Which telescope is typically shorter and lighter?

• A. Galilean
• B. Keplerian
• C. Both
• D. None

Answer: (A)

When thinking about telescope length and weight, the arrangement of the lenses matters most. A Galilean telescope uses a convex objective and a concave eyepiece. The tube length is roughly the difference of their focal lengths, not the sum. For the same magnification, if you set f_o = M f_e (with the eyepiece focal length negative for the Galilean case), the tube length becomes about (M − 1) times a focal-length unit.

In contrast, a Keplerian telescope uses two convex lenses, so the tube length is the sum of the focal lengths, about (M + 1) times that unit for the same magnification. That extra bit, plus needing both lenses to be reasonably large to keep aberrations in check, makes the Keplerian design longer and typically heavier.

So the Galilean arrangement is typically shorter and lighter for the same level of magnification. (Also, it produces an erect image, unlike the Keplerian, which inverts the image.)