telescopes designs and optics

1. Telescopes: Design, Optics
and Handling Techniques
Presenter
Harshita Thapa
B.Optometry
Third Year

2. Objectives
• To discuss about the designs and optics of telescope
• To illustrate different handling techniques for telescope

3. Presentation Layout
• Introduction
• Optical Principle of Telescope
• Types of Telescopes
• Optical Design of Telescopic system
• Methods Adapted by Telescopes for near use
• Handling Techniques
• Summary
• Reference

4. Introduction
• Optical instrument used to magnify the apparent size of a distant
object.
• Generally, focused for infinity, but can be adapted for near vision use.
• Contains two elements;
1. Objective lens (convergent lens)
2. Ocular or eyepiece lens(either convergent or divergent lens)

5. Optical Principle of Telescope
Afocal Telescope (normal adjustment)
• Separation between objective and ocular lens by their absolute value
of focal length.
• Produces only angular magnification.
• Parallel rays of light incident from infinity and emerges.
• Equivalent power of telescope is zero.
• Magnification= -Doc / Dobj

6. Optical Principle of Telescope
Focal Telescope
• Made focal for intermediate or near use.
• Made focal due to relative distance magnification.
• Referred as Telemicroscope.

7. Types of Telescopes
• Galilean telescope
• Keplerian telescope

8. Galilean Telescope
• Primary focal point of ocular lens is coincident with the secondary
focal point of objective lens .
• Ocular lens (eye piece): Negative
• Pocular > Pobj
• Magnification: Positive
• Image characteristics: Upright and virtual

9. Galilean telescope
• Mang = tanθ’/ tanθ
= (h/f2)/(h/f1’)
Or,
• Mang = - F2/F1’

10. Keplerian Telescope (Astronomical Telescope)
• Primary focal point of ocular lens is coincident with the secondary
focal point of objective lens.
• Ocular lens(eye piece): Positive
• Magnification: Negative
• Image characteristics: Inverted in astronomical telescopes.
• Note: Prisms are added to upright the final image in terrestrial
telescopes.

11. Keplerian telescope
• Mang = tanθ’/ tanθ
= (h/f2)/(h/f1’)
Or,
• Mang = - F2/F1’

12. Aperture stop
• Physical aperture that limits the light entering into the system from an
axial object.
• Objective lens serves as aperture stop in both Galilean and Keplerian
telescope.
Uses
• Control the amount of light passing through system and limit the field
of view of the system.

13. Entrance pupil and Exit pupil
• Definition of entrance pupil: Entrance pupil is the image of an
aperture stop formed by any lenses in front of the aperture stop in an
optical system.
• In telescopes, the entrance pupil is the image formed by an objective
lens of the aperture stop.
• Location of the image: in front of the aperture stop
• Definition of exit pupil: An exit pupil is the image of an aperture stop
formed by lenses following the aperture.

14. Exit Pupil in Galilean telescope
• In Galilean telescope,
objective lens is the
aperture stop which is
imaged by an ocular lens
(minus power).
• Exit pupil: within the
tube (near ocular lens)

15. Exit pupil in Keplerian telescope
• In Keplerian telescope,
objective lens is the
aperture stop which is
imaged by an ocular lens
(plus power).
• Exit pupil: outside the tube
(near ocular lens)
• A telescope’s exit pupil is
also called Ramsden circle.

16. Comparing Galilean and Keplerian Telescopes
Characteristic Galilean Keplerian
Objective Positive Positive
Eyepiece Negative Positive
Image orientation Upright Inverted or upright
Location of exit pupil Within tube Outside of tube
Field of view Less Greater
Tube length Shorter Longer
Shape Straight May be bent
Weight Generally lighter Generally heavier

17. Ideal Features of Optical Design of Telescope
• Appropriate magnification
• Wide field of view
• Minimal or no aberrations
• Excellent light transmission
• High retinal illuminance
• Light weight
• Compactness
• Low cost
• Focusability

18. Calculating Telescopic Magnification
1. Determine patient’s best corrected visual acuity (BCVA, e.g., 6/60)
2. Determine target acuity (e.g., 6/12)
3. Use the formula,
Amount of magnification = MTS = BCVA/ Target Acuity
= 60/12
= 5X
4. Try telescope with low magnification (e.g., 3X – 5X)
(A trade of between visual requirement, magnification, and FoV)
Note: Too much telescopic magnification increase inherent limitations of field and
lessen optical quality.

