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OD3 (S1&2) > LV - Distance Aids: Telescopes - Week 6 > Flashcards

LV - Distance Aids: Telescopes - Week 6 Flashcards

1
Q

In what form are low vision aids telescopes typically prescribed?

A

Hand-held monoculars

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2
Q

Can low vision aids telescopes be binocular?

A

Yes, low mag ones can be spec mounted

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3
Q

What is the typical magnification range for distance telescopes?

A

1.5x to 8x

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4
Q

What does a distance telescope do?

A

Magnifies the apparent size of a distance object

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5
Q

What are the two component lenses of a distance telescope and what power do they have (+ or -)?

A

Objective lens
-positive and closest to the object
Eyepiece/ocular lens
-either positive or negative

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6
Q

What power do eyepiece lenses of keplerian and galilean telescopes have?

A

Positive - keplerian
Negative - galilean

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7
Q

When are telescopes considered afocal? Explain in terms of focal planes and what arrangement the rays have going in and out of the telescope.

A

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

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8
Q

What is Fe when telescopes are afocal?

A

0

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9
Q

What kind of magnification do telescopes do?

A

Angular magnification

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10
Q

What is the formula for magnification of an afocal telescope?

A

M = -Feyelens / Fobjective

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11
Q

What can telescopes focus on when focal and what kind of magnification is produced? Give three examples of these kinds of microscopes.

A

When focal, focussed at finite/near distance and magnification produced by relative distance magnification
-telemicroscope
-reading microscope
-surgical telescope

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12
Q

What is the absolute dioptric power of a galilean eyepiece lens vs the objective lens?

A

Eyepiece is negative and higher absolute power

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13
Q

Between keplerian and galilean telescopes, which provides an erect image? Use the formula for M to prove this.

A

Galilean
M = -Fe/Fo
-in galilean telescopes, Fe is -
-so M = -(-Fe)/Fo means a positive M

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14
Q

What is te formula for tube length of galilean telescopes. Is fe positive or negative and how does this compare to keplerian telescopes?

A

d = fe + fo
fe is negative, so d is shorter compared to keplerian telescopes of similar M and fo

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15
Q

Is the exit pupil of a galilean telescope virtual or real? Where is it located if real?

A

Virtual

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16
Q

What is the dioptric power of a kelerian eyepiece lens vs the objective lens?

A

Numerically stronger than the objective lens

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17
Q

How can an image from a keplerian telescope be made erect?

A

Prisms

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18
Q

Is the exit pupil of a keplerian telescope virtual or real? Where is it located if real?

A

Real and located behind the eyepiece outside the telescope and close to the patients eye

19
Q

Which is lighter, keplerian or galilean telescopes? Explain why.

A

Galilean
-keplerian tend to have prism

20
Q

Between keplerian and galilean telescopes, which provides higher magnification?

A

Keplerian

21
Q

Between keplerian and galilean telescopes, which provides higher field of view? Explain why.

A

Keplerian
-exit pupil is outside the system and close to the patients entrance pupil

22
Q

Between keplerian and galilean telescopes, which provides better image quality?

A

Keplerian

23
Q

Between keplerian and galilean telescopes, which is more expensive?

A

Keplerian

24
Q

What is field of view limited by in telescopes (2)?

A

Diameter of the objective lens
Vertex distance

25
Q

Is it more desirable to have a large or small exit pupil for telescopes?

A

Large, allows more light to reach the patients eye and easier to align

26
Q

What is the formula for exit pupil diameter?

A

Exit pupil diameter (mm) = objective lens diameter (mm) / M

27
Q

What is the formula for eye relief?

A

Eye relief (mm) = d (mm) / M

28
Q

How can you tell between keplerian and galilean telescopes by looking at it?

A

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

29
Q

What does the weight of a telescope typically depend on and how can it be minimised (2)?

A

Depends on the objective lens
-minimise with plastic lens and housing

30
Q

How can aberrations in telescopes be minimised?

A

Using doublet lenses

31
Q

How can loss of light by reflection be minimised in telescopes?

A

Antireflective coatings on lenses

32
Q

What is retinal illuminance proportional to in a telescopic system?

A

Area of the exit pupil

33
Q

What is the formula for reduction in brightness in telescopes?

A

exit pupil diameter^2 / patients pupil diamter^2

34
Q

What is the formula for retinal illuminance?

A

Objective lens diameter x M

35
Q

What is the approximation formula for the amount of accommodation required when viewing an object at a finite distance with a focal telescope?

A

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)

36
Q

What are three options if a patient uses a telescope for some near distance?

A

Patient must accommodate
Increase power of the objective lens by adding a near cap
Increase separation or tube lendgth (d)

37
Q

What are two things that can be done if the patient needing the telescope has a significant refractive error?

A

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

38
Q

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.

A

Myopic patients obtain less magnification with galilean telescopes and more with keplerian
Hyperopic patients obtain more magnification with galilean telescopes and less with keplerian

39
Q

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?

A

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

40
Q

What is the most common form of prescribed telescopes for magnification and why?

A

Handheld as its easily concealed and doesnt interfere with mobility

41
Q

When is spectacle-mounted telescope magnification appropriate?

A

Prolonged viewing
-television

42
Q

Describe the comparison method for verifying the magnification of a telescope.

A

Spot an object with a repeating pattern, comparing the number of objects seen unaided with the number seen through the telescope

43
Q

Describe the direct method for verifying the magnification of a telescope. What kind of telescope does this apply to?

A

M = objective lens diameter / exit pupil diameter
-keplerian

44
Q

List the 6 components of instructions on using telescopes.

A

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

OD3 (S1&2) (114 decks)

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