Telescope, Distance Optical Aids For Low Vision Patient, Types, Advantages, Disadvantage and How To Guide The Patient..

1. Telescope
PRADEEPA B V
FACULTY OF M M JOSHI EYE INSTITUTE( DEP OF OPTOMETRY)

2. Telescopes
The telescope is an optical instrument used to magnify the apparent size of
distant objects making them appear closer to the patient then they actually
are. It is the non-electronic device that is available for improvement of the
distance & intermediate resolution when:-
• Patient can’t move closer to the object
• The best conventional spectacle or contact kens do not provide the
needed resolution
• The optical system consists of two lenses called ocular lens & objective
lens separated by a short distance in a metal tube.

3. The Objective Lens ;
It is a positive or convergent lens is positioned closest to the object. It is
generally large in diameter & is used to collect a large quantity of light.
The Ocular Lens (eye piece);
• It is closest to the eye, it may be either a convergent in Keplarian
telescope or divergent in Galilean telescope.
• The power of ocular lens is more than objective lens.
• If a negative lens is placed in Galilean telescope erect & virtual image is
formed within the system.
• If a positive lens is placed in Keplarian telescope real & inverted image is
formed outside the system

4. Principle ;
Of telescope When a parallel ray of light strikes the convergent objective
lens, the image formed by the objective lens is located in the plane of
secondary focal point. In an a focal system, the ocular lens is positioned so
that its primary focal point is located coincident with the image formed by the
objective lens.
There are two types of telescope
1. Galilean telescope
2. Keplarian telescope

5. 2. Keplarian telescope ;
• Commonly used to view stars & planet.
• It consists of both positive objective lens & ocular lens.
• The parallel rays of light from a distance object forms the image AB of the
objective at the principal focus of the objective lens, which is also the
principal focus of eyepiece, & hence the eyepiece will form the image at
infinity.
• Basic principal : - secondary focal point of the objective lens coincides with
the primary focal point of the eye piece.
• m=-Docl/Dobj , magnification is negative indicating a real & inverted
image.
• Magnification of the telescope is defined as the ratio of angle subtended
by the final image at the eyepiece to the angle subtended by the rays of
light from the object at the objective lens.
• The length of the telescope is fobj + focl
• Additional elements(usually prism) must be added to Keplarian telescope
to make the final image upright called the terrestrial telescope (usually
used in low vision).

6. 1. Galilean telescope;
• It consists of positive objective lens & minus negative ocular lens or eye piece.
• Before the image AB of a distant object is formed at the principal focus of the
objective lens, they are incident on the concave lens eyepiece & diverging out of
the eyepiece when these rays of light are produced back, the virtual erect image of
the object is formed within the optical system.
• Principle : - secondary focal point of objective lens coincides with the primary focal
point of eyepiece. m= -Docl/Dobj
• Magnification is positive indicating the image is erect & upright.
• The length of the telescope (secondary focal length of objective lens – primary
focal length of ocular lens) d=fobj+focl
• Increasing the power of both objective & eyepiece produce a shorter telescope of
constant magnification.
• They produce a bright image but offer rather narrow field of view.
• Simple optical system, small & lightweight & hence they are convenient for
binocular prescription.
• They are available as both fixed focus & focusable versions & are usually
prescribed in power between 1.5x & 3x though other powers are available.

7. Difference between Keplarian & Galilean telescopes
characteristics Keplarian Galilean
1. Objective lens Positive Positive
2. Ocular lens Positive Negative
3. Eyepiece system Compound system Simple lens system
4. weight Heavier Lighter
5. length Longer Shorter
6. exit pupil location Outside the system Inside the system
7. Magnification available Low & High Low
8. Effect of focusing for an
uncorrected myopic
Increased magnification Decreased magnification
9. Effect of focusing for an
uncorrected hyperopia
Decreased magnification Increased magnification
10. Field of view More Less
11. Image quality Better Poorer
12. Cost Higher Poorer

9. Exit pupil ;
• Image of objective lens as seen by the eyepiece
• Galilean ; inside the telescope
• Keplarian ; outside the telescope