19. Field of View (FoV)
• Field of view of telescope is determined by
Characteristic Galilean Keplerian
Size of objective lens ↑ diameter of objective lens = ↑ FoV
diameter of exit pupil =
𝐷𝑜𝑏𝑗
𝑀𝑇𝑆
Magnification of system ↑ Magnification = ↓FoV
Separation between lens ↑TL = ↑ FoV ↑TL = ↑ FoV
FoV in Galilean T > FoV in Keplerian T [𝑀 =
1
1−𝑑𝐷𝑜𝑏𝑗
]
Vertex distance (VD) ↑VD = ↓ FoV (Galilean > Keplerian)
Exit pupil ↓ FoV (exit pupil: inside the tube) ↑ FoV (exit pupil: outside the tube)
↑exit pupil diameter = ↑𝐹𝑜𝑉 [𝐸 =
𝐷𝑜𝑏𝑗
𝑀𝑇𝑠
]

20. Aberrations
• With increase in magnification, aberrations
increases.
• Mostly chromatic aberration increases and spherical
aberration also increases minimally.
• Way to minimise aberration
o Use of cemented doublet objective and eyepiece,
reduces chromatic aberration.
o Small aperture lenses reduces spherical
aberration.

21. Light transmission
• Anti-reflective coating increases light transmission effectively by 4% at
each optical surface compared to uncoated lens.
• Anti-reflective coating of wavelength of maximum visibility is
generally selected.
• Anti-reflective coated lens appears purplish or purple green reflective
colour.

22. Retinal illuminance
• Telescope reduces the brightness of extended light sources.
• Retinal illuminance is directly proportional to the area of exit pupil of
the eye-telescopic system.
• Exit pupil= Diameter of objective lens/MTS
• Relative light efficiency = (Exit pupil)2

23. Retinal illuminance
Condition Effect Result
Exit pupil of telescope >
Eye pupil diameter
Eye pupil is filled with light Image brightness same as
unaided eye
Exit pupil of telescope =
Eye pupil diameter
Eye pupil is filled with light Image brightness same as
unaided eye
Exit pupil of telescope <
Eye pupil diameter
Eye pupil is not filled with
light
Reduction in image
brightness

24. Weight
• The diameter of objective lens, is if increased, the weight of telescope
increases.
• Galilean telescope: has certain range magnification , weight and tube
length is limited.
• Keplerian telescope: heavier with longer tube length, require a mirror
or prism (to upright the image).

25. Expense and Compactness
• Galilean telescopes are smaller and less expensive than Keplerian
telescopes.
• Keplerian telescopes are more costly because of expense of
incorporating upright prisms or mirrors.
• Compactness is related to tube length.
• Keplerian telescope is less compact than Galilean telescope because
both lenses in Keplerian telescope are positive.

26. Methods adapted to telescopes for near use
1. Increasing the power of the objective lens
• Diverging wavefront striking the objective lens can be neutralized by
adding a reading cap to the front of telescope.
• Working distance of near telescope is equal to focal length of
reading cap or additional plus power added to objective lenses.
• Equivalent power= (Dcap)(MTS)

27. Methods adapted to telescopes for near use
2. Decreasing the power of the ocular lens
• A significant amount of accommodation is required to clear the image
blurred by divergent rays emerging from eyepiece.
• Required amount of accommodation is eliminated by changing power
of ocular lens.
3. Both Galilean and Keplerian telescopes can be made focal by
increasing the tube length or separation of lens elements.

