Telescopes in low vision

Question 1

What is low vision?

Answer 1

VA that is less than 6/18 and equal to or better than 3/60 in the better eye with best corrected. Or visual fields less than 20 degrees in diameter

Question 2

Why telescopes for low vision?

Answer 2

they increase angular substance without requiring a change in distance. They are the only visual aid for distance magnification

Question 3

What is the objective lens?

Answer 3

closer to the object
positive (convex) lens
forms a real image of an object at infinity
longer focal length

Question 4

What is the eyepiece/ocular lens?

Answer 4

close to the eye
enlarges the image produced by the objective
shorter focal length
depending on the telescope it is positive or negative

Question 5

What are the two types of telescopes?

Answer 5

Keplerian
Galilean

Question 6

What are the features of a Keplerian telescope?

Answer 6

positive ocular - positive image focal length
telescope length - t = f’o + f’e
image inverted: M =-Fe/Fo < 0
real exit pupil (image space, converging rays)

Question 7

What are the features of a Galilean telescope?

Answer 7

negative ocular - negative image focal length
telescope length - t = f’o + (-f’e)
image upright: M= -Fe/Fo > 0
virtual exit pupil

Question 8

What is exit pupil (EP)?

Answer 8

the image of the object as seen through the eyepiece.
Keplerian: real EP, outside the instrument
Galilean: virtual EP, inside the instrument

Question 9

What is the field of view?

Answer 9

The field of view increases in proportion to the working distance of the object. the larger the EP the smaller the field of view.

Question 10

How can you get maximum field of view?

Answer 10

when the eye pupil = the exit pupil size and placed at the same position