LV: Telescopes!

These are a method of optical magnification which is more versatile than the plus lens.

• Distance, intermediate or near
• Hand held for spotting or spec mounted but rarely worn constantly/whilst mobile

General formula (for afocal telescopes) is M = -Fe / Fo and also t (tube thickness) = fe' + fo'

Two types of telescope - astronomical and galilean

Astronomical

• Both eyepiece and obj lens are +ve, this gives -ve magnification and an inverted image
• Both focal lengths positive so the telescope is long
• Prism erecting system needed too
• Heavier, longer and more expensive than galilean
• Larger field of view and better image quality though
• Exit pupil outside system
• Mag range from low - high. Up to 10x
  

Galilean

• Eyepiece lens is negative and objective lens positive. +ve mag so erect image
• One focal length is negative so the system is shorter
• Lighter, shorter and cheaper than astronomical
• Poorer image quality and small field of view though
• Low magnification only - 4x maximum
• Preferred for spec mounted due to lower weight and shorter length

Labelling

• Magnification 2.2x, 3x etc
• Weight in grams
• Visual field 250m/1000m linear OR eg 12.5 degrees (200 degrees without scope)
• Working distance 45-200cm
• Sometimes written 8 x 40 where first number is mag and second number is diameter of objective lens in mm

Exit Pupil

• This is the image of the objective lens seen through the eyepiece. All of the rays entering the objective lens pass through the exit pupil.
• The exit pupil defines the field and the amount of illumination. These can be determined by direct measurement or calculation. Ideally the patient's pupil should be exactly the same size
• Astros have EP behind Fe so patient's eye can get close to it, with Gal the exit pupil is within the telescope so there's a greater distance between it and the eye

diam of exit pupil = diam of obj lens / magnification of telescope

eg size and location of the exit pupil for a 3x24 gal telescope with an Fo of +10.00D

Fe = -3x10.00 = -30.00

1 / l' = 1 / l + F = 1 / -0.0667 + (-30) = -45

1 / -45 = -22.22cm SO EXIT PUPIL IS 22.22cm IN FRONT OF THE EYEPIECE

24/3 = 8mm SO EXIT PUPIL IS 8mm IN DIAMETER

Factors Affecting Field of View

• 1/Mag - use minimum power - use min until px becomes practiced
• 1/vertex dist - 5mm = smallest dist w/o specs, 20mm = w/specs & eyecup
• Object distance
• Object diameter - but telescope becomes heavier and more difficult to handle

Matching the size of the exit pupil to the px's pupil optimises the field BUT if alignment slips even a tiny bit part of the field will go dark, so in practice it's actually better if the exit pupil is greater than the patient's pupil despite some loss of field as a result - the misalignment is more easily tolerable. Better to measure in practice than calculate as more closely related to px experience.