Which design has a longer tube length?

Which design has a longer tube length?

• A. Keplerian
• B. Galilean
• C. Both
• D. Unknown

Answer: (A)

Tube length grows with how the lens focal lengths add up. In a Keplerian design both lenses are positive, so the distance between them for the focused-at-infinity setup is the sum of their focal lengths, which makes the instrument longer. In a Galilean design the eyepiece has a negative focal length, so the effective separation needed is reduced by the magnitude of that negative focal length, allowing the tube to be shorter. So, for the same basic setup, the Keplerian arrangement ends up with the longer tube.

Tube length grows with how the lens focal lengths add up. In a Keplerian design both lenses are positive, so the distance between them for the focused-at-infinity setup is the sum of their focal lengths, which makes the instrument longer. In a Galilean design the eyepiece has a negative focal length, so the effective separation needed is reduced by the magnitude of that negative focal length, allowing the tube to be shorter. So, for the same basic setup, the Keplerian arrangement ends up with the longer tube.