LV - Distance Aids: Telescopes - Week 6 Flashcards
In what form are low vision aids telescopes typically prescribed?
Hand-held monoculars
Can low vision aids telescopes be binocular?
Yes, low mag ones can be spec mounted
What is the typical magnification range for distance telescopes?
1.5x to 8x
What does a distance telescope do?
Magnifies the apparent size of a distance object
What are the two component lenses of a distance telescope and what power do they have (+ or -)?
Objective lens
-positive and closest to the object
Eyepiece/ocular lens
-either positive or negative
What power do eyepiece lenses of keplerian and galilean telescopes have?
Positive - keplerian
Negative - galilean
When are telescopes considered afocal? Explain in terms of focal planes and what arrangement the rays have going in and out of the telescope.
Afocal when objective and eyepiece lenses are separated by a distance equal to the absolute values of their focal lengths
-eyepiece is positioned so that its front focal plane is coincident with the back focal plane of the objective
-parallel rays from infinity go in, parallel rays come out
What is Fe when telescopes are afocal?
0
What kind of magnification do telescopes do?
Angular magnification
What is the formula for magnification of an afocal telescope?
M = -Feyelens / Fobjective
What can telescopes focus on when focal and what kind of magnification is produced? Give three examples of these kinds of microscopes.
When focal, focussed at finite/near distance and magnification produced by relative distance magnification
-telemicroscope
-reading microscope
-surgical telescope
What is the absolute dioptric power of a galilean eyepiece lens vs the objective lens?
Eyepiece is negative and higher absolute power
Between keplerian and galilean telescopes, which provides an erect image? Use the formula for M to prove this.
Galilean
M = -Fe/Fo
-in galilean telescopes, Fe is -
-so M = -(-Fe)/Fo means a positive M
What is te formula for tube length of galilean telescopes. Is fe positive or negative and how does this compare to keplerian telescopes?
d = fe + fo
fe is negative, so d is shorter compared to keplerian telescopes of similar M and fo
Is the exit pupil of a galilean telescope virtual or real? Where is it located if real?
Virtual
What is the dioptric power of a kelerian eyepiece lens vs the objective lens?
Numerically stronger than the objective lens
How can an image from a keplerian telescope be made erect?
Prisms
Is the exit pupil of a keplerian telescope virtual or real? Where is it located if real?
Real and located behind the eyepiece outside the telescope and close to the patients eye
Which is lighter, keplerian or galilean telescopes? Explain why.
Galilean
-keplerian tend to have prism
Between keplerian and galilean telescopes, which provides higher magnification?
Keplerian
Between keplerian and galilean telescopes, which provides higher field of view? Explain why.
Keplerian
-exit pupil is outside the system and close to the patients entrance pupil
Between keplerian and galilean telescopes, which provides better image quality?
Keplerian
Between keplerian and galilean telescopes, which is more expensive?
Keplerian
What is field of view limited by in telescopes (2)?
Diameter of the objective lens
Vertex distance
Is it more desirable to have a large or small exit pupil for telescopes?
Large, allows more light to reach the patients eye and easier to align
What is the formula for exit pupil diameter?
Exit pupil diameter (mm) = objective lens diameter (mm) / M
What is the formula for eye relief?
Eye relief (mm) = d (mm) / M
How can you tell between keplerian and galilean telescopes by looking at it?
Hold it away from you, you will see a circle of light - the exit pupil
-if its within the telescope, it is galilean
–move the telescope side to side, makes it easier
What does the weight of a telescope typically depend on and how can it be minimised (2)?
Depends on the objective lens
-minimise with plastic lens and housing
How can aberrations in telescopes be minimised?
Using doublet lenses
How can loss of light by reflection be minimised in telescopes?
Antireflective coatings on lenses
What is retinal illuminance proportional to in a telescopic system?
Area of the exit pupil
What is the formula for reduction in brightness in telescopes?
exit pupil diameter^2 / patients pupil diamter^2
What is the formula for retinal illuminance?
Objective lens diameter x M
What is the approximation formula for the amount of accommodation required when viewing an object at a finite distance with a focal telescope?
A~(M^2 x L)
A - accommodation required
M - magnification
L - distance between the object and objective side of the telescope 1 / (distance in m)
What are three options if a patient uses a telescope for some near distance?
Patient must accommodate
Increase power of the objective lens by adding a near cap
Increase separation or tube lendgth (d)
What are two things that can be done if the patient needing the telescope has a significant refractive error?
Can be corrected by incorporating refractive error into the eyepiece or as an eyepiece cap
Alter tube length
-shorter tube for myopia, longer for hyperopia
When altering telescope tube length to correct for refractive error, explain what kind of magnification myopes and hyperopes can expect with both galilean and keplerian telescopes.
Myopic patients obtain less magnification with galilean telescopes and more with keplerian
Hyperopic patients obtain more magnification with galilean telescopes and less with keplerian
How should magnification be prescribed? Explain with the formula for M and give an example. What target VA is usually adequate for most distance tasks?
Prescribe the least amount of magnification to meet the patients goals
M = best VA / target VA
i.e. patient reads 6/30 and wants 6/12, M = 30/12 = 2.5x
6/12 is adequate for most patients
What is the most common form of prescribed telescopes for magnification and why?
Handheld as its easily concealed and doesnt interfere with mobility
When is spectacle-mounted telescope magnification appropriate?
Prolonged viewing
-television
Describe the comparison method for verifying the magnification of a telescope.
Spot an object with a repeating pattern, comparing the number of objects seen unaided with the number seen through the telescope
Describe the direct method for verifying the magnification of a telescope. What kind of telescope does this apply to?
M = objective lens diameter / exit pupil diameter
-keplerian
List the 6 components of instructions on using telescopes.
Localisation
-aligning eye, telescope, target
Focussing
Spotting
-locating an object large enough to be seen unaided and using the telescope to examine in detail
Tracing
following a stationary line (street curb)
Tracking
-following a moving object
Scanning
-finding and reading information that can not be seen without the aid of the telescope
