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Newtonian Reflectors

         This beautiful drawing is by Russell W. Porter, the architect of the 200" Palomar Telescope. Notice the astronomer sitting in the Prime Focus cage inside the telescope tube. The 200" mirror beneath him reflects the secondary diagonal mirror affixed to the bottom of the Prime Focus cage.



         All telescopes work on the general principle of collecting light and then magnifying the collected image. This magnification is accomplished either with an eyepiece (as in many amateur telescopes), or with millions of transistors on a silicon wafer called a CCD (as in professional or research instruments).  If the light is focused by a lens, the telescope is known as a refractor. Lenses of refracting telescopes start to deform due to the force of gravity when they exceed about 40" in diameter. Since modern research instruments start at that size to as much as ten times that diameter (the twin 10-meter Keck telescopes), they must therefore be built another way in order to gather, or to focus, their light.

         This type of telescope was invented by Sir Isaac Newton in 1668, hence its name: the Newtonian reflector. Shown below, light rays represented by blue lines enter the telescope at its open end on the left. The light hits a parabolic mirror at the end of the telescope tube, from whence it is reflected and converges to a focus back towards the opening of the tube.



         An automobile headlight is a more familiar application of a parabolic mirror. There light emitted at the focus from a headlight bulb strikes the mirror behind it and all the reflected rays emerge parallel to each other along the central axis of the mirror illuminating objects ahead.  A telescope mirror acts exactly the same way in reverse. Parallel rays of light coming from celestial objects essentially at infinity are focused by the telescope's parabolic back in the direction from whence they came. Just before they reach a focus within the tube, they are deflected 90 by a small, diagonal "secondary" mirror placed in their path. The deflected light finally reaches a focus at the eyepiece, which magnifies the focused image.

         The secondary mirror obstructs a small percentage of the incoming light, but is a far better solution then putting your head in the way of the incoming light to see the focused image. It is also responsible for the diffraction spikes that you see around bright stars in astronomical photographs.  This tells you that the image was made using a reflecting or catadioptric telescope, and is the result of the scattering of light around the four secondary (or primary focus) mirror support vanes that cross the path of the incoming light.  In the 200" Palomar Hale reflector, the prime focus mirror is in a cage so large (see drawing at top) that a man sits inside the telescope at its focal point without appreciable loss of light!  Note the four sets of orthogonal vanes holding the cage in place. These vanes give rise to the diffraction spikes around bright stars in all of the Hale telescope's photographs.


go to . . .

telescopes in general
refracting telescopes
compound- catadioptric telescopes
charge-coupled devices (CCDs)
adaptive optics





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Carl Woebcke: Newtonian Reflectors, 1991-2017. All rights reserved.