Cassegrain Telescopes

The original Cassegrain reflector design was attributed to Laurent Cassegrain of France in year 1672. The Cassegrain based telescopes design have a concave (shaped inwards) primary mirror (that “Zooms in” or magnify the image) converging the light rays with a focal ratio of f/2 to f/2.5 and a convex (shaped outwards) secondary mirror with a focal ratio of f/4 to f/5. The secondary mirror “zooms out” the image diverging the light rays to increase the telescope’s effective focal length just like cars’ side mirrors with the “Objects in mirror are closer than they appear” warning do. Both primary and secondary mirrors are squared facing each other frontally. The secondary mirror reflects the concentrated light from the primary mirror perpendicularly through a hole in the same primary mirror and the focal plane is behind of it. Cassegrain reflector telescope variants that are commercially available are the Schmidt-Cassegrain, Maksutov-Cassegrain, Ritchey-Chretien, Corrected Dall-Kirkham (CDK) and the Classical Cassegrain.