Keplerian Telescope: Angular Magnification

Theory

An objective ($f_1$) and an eyepiece ($f_2$) separated by $f_1+f_2$ form an afocal system: a parallel beam comes out parallel, narrower and inverted. The angular magnification is $M=-\frac{f_1}{f_2}$ — here $-4$.

What to observe

• The beam leaving the eyepiece is 4× narrower and flipped.
• Both lenses share the same focal point between them.

Try this

• Rotate the light source slightly: the output beam tilts 4× more.
• Use f₂ = −1 m (Galilean telescope) at distance f₁ + f₂.

Also available in: Luneta astronômica (Kepler): aumento angular (Português) · Telescopio kepleriano: aumento angular (Español)