Questions on mirrors.

It seems to me that a glass mirror with a metal coating on the back
surface tends to reflect better than a strip of aluminum or aluminum
foil.

Telescope mirrors, however, are coated with aluminum or silver on the
front surface and not the back, to avoid refraction within the glass
or other material, and to allow the optical qualities of the interior
of the reflector support, to be much poorer than that of optical
glass.

The question that I have is this. Does the reflective qualities of
mirrors with the silvering surface on the back of the glass in regular
mirrors, have anything to do with the index of refraction of the
transparent material at the surface of the metal?

Would an optical quality, reflective sheet of aluminum immersed in oil
or in some other transparent material reflect better than one exposed
to air?

Or is the entire reason why mirrors have metal coatings on the back
surface to protect the metal from tarnishing?

Did they used to coat polished metal or foil with oil, and then
overlay the oiled foil with glass before they were able to coat
mirrors with metal back in somewhat earlier time periods?

What is the best way to clean dust from a telescope mirror, without
potentially introducing scratches into the aluminum or silvered
surface?

Gulliver wrote:

It seems to me that a glass mirror with a metal coating on the back
surface tends to reflect better than a strip of aluminum or aluminum
foil.

No. You get a double image from surface reflection, the seven optical
aberrations, EM absorption by the intervening medium, and scattering
by optical inhomogeneities including strain, birefingence,
particulates, bubbles... A first class mirror is a first surface
mirror, or an anti-reflection dielectric-coated first surface mirror,
or a multi-layer dielectric mirror on an inert backing.

Telescope mirrors, however, are coated with aluminum or silver on the
front surface and not the back, to avoid refraction within the glass
or other material, and to allow the optical qualities of the interior
of the reflector support, to be much poorer than that of optical
glass.

To maintain the figure of the mirror you want zero coefficient of
thermal expansion. Zerodur and titanium silicate (Corning ULE) are
not clear, colorless, and optically homogeneous. To have a usable
mirror at all it must be both stiff and lightweight - honeycomb - or a
dynamically supported membrane. No matter how you slice it, it must
be first surface mirror.

The question that I have is this. Does the reflective qualities of
mirrors with the silvering surface on the back of the glass in regular
mirrors, have anything to do with the index of refraction of the
transparent material at the surface of the metal?

Sure. First surface reflection is controlled by refractive index.
Chromatic aberration is controlled by Abbe number. You would want the
smallest possible refractive index with the largest possible Abbe
number. You would want a vacuum intervening - a first surface mirror.

Would an optical quality, reflective sheet of aluminum immersed in oil
or in some other transparent material reflect better than one exposed
to air?

Read up on dielectric anti-reflection and forced-reflection coatings.

Or is the entire reason why mirrors have metal coatings on the back
surface to protect the metal from tarnishing?

Did they used to coat polished metal or foil with oil, and then
overlay the oiled foil with glass before they were able to coat
mirrors with metal back in somewhat earlier time periods?

What is the best way to clean dust from a telescope mirror, without
potentially introducing scratches into the aluminum or silvered
surface?

All this info has flooded the world. Look up amateur telescope sites.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!

In article , (Gulliver) writes:
It seems to me that a glass mirror with a metal coating on the back
surface tends to reflect better than a strip of aluminum or aluminum
foil.

A matter of smoothness. Coat the *front* of a smooth piece of glass
with a thin metal layer and it'll reflect even better than the one at
the back.


Telescope mirrors, however, are coated with aluminum or silver on the
front surface and not the back, to avoid refraction within the glass
or other material, and to allow the optical qualities of the interior
of the reflector support, to be much poorer than that of optical
glass.

The question that I have is this. Does the reflective qualities of
mirrors with the silvering surface on the back of the glass in regular
mirrors, have anything to do with the index of refraction of the
transparent material at the surface of the metal?

No.

Would an optical quality, reflective sheet of aluminum immersed in oil
or in some other transparent material reflect better than one exposed
to air?

Or in vacuum. yes.

Or is the entire reason why mirrors have metal coatings on the back
surface to protect the metal from tarnishing?

This and mechanical protection (from scratches and the like).

Did they used to coat polished metal or foil with oil, and then
overlay the oiled foil with glass before they were able to coat
mirrors with metal back in somewhat earlier time periods?

I don't know, but this is quite possible.

What is the best way to clean dust from a telescope mirror, without
potentially introducing scratches into the aluminum or silvered
surface?

Gently. Air jet, at not to high pressure. Preferably in a clean
room.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

"Gulliver" wrote in message
m...
What is the best way to clean dust from a telescope mirror, without
potentially introducing scratches into the aluminum or silvered
surface?