11. Telescopes ;
• Telescopes with magnification power from 2X- 10X are prescribed
• They can be prescribed for near, intermediate and distant tasks
• Field of view decrease with magnification
• Magnification of telescope is given by the formula M= Fo/Fe
• Telescope focus near object by:
- changing the distance between objective and ocular lens
- increasing the power of the objective lens
• Types of telescopes used as low vision aids are:-
-Hand held monocular
-Clip on design
-Bioptic design: mounted on a pair of eyeglasses
-spectacle mounted telescope
-binocular telescope

12. 1)Monocular hand held telescope ;
-Are healed with hand infront of eye
-Available in 2X-10X magnification
-Useful specially for students.
-Light weight & inexpensive
-Patient have to spot the target and then focus
Disadvantage:
hand pain for prolonged use.

13. 2) Spectacle Mounted Telescope;
- Telescope is mounted on the telescope on the spectacle
- Helpful for prolonged viewing task
- Good for television or sport events
- Hands free
- Eg school children
3) Binocular Telescope;
- Are prescribed if patient have approximately
- same visual acuity
- Are available upto 3.5X
- Have greater field of view

14. 4) Clip on Telescope;
- Have a clip on a monocular telescopic
- Can be attached on the spectacle
- Can also be used as hand held telescope
- Weight is an disadvantage
5) Bioptic Telescope ;
- Maintained high on the spectacles
- Are fitted on superior part of spectacles
- Gives vision down gaze too
- Monocular or binocular depends on purpose
- Limited avaibility

15. Advantage of telescope;
• only possible device to enhance distant vision
• Available with focusing ability
• For students who want to see chalk board in class room
• For viewing distance sign board while walking
• Greater flexibility to meet constantly changing visual ability
Disadvantage of telescope;
• Restricted field of view
• Expensive and costly
• Depth perception id distorted
• Cosmetically unappealing.
• Not the choice of aid in diseases with constricted field

16. An ideal telescope used for low vision would have following features;
• Wider field of view
• Light weight
• Limited or no aberrations
• Excellent light transmission
• Effective retinal illumination
• Compactness
• Low cost
• Focus ability
• Appropriate magnification

17. Instruction in the use of telescope;
• It is important that proper instruction in the use of a telescope be
provided so that the patient is able to realize the full potential of the
device.
• The patient’s pathology, refractive error, visual acuity, age & motor skills
should have also been taken into account.
• Instruct the patient not to attempt to walk while looking through the
telescope. It should only be used while stationary.
• The ideal sequence for instructions should be, move from simple targets
to more complex ones, move from static activities to dynamic ones.
• Instruction & practice sessions should b kept short & success oriented.
• Instruction should begin with the introduction of the telescope & its parts
like eyepiece, objective lens, ocular lens & focusing ring.
• Allow the patient to handle the telescope & to focus in all the way in &
out, patient should be encouraged to describe what is seen through the
telescope.

18. • If using handheld telescope patient should be encouraged to tuck it into
the ‘’V’’ made by tumbe & forefinger of the right hand if using with right
eye or left hand if using with let eye.
• When brought up near to the eye the tumbe or forefinger should be
rested against the face for stability.
The following steps or procedures are recommended when instructing
the patient to use a telescope.;
1. Localization
2. Focusing
3. Spotting
4. Tracing
5. Tracking
6. Scanning

19. 1. Localization ;
• The first step in using telescope is localization of the target that is finding the
target & being sure the target, telescope & eye are aligned. The should be
instructed to view the target unaided & then to keep viewing it while a handheld
telescopes brought up to the eye.
• If patient using a mounted biopic system the target should be first localized
through the carrier lens & then the patients head should be dropped to bring the
target into view with the telescope.
• A head strap is often useful for stability of the spectacle & telescope & also
increases patients comfort. Patient may benefit from closing or patching other eye.
• The patient should be encouraged to think about the best viewing position for any
given target & to move to that ideal position. Normally the best viewing position is
perpendicular to the target.
• Alignment of target should be verified by asking the patient is the image is fully
circular (A mounted system may need some adjustment of the telescope position
or the nose pads for alignment)
• The patient should sit or stand while viewing through it. While sitting, the elbow
may be rested on the table to aid stability. When standing the patient may steady
the telescope by holding his/her upper arm against his/her body or by supporting
the elbow of the arm holding the device with the opposite arm.
• Leaning against a wall may also provide additional support & stability

