Observatories and Instruments

Reflecting telescopes use mirrors to gather light.  A large concave mirror (the “objective” mirror) collects the light as it enters the tube of the telescope.  It then reflects the collected light onto a smaller mirror, which in turn reflects it into the eyepiece for magnification.

re·fract  (r-frkt)

Refracting telescopes use convex lenses to gather light.  The lenses refract (bend) the light to bring it into focus.  The lens is thicker in the center than it is at its edges.  This means that the light bends more at the edges, sending to toward a central focus point.  The point of focus is where the image is created.

The Yerkes 40-inch refractor is the largest refracting telescope in the world.

Cassegrains are reflectors.  The classic Cassegrain has a parabolic (concave) primary mirror and a hyperbolic (convex) secondary mirror.  As you can see from the diagram below, the mirrors are aligned with the focal point behind the primary mirror (outside the telescope tube, at the end of the tube).  The light is gathered through the open end of the tube on the left and bounced off the primary mirror onto the secondary mirror. The secondary mirror then bounces the light to the central focal point, through a hole in the primary mirror at the right.

Cassegrain Telescope

After some searching, I found the original description of the Cassegrain telescope in the 29 February 1672 issue of the Journal des Sçavans.* Here’s the .pdf, Cassegrain Telescope, 1672 Journal of Sçavans, courtesy of Gallica Bibliothèque Numérique.