Reflecting Telescopes
Anna Edmonds
Refracting and reflecting telescopes are two kinds used commonly by amateurs to help us see objects more clearly in the night sky. The problem of blurred colors caused by the lenses of refracting ‘scopes, as we mentioned in the last Newsletter, prompted Isaac Newton to develop a reflecting ‘scope. His solution was a mirror which reflected the light rays equally, regardless of their wavelength (color). Because the glass of the mirror doesn’t refract the light, the colors aren’t broken into a prism.
With a mirror, the angle at which the beam of light strikes each place is reflected off it at exactly the same angle. In order to concentrate the light, the mirror ground down until the surface us concave. For all the reflected light to be focused, the concavity needs to be in the shape of a parabola. (Remember your geometry and the names for conic sections? a: circle; b: parabola)
Newton placed a second, flat mirror to deflect the light rays to a point at the side of the mirror to where he could see the object through an eyepiece. This design is called a Newtonian reflector.
Like glass lenses, the surface of the mirror must be completely free of irregularities. Newton used a tin-copper alloy to plate his mirror; today the plate is usually either silver or aluminum.
A parabolic shape and the proper surface present their own problems for telescope makers: Secondly, achieving a perfectly uniform parabola takes a lot of very careful work. Ed Ritchie designed and made his own grinder for our 27.5-inch ‘scope. But with its help, he took over a year to grind and figure the glass before he was satisfied. (To say nothing of the quantities of glass dust that had to be swept out of his basement and out of Gena’s kitchen.) Whatever metal is used for the surface, over time it develops irregularities with exposure to the air. Thus the surface then has to be renewed, involving disassembling the telescope and sending the mirror to a distant place for re-plating. Silver plating lasts about 6 months. Aluminum plating plus a second shield of plastic (such as is on our ‘scope) is more complicated and more expensive to do, but should last 20 years.
By being able to reflect the light either to the bottom of the ‘scope or away from it, the designers are able to attach heavy instruments like cameras without pulling it off balance.
In their desire to see better and farther into space, scientists are continuing to develope increasingly versatile telescopes, including those that search electromagnetic wavelengths other than the ones our eyes can see.