TOPIC 13: TELESCOPES

Question 1

What are the 2 types of telescopes and how to calculate their magnification

Answer 1

1. Astronomical (2 convex (plus) lenses)
2. Galilean (convex Fo (objective lens) and a concave Fe (eyepiece lens)

Magnification (M) = -Fe / Fo
Fe= eye piece lens power
Fo= objective lens power
NOTE: For Astronomical: the resultant Magnification, M is NEGATIVE, indicating image is INVERTED

For Galilean: the resultant Magnifiation, M is POSITIVE, indicating image is UPRIGHT

Question 2

How is the magnification affected if an ametropic patient uses the telescope without refractive correction?

Answer 2

Since Magnification (M) = - Fe/ Fo ,

An uncorrected hyperope who borrows the (+) Fe in an Astronomical telescope will cause the overall Magnification of the telescope to be reduced.

An uncorrected myope gives back the (+) Fe in an Astronomical telescope will cause the overall Magnification of the telescope to be increased.

E.g. A hyperopic client has a refractive error of 5D
He uses an Astronomical telescope with Fe = +20D and Fo = +10D

If he doesn’t wear his SVD, he borrows 5D from the Fe lens. The ‘new’ Fe is now 20D-5D=15D.

Question 3

Compare and contrast astronomical and Galilean telescopes (magnification, overall length, cost, weight, optical quality, field of view, exit pupil)

Answer 3

magnification:
The Astronomical design is available in a larger range of magnifications (2.75x to 14x) but the Galilean is only available at up to 3-4x for distance use, and up to 8x for near use.

overall length:
For the same magnification, the Galilean has shorter overall length, although folding of the light path can make it difficult to tell.

cost/weight:
Astronomical telescopes are more complex optical systems, are more expensive and heavier

optical quality:
Astronomical telescopes have a better image quality than galilaen telescopes

field of view/exit pupil:
Astronomical telescopes have larger field of view as the user’s pupil is closer to the telescope’s exit pupil than galilean telescope

Question 4

How to calculate size of the exit pupil?

Answer 4

Exit pupil diameter (mm) = diameter of objective lens (mm) / magnification

smaller exit pupil=dimmer image

Question 5

How to read the markings on a telescope (i.e. 2.8 X 9 220/1000m 12,5 deg)

Answer 5

markings on a telescope (i.e. 2.8 X 9 220/1000m 12,5 deg)
The 1st number = magnification
The 2nd number = diameter of objective lens
12.5 degrees refers to the field of view (220m at 1000m)

in this case 2.8x magnification and 9mm diameter obj lens

Question 6

What are the advantages and disadvantages of near telescope vs plus lens magnifiers

Answer 6

Advantage:
Allows greater working distance between the object and the client’s eye. This is especially useful if some distance is necessary between the client and the object (e.g.writing, painting, for safety reasons)

Disadvantage:
The field of view with a telescope is much smaller and this can limit the practicality of using telescopes for near tasks (especially craft work where it is necessary to manipulate the object)

Question 7

What are the advantages and disadvantages of telescopes in general?

Answer 7

advantages:
1. Magnification achieved at a LONGER working distance
2. Many can be focused to give both distance and near magnification – theoretically yes, but hard for patient to manipulate the focusing

disadvantages:
1. NARROW field of view – challenges in finding target
2. SHALLOW depth of field – cannot perform manipulative tasks such as sewing
3. Generally more EXPENSIVE compared to plus lens magnifiers
4. BINOCULARITY is difficult to achieve – for near work, the tubes need to be converged
5. REDUCED light gathering, NOT so suitable for patient with low contrast sensitivity