Telescopes 2

Angular subtense

Unlike spectacle magnifiers that rely on bringing objects closer, telescopes increase what

Working distance

Telescopes magnify the image without forcing the patient to change their what

Longer Working Distance

Which feature refers to the telescope that magnifies objects while keeping them far away?

Task-specific

Which feature refers to the telescope that is ideal for spotting bus numbers, reading music, or watching TV

Focusable

Which feature refers to the telescope that can be adjusted for both distance and near tasks

Restricted Field of View

Which feature refers to the telescope where it is dangerous to use while walking or mobile.

Dynamic Magnification

Which feature refers to the telescope where it magnifies the speed of moving objects, causing motion sickness or disorientation

Cost & Complexity

Which feature refers to the telescope where it is considered generaly expensive and very difficult to use binocularly

Afocal

Telescopes are essentially what type of systems.

Parallel

The rays of light entering from a distant object, and the rays leaving the telescope are what

Galilean Telescopes

Convex (+) objective lens and a Concave (-) eyepiece.

Negative eyepiece

In a Galilean telescope what intercepts the converging light before it reaches its focal point, diverging the rays so they exit parallel

Erect

In a Galilean telescope the image remains natively what

Keplerian telescope

Two Convex (+) lenses

Objective lens

The what converges light past its focal length . The eyepiece then converges those divergent rays back to parallel

Inverted

In a Keplerian telescope the image is what

Porro prisms or roof prisms

Fold the light path and flip the image right-side up

Galilean enlargement range

Lower (typically 1.5x 4.0x)

Galilean quality & field

Poorer edge quality, smaller field

Galilean size

Shorter & Lighter

Galilean complexity

Simpler (no prisms needed)

Keplerian enlargement range

Higher (typically 2.0x– 10.0x)

Keplerian quality & field

Better image quality, wider field

Keplerian size

Longer & Heavier (due to prisms)

Keplerian complexity

More complex

For myopes

They must shorten the tube. This changes the exiting light from parallel to divergent.

For hyperopes

They must lengthen the tube. This changes the exiting light from paralel to convergent.

Telemicroscopes

Are prescribed for one major reason: Working Distance. Will always provide a longer working distance than a simple spectacle magnifier of the exact same equivalent power.

Method 1: Eyepiece Correction

Placing a very strong plus lens over the eyepiece. Rarely used unless the patient already wears high-powered reading spectacles.

Method 2: Lengthening the Tub

Twisting the dial to extend the telescope barrel. Lengthening the tube to focus on near objects actualy increases the overal magnification for both Galilean and Astronomical telescopes.

Method 3: Objective Lens Caps

Placing a plus lens (reading cap) over the front lens. This neutralizes the divergence of the near object so the telescope remains perfectly afocal.

Exit pupil

Is the bright circle of light that leaves the eyepiece and enters the patient's eye. It is always smaler than the front lens.