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Many websites say this is because the effective focal length of the objective is increased by making the secondary mirror convex. This allows a Cassegrain telescope to be shorter than a similarly powered Newtonian.

What I don't understand is that if the focal length is made longer, how come the telescope doesn't have to be made longer too? Could someone explain how the focal length is increased and how this leads to shorter telescopes?

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Basically, the light has a longer way to go after hitting the secondary mirror in a Cassegrain than in a Newton telescope.

In a Newton telescope (top image), the light hits the primary mirror and goes through the telescope tube once. In a Cassegrain telescope (bottom image), the light hits the primary mirror and goes through the entire length of the tube twice.

The focal length of a telescope is the distance between the telescope's primary mirror and the point where the light rays come together in focus. This is how far the light has to go after hitting the primary mirror before reaching the eyepiece.

Since the light goes a longer distance within a Cassegrain telescope than a Newton telescope of the same length, the focal length of the primary mirror can be made longer. Conversely, for two telescopes with the same focal distance, the Cassegrain will be shorter than the Newton telescope.

You can imagine the Cassegrain telescope folding the focal length in two, whereas the Newton telescope leaves the focal length stretched out flat.

enter image description here

In this image, light has a longer path to go in the bottom (Cassegrain) telescope than in the top (Newton) one. Even though they are the same size, the bottom telescope has a longer focal length.


This would make it seem that a Cassegrain can have a focal length twice the length of the tube, whereas a Newton can have a focal length roughly the length of the tube. In fact, Cassegrain have an extra trick up their sleeve to increase the focal length, and can have a focal length equal to five times the length of the tube.

The secondary mirror of a Cassegrain telescope is convex, and has its own focal length. The effective focal length of the primary mirror + the secondary mirror can thus be made longer than that of the primary mirror alone.

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  • $\begingroup$ So if light travels a greater distance within the cassegrain, why is it shorter? $\endgroup$ – XXb8 Apr 20 at 8:45
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    $\begingroup$ @XXb8 If you take a cassegrain and a newton that are the same length, the light will travel a longer distance in the cassegrain. If you take a cassegrain and a newton that have the same focal distance, the cassegrain will be shorter. $\endgroup$ – usernumber Apr 20 at 8:50
  • $\begingroup$ I don't understand why. Maybe it's because of the misconception I have on what the 'focal length' actually is (is it the focal length of the concave or the convex mirror?) $\endgroup$ – XXb8 Apr 20 at 8:52
  • $\begingroup$ sorry I meant mirrors! I will edit it now $\endgroup$ – XXb8 Apr 20 at 8:53
  • $\begingroup$ @XXb8 I added an description of the focal length in the answer. Does this make things clearer? $\endgroup$ – usernumber Apr 20 at 8:53
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The Focal Length is not really the distance the light travels from the objective. The Focal Length is the diameter of the aperture divided by the tangent of the angle at which light rays from the edge of the objective converge at the telescopes focal point. The curved secondary mirror of a Cassegrain telescope decreases that angle, and thus increases the effective focal length.

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    $\begingroup$ this answer is more correct. the internal light path works in the simplest sense with a primary and flat mirrors. a more intuitive answer would have to involve how far from your eye a disk the diameter of the primary would to be in order to form the cone that the optical train of your telescope does produce (at the eyepiece). $\endgroup$ – tomc Apr 21 at 0:18
  • $\begingroup$ While this is certainly more correct, as an answer to the OP's question "Why are Cassegrain telescopes shorter than Newtonian reflecting telescopes?" it's still incomplete. $\endgroup$ – uhoh Apr 21 at 1:03

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