telescope 1. Telescopes 2. 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. There are two basic kinds of Telescope. Keplerian or Astronomical 4. Galilean Telescopes

1. Telescopes

2. 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. There are two basic kinds of Telescope.
Keplerian or Astronomical

4. Galilean Telescopes

5. fo’ fe
t
FeFo
A
B
C
D
D
C
B
A
Keplerian or Astronomical Telescopes

6. +ve Objective lens (Fo) and +ve Eyelens (FE)
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)

7. The Exit Pupil in the Keplerian Telescope

8. Galilean Telescopes
fo’
t
fe
A
B
A
B
Fo
Fe

9. Galilean Telescopes
+ve Objective lens (Fo) and -ve Eyelens (FE)
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.

10. 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

11. 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. 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

13. 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. 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. Telemicroscopes
The accommodation required to view an object at a particular
working distance through a telescope is given by:
Accomn
= Working Dist. (in Dioptres) x (M)2
e.g. to view an object @ 40cm using a telescope with M = 4x
Accomn
= 2.50 x (4)2
= 2.5 x 16.00
= 40 Diopters!
Vergence Amplification (Freid 1977)

16. 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:
3. Increase the tube length
5. Introduce a reading cap into the device.
- this is a +ve lens of power equivalent to the required
working distance

17. Fo
Fe
RC
FRC
Telemicroscope
Telescope
Telemicroscopes

18. The magnification provided by such a system is the
product of the individual components:
M total = M telescope x FRC / 4

19. 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

21. 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