

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The magnification provided by such a system is the
product of the individual components:
M
total
= M
telescope
xF
RC
/ 4

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Characteristics



Monocular



Binocular

Magnification Range


Focussing Distance


Mounting Available




Special Features





2.75x - 12x


Infinity to 20 cm


Finger ring, neck/wrist cord
Clip on or fixed frame mounted
(up to 6x)
Table top tripod mounted ( > 8x)

Can have additional +ve lenses to
make a very high powered stand
mag

6x to 8x


Infinity to 2 m


Hand held
Spectacle Mounted (rare)



Limited adjustment for PD.

Astronomical
Prescribing Telescopes

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21. Gaurav Bhardwaj





Characteristics



Distance



Near


Fixed focus
With Caps


Fixed focus for
Near

Magnification range

Binoc. or Monoc.

Viewing Dist
(D, I, N)


Mounting available





Examples



1.7x to 3x

B or M

D to I



Clip-on
Spec
mounted
Hand-held


Eschenbach,
Keeler
And Rayner
rapid-focus


1.7x to 4x

B or M

I to N



Clip-on
Spec
mounted



Eschenbach

1.6x to 8x

M

D, I and N if
appropriate
Caps are available

Spec-mounted
Clip on




Keeler Multi-cap
Nikon

1.6x to 8x

B or M

N



Spec mounted
Clip-on




Keeler Bar type


Limited Focussing
Ability
Galilean Telescopes

22. Gaurav Bhardwaj