The Afocal Telescope: Two Lenses, No Net Focusing

1 · Predict

An afocal (Keplerian) telescope places its objective and eyepiece so their focal points coincide — parallel light in, parallel light out. Does a shorter eyepiece focal length give higher or lower telescope magnification?

• Higher — magnification is the ratio of objective to eyepiece focal length, so shrinking the eyepiece's focal length increases the ratio.
• A shorter eyepiece gives lower magnification.
• Eyepiece focal length doesn't affect magnification.

2 · Set Up

1. Open the telescope preset and press Reset. The objective has f = 100 cm; the two lenses are separated by their combined focal lengths (afocal condition).
2. Enable the angular-magnification readout.
3. Set the eyepiece's focal length for each trial (objective fixed at f = 100 cm) and record the resulting magnification.

3 · Collect Data

Plot M (y-axis) against 1/f_e (x-axis) for your three trials.

4 · Analyze

1. For one trial, compute M = −f_o/f_e using f_o = 100 cm. Compare to the table.
2. Explain why the magnification is negative (an inverted image, typical of a simple Keplerian telescope) and why a shorter eyepiece focal length increases the magnitude of M.

5 · Extend

1. Unlike the eyepiece (which sets magnification), the OBJECTIVE lens's diameter (not shown in this focal-length-only model) determines how much light the telescope gathers and its ultimate resolving power. Explain why astronomers care more about a telescope's aperture (objective diameter) than its magnification alone.
2. 'Afocal' means parallel light in produces parallel light out — the telescope has no single overall focal length itself, unlike a camera lens or magnifying glass. Explain why this design is ideal for viewing very distant objects (effectively at infinity) with a relaxed eye.

The Physics Behind This Experiment

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