TELESCOPES FOR LOW VISION Basic and Advanced
Abstract
Telescopes have a multitude of applications. They may be monocular or binocular, head mounted or
hand-held. Important optical properties are magnification, depth of focus, field of view, field of fixation
and image brightness. Patient factors, ametropia, focusing distance, eye relief and pupil size are
important to telescope selection and design.
Learning Objectives HEY ROBERT CHANGE THESE AS MUCH AS YOU LIKE
1. Revision on basic theory or telescopes
2. Learn new approaches to apertures and stop theory and understanding image illuminance and field of
view in telescopes
3. Understand vergence amplification with telescopes and the implications for ametropia correction,
depth of field, and novel methods for checking optical parameters.
4. Learn techniques for fitting teslcopes and determining mounting angles.
5. Learn about the properties and features of commonly available telescope systems
Key Words
1.Low Vision
2.
3.
Special Instructions to AAO staff
I wish to have Dr Ian L Bailey (OD, DSc, FAAO – UC Berkeley) as a co- instructor)
BELOW IS COURSE OUTLINE
VERY VERY similar to last time (2007) Edit Freely
TELESCOPES FOR LOW VISION Basic and Advanced
Robert B Greer, OD, FAAO
Ian L Bailey, OD, DSc, FAAO
School of Optometry
University of California, Berkeley
I
BASIC THEORY OF TELESCOPES.
A Galilean (+/-) and Keplerian (+/+)
Basic formulae M = - Poc / Pobj
d = 1/ Poc + 1/ Pobj
M = 1/ (1-d.Pobj )
M = 1-d.Poc
Terrestrial telescopes Porro and Pechan prism systems for Keplerian telescopes
Spectacle magnification is analogous to telescope magnification.
Useful formula for spectacle magnification M% = zPspec ( if z in cm)
EVD concept Alternative to quantifying magnification
EVD is the distance at which the object would be places in order to subtend the same angle
that is subtended by the image
B Adapting telescopes for ametropia
Add plus or minus power to the ocular to correct ametropia
Change telescope length.
Longer gives more plus for hyperopia or close focus
Shorter gives more minus for myopia
C
Achieving a close focus
Lens cap on objective
If focal length of lens cap = fv
Then EVD = fv / M
If lengthening telescope to focus on near object
Then EVD = (fv /M) - foc
D Measuring optical properties of telescopes
Measuring Magnification
Comparison
Entrance/ exit pupil ratio (diameters objective/ ramsden)
Lateral magnification (object near exit pupil plane)
Lateral magnification (lensometer target)
Longitudinal magnification ( = M2)
Vergence change when lens added to objective
Measuring Back Vertex Power
Lensometer often gives erroneous measures.
Using close focusing telescopes to check the BVP of a system
II APERTURES AND STOPS- RESTRICTING LIGHT AND FIELD.
Ramsden disc - an image of objective lens
It is smaller in size by M times. Its position is d/M from ocular lens
A. Image brightness
Telescopes reduce brightness if Ramsden Disc is smaller than the observer’s pupil
Telescopes only enhance brightness of point sources (tars)
B. Field limitation.
Very different for Keplerian and Galilean
The image field depends on the angle that the field limiter subtends at the pupil.
Image Field Aspect Ratio (IFAR) = diameter of field limiter/ its distance from the eye
For Galiean telescopes, Field limiter is Ramsden disc
Ramdsen disc is small and “inside” telescope.
IFAR <<0.5 (usually)
For Keplerian systems field limiter is either a field lens or theocular lens
For Keplerian telescopes, IFAR = approximately 1
C, Field of View.
Determined by intersection of the Image Field cone (angle given by IFAR) and the EVD
plane FoV width = EVD. IFAR
D Field of fixation
Defines the area that may be foveated. Theory is the same as for field of view, except that the
reference point is the center of eye rotation rather than the center of the pupil.
Determining the IFAR for a field lens in a Keplerian syatem.
Illuminate the ocular lens (laser pointer) and project to a remote flat surface.
Measure the diameter
E Vergence amplification.
v2 = u1/M2 - d/M
V2 = U1M2 /(1-d M U1)
Implications for in-office testing.
Depth of focus .Effective amplitude of accommodation is reduced by approximately M2.
Depth of focus for near vision telescopes
Calculate far point and near point of the range of clear vision, if the patient had a simple
spectacle correction with an addition corresponding to the telescope’s viewing distance.
Divide this range by M2 to calculate the approximate depth of field with the telescope system.
III DESIGNING TELESCOPE SYSTEMS FOR INDIVIDUAL PATIENT NEEDS
A. Mounting telescopes in Spectacle Frames.
Use stenopaic apertures to determine PD and mounting angle.
Position stenopaic pinhole at telescope viewing point.
Have patient observe a distant object at eye level.
Measure the spectacle plane angle with a protractor and plumb line
This determines mounting angle for viewing along the axis.
B More advanced technologies
Zoom telescopes
Autofocus and auto convergence systems
Video telescopes and camcorders have some special advantages
Autofocus, easy optical and electronic zoom,
constant brightness, contrast enhancement,
image may be recorded.
C.
Choices for the Clinician
Magnification
Field and exit pupil size
Focus adjustability
Image quality
Coatings
Binocular/Monocular
Hand held/ mounted
Portability and ergonomics
Conspicuity and cosmesis
Durability and cost
INFORMATION WILL BE PROVIDED ON THE TECHNICAL PROPERTIES OF COMMONLY USED TELESCOPES.