Jim asked:
I've already spoken my peace but I'm wondering if one of our astronomers
would describe how a front-surface mirror is made? How is it different
from a bathroom mirror?
--------------------------
With some rare exceptions, mirrors used in optical instruments are
"first surface" mirrors where the reflective coating is on the front of
the glass. This is so that light does not pass through the glass and
introduce problems of refraction, glass quality, etc. It is not
uncommon for telescope mirrors to have bubbles in the glass but, unless
they are within the curve ground into the glass, they do no harm since
the light does not pass through the glass. The front surface, however,
leaves the reflective coating relatively exposed to physical damage and,
in the case of silver, readily exposed to oxidation. Silver oxide is
black and undesirable for passing light. Therefore, when silver is used
it must be overcoated to protect the surface.
Most first surface mirrors use aluminum as the reflective surface.
Aluminum is deposited in a process called "sputtering". The glass to be
coated is placed in a vacuum chamber, the aluminum source is heated to a
high temperature to create a vapor above the glass and the aluminum
vapor falls onto, cools off, and coats the surface. The advantage of
aluminum is that, although it also oxidizes quickly like silver,
aluminum oxide is clear and tough as nails (check to see what your
grinding wheel is made of). This is a pretty robust surface but it is
microscopically thin and can be scratched through with something
abrasive. For an SLR though, the mirror is only used for viewing and
contributes nothing to the image on film. A few "sleeks" (fine
scratches) are of no consequence in practical usage.
For first surface mirrors the disadvantage of aluminum over silver is
that aluminum is not quite as reflective as silver. I don't recall the
precise values but is something like 93 vs 97 percent. In practice,
aluminum is superior which is why you would be hard pressed to find a
silver coated first surface mirror today.
Second surface mirrors like your typical bathroom mirror are coated on
the back, usually with silver. Silver is deposited on the glass out of
a chemical solution containing silver nitrate as the source of the
silver. Care must be taken as, if not done properly, the chemical
reactions might form silver fulminate which is an explosive. After the
glass is coated with silver a layer of paint is applied to keep the
silver from oxidizing. Some prisms are also made like this as are some,
if not all, of our OM prisms.
Chuck Norcutt
Woburn, Massachusetts, USA
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