Download outline4339

I
Basic Theory of Telescopes.
A
Galilean (+/-) and Keplerian (+/+)
Basic formulae M = - Poc / Pobj
M = 1/ (1-d.Pobj )
d = 1/ Poc + 1/ 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 disk - 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 Disk 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 disk
Ramdsen disk 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
D
FoV width = EVD. IFAR
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.
III
Designing telscope systems to meet the needs of the individual patients
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.