20. 2. Focusing ;
• Once the patient has mastered in localization techniques, instruction should proceed to
focusing the telescope.
• Patient should also be informed about the range of focus of the telescope (e.g. from
infinity to 12 inches) Initially the telescope should be focused for the patient when the
telescope is in focus, the patient should view the target.
• The telescope should then be taken out of focus so that the patient can see the
difference between in focus & out focus.
• A patient should practice focusing on targets that are locate at various distance & also
experiment the closest & furthest distance that can be focus.
• Glasses should be worn while focusing the telescope if there is a refractive error.
• Patient should be encouraged to experience blurring on either side of the clear focus.
• This will ensure that the clearest focus will be obtained. On binocular system, each
telescope should be focused independently of the other, accomplished by closing the
eye behind the telescope not being focused.
• If this step is difficult for the patient, either an autofocus could be recommended which
obviously would eliminate any focusing problem or to make a marking on the telescope
that would indicate the described focus for only one fixed distance.
• If the telescope goes out of focus its simply should be rotated in one direction or the
other until the markings are realigned.

21. 3. Spotting ;
• It describes the result of localization & focusing. It is the process of finding a
target without benefit of the telescope, positioning the telescope so that it is
aligned between the eye & the target & focusing the telescope until the
image is as clear as possible.
4. Tracing;
• It is a skill must be mastered after proficiency in spotting. It involves the
following of stationary line in the environment like telescope poles,
telephone wires, streets & curbs etc.
• The patient should move his/her head smoothly & slowly when tracing. The
motion will allow him/her to maintain his/her orientation to the
environment.

22. 5. Tracking;
• It is a slightly more advanced skill than is tracing, as it involves moving the
head & telescope smoothly while following a moving target. The patient
must be preferably seated & observe the target that is perpendicular to
the straight-ahead direction & moving at a constant rate.
• As this skill are mastered, diagonal movements, movements towards &
away from the patient, making the target smaller, reversal of direction &
curved paths are introduced at different rates. Losing sight of the target is
not unusual (initially).
• Patient must be encouraged to find the target again by following the
localization. Step followed by tracking. If the distance between the target
& patient changes then the telescope must be refocused along with
tracking, (Such an activity should not be used for practice until the patient
has demonstrated good initial tracking skills)

23. 6. Scanning;
• Most difficult but most valuable skill to master in the hierarchy of telescopic skills
Scanning is using an organized search pattern to locate the target that can’t be
locate without the use of the telescope For scanning to be effective, the patient
must first use some objects in the environment to define his/her location
(environment reference point).
• Kinesthetic awareness (Perception of one’s own body movements of limbs,
muscles etc) should be used to define how mush to turn the body so that scanning
can b then performed in a grid pattern.
• When targets are found in vertical plane, the patient must scan horizontally & vise
versa E.g., If street sign must be scanned vertical plane the patient must scan
horizontally to find the pole & then vertically up the pole until patient finds the
sign itself.
• Scanning can begin indoors with simple tasks such as finding printed sentence on a
chalk board & then can graduate to the outdoor environment.
• The technique that needs the most attention is the ability to overlap the scanning
paths completely.
• When tracking or scanning, patient may complain of discomfort due to the
movement of the image.
• The patient should be reminded that the telescope magnifiers both image size &
motion & therefore he/she should be advised to move the head smoothly &
slowly.

24. Reference :
1) Raju Kaiti – Optometrist at kathmandu university , medical
school slide share.
2) Google
3) Monica Chowdary Low vision Book.
4) Telescope related articals.

25. Thank You