28. Refractive Error Correction in Telescope
• Can be done by incorporating refractive correction directly into ocular
lens of Galilean or Keplerian telescope.
• By altering the tube length of telescope,

29. Monocular vs Binocular telescopes
Monocular Telescopes
• Low vision patients may not
have binocular vision.
• In such case, monocular
telescopes are prescribed in the
better seeing eye.
• Mostly, they are handheld
telescope.
Binocular Telescopes
• Prescribed in patient have
approximately same visual acuity
in both eyes and demonstrating
some degree of binocularity.
• Example: spectacle-mounted
telescope

30. Handheld Telescopes
• Can be conveniently carried around neck
(Often come with cords and straps).
• Available mostly in Keplerian (FoV, range
of magnification, clarity, retinal
illumination)
Uses
• For temporary or occasional use
• Reading street signs
• Viewing scenery

31. Spectacle Mounted Telescopes
• For the task involving prolonged viewing,
spectacle mounted telescopes are more suitable.
• e.g., Clip on telescopes
• Can be secured directly into patient’s line of sight.
Uses
• Watching television
• Writing
• Driving

32. Advantages of clip on telescope
• Can be easily removed when not desired and used as handheld
telescope.
• Can be essential low vision aid in patients with Parkinson’s disease,
Multiple sclerosis, Hyperthyroidism ,etc
• Easily allows for a patient's spectacle correction to be
incorporated into a spectacle mounted telescope.

33. Disadvantages of clip on telescope
• Possibility of scratching spectacle lenses.
• Mechanical difficulties encountered in clipping it.
• Determining the correct position and maintaining the
proper alignment.
• Reduction of field of view, exit pupil being placed far from eye.

34. Other Spectacle Mounted Telescope Designs
Full field position
• When telescope is positioned in
the centre of carrier lens.
• Designed for distance tasks
when patient is stationary.
• Used for watching television
Bioptic and Trioptic
• When telescopic unit is positioned
superiorly within the frame.
• Especially designed system in
which a superiorly placed telescope
is combined with an inferiorly
mounted telemicroscope.
• Prescribed for driving

35. Verification of Telescopic devices
• Visual inspection of device is an important part of verification
process.
• If telescope is focusable, it should be turned smoothly and must be
capable of focusing at expected distances.
• Should be positioned in carrier lens at recommended angles and inter
pupillary distance.
• Proper angle can be verified with a protractor.

36. Verification of magnification of an afocal
telescope
Can be done in number of ways:
• Comparison method
• Actual measurement of both the diameter of objective lens and the
exit pupil of the telescope; MTS= dobj/diameter of Exit pupil
• By the principle of vergence amplification,
Aocular= M2U/1- dMU

37. Procedure to determine the magnification
1. Position an unknown telescope in the lensometer with ocular lens
against lens stop.
2. The Galilean telescope should be focused for infinity or it’s back
vertex power must be adjusted to zero.
3. Hold trial lens of +1.00 D without removing the telescope from
lensometer.

38. Procedure to determine the magnification
4. Focus the lensometer to obtain new back vertex power reading.
5. Remove the telescope and measure tube length.
6. Calculate the magnification of telescope by following formula:
M =
− 𝐴0 d + 𝐴0𝑑 2 − 4𝐴0
2

39. Handling Techniques
Following procedures recommended in sequence to handle the
telescope:
1. Localization: Finding the target and aligning the target, eye and
telescope.
2. Focusing: Viewing the target by focusing the telescope.
3. Spotting: Result of finding the target, aligning and focusing until the
image is clear.

40. Handling Techniques
4. Tracing: Following the stationary line along with slow and smooth
movement of head.
5. Tracking: Following a moving target with smooth movement of head
and telescope.
6. Scanning: Using a reference point or some objects in the
environment as a search pattern to locate a target.

41. Summary
• Telescope is the only device available for improvement of distance
and near vision.
• Keplerian telescope provide larger field of view than Galilean
telescope of equal magnification and equal objective lens.
• Telescope devices must be prescribed according to the goals and
needs of the patient.
• Magnification of telescope must be as low as possible to lessen the
optical quality and enhance the field of view.

42. References

43. THANK YOU !