69 × 5 × 5 cm

This is a simple demonstration telescope made with two lenses mounted in a cardboard mailing tube.

A Keplerian telescope uses a positive lens for the eyepiece. It produces an inverted image but has a wider field of view than Galileo's telescope invented two years previously in 1609.

Modern terrestrial telescopes are usually Keplerian with an extra lens to render the image upright.