Thanks, Chuck,
I appreciated this information about mirrors. Never really thought about
how they were made before, but it was interesting to learn about it.
It's this kind of information that makes this list so educating (and
entertaining).
-poo
At 14:25 2003-04-16, Chuck Norcutt wrote:
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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