Buy a can of compressed air from a camera shop or Radio Shack.
Be careful to hold it upright because the propellant can mess up
an antireflective coating.

(Gulliver) wrote:

Others have answered your questions but I have a related question that
the experts might be able to help with. I hope you don't mind if I ask
it here.

When using a sextant on land (an odd thing to do nowadays since they
cost far more than a gps unit) one uses an artificial horizon in place
of the sea horizon. This is a small pool of water or mercury with a
couple of pieces of glass arranged in an A frame overtop to keep the
wind from disturbing the liquid. Water doesn't reflect very well and
mercury is hard to get these days (and hard to keep clean as well) so
it's tough to do star sights. Does anyone know of a liquid (or
something to form a film on top of a liquid) that might do better than
a simple pool of water?

Ken Muldrew

(remove all letters after y in the alphabet)

In sci.physics Ken Muldrew wrote:
(Gulliver) wrote:

Others have answered your questions but I have a related question that
the experts might be able to help with. I hope you don't mind if I ask
it here.

When using a sextant on land (an odd thing to do nowadays since they
cost far more than a gps unit) one uses an artificial horizon in place
of the sea horizon. This is a small pool of water or mercury with a
couple of pieces of glass arranged in an A frame overtop to keep the
wind from disturbing the liquid. Water doesn't reflect very well and
mercury is hard to get these days (and hard to keep clean as well) so
it's tough to do star sights. Does anyone know of a liquid (or
something to form a film on top of a liquid) that might do better than
a simple pool of water?

Indium/gallium/tin eutectic melts around 10.7C.
Probably not much use.

"Gulliver" wrote in message
m...
It seems to me that a glass mirror with a metal coating on the back
surface tends to reflect better than a strip of aluminum or aluminum
foil.

Telescope mirrors, however, are coated with aluminum or silver on the
front surface and not the back, to avoid refraction within the glass
or other material, and to allow the optical qualities of the interior
of the reflector support, to be much poorer than that of optical
glass.

The question that I have is this. Does the reflective qualities of
mirrors with the silvering surface on the back of the glass in regular
mirrors, have anything to do with the index of refraction of the
transparent material at the surface of the metal?

Yes.

Would an optical quality, reflective sheet of aluminum immersed in oil
or in some other transparent material reflect better than one exposed
to air?

You would have problems with surface tension, eddy currents in the medium
etc. However, if you had a liquid with the same index of refaction and
similar dispirsive properties, you could form a decnet mirror (but on that
would point only up).



Or is the entire reason why mirrors have metal coatings on the back
surface to protect the metal from tarnishing?

No. There are also problems of heat load (heat transfer) to the mirror, and
quite a few other optical effects that are cured (or created) with a front
film mirror.

Did they used to coat polished metal or foil with oil, and then
overlay the oiled foil with glass before they were able to coat
mirrors with metal back in somewhat earlier time periods?

That is a historical question, that can be answered with a google search.


What is the best way to clean dust from a telescope mirror, without
potentially introducing scratches into the aluminum or silvered
surface?

Very carefully.

Ian Stirling wrote:

In sci.physics Ken Muldrew wrote:

When using a sextant on land (an odd thing to do nowadays since they
cost far more than a gps unit) one uses an artificial horizon in place
of the sea horizon. This is a small pool of water or mercury with a
couple of pieces of glass arranged in an A frame overtop to keep the
wind from disturbing the liquid. Water doesn't reflect very well and
mercury is hard to get these days (and hard to keep clean as well) so
it's tough to do star sights. Does anyone know of a liquid (or
something to form a film on top of a liquid) that might do better than
a simple pool of water?

Indium/gallium/tin eutectic melts around 10.7C.
Probably not much use.

Yeah, I think mercury would be much better. On the other hand, I
wonder if one could obtain a plastic that is light enough to float on
water with perfectly parallel sides and just silver the top. That
might do the trick.

Ken Muldrew

(remove all letters after y in the alphabet)

In sci.physics Ken Muldrew wrote:
Ian Stirling wrote:

In sci.physics Ken Muldrew wrote:

When using a sextant on land (an odd thing to do nowadays since they
cost far more than a gps unit) one uses an artificial horizon in place
of the sea horizon. This is a small pool of water or mercury with a
couple of pieces of glass arranged in an A frame overtop to keep the
wind from disturbing the liquid. Water doesn't reflect very well and
mercury is hard to get these days (and hard to keep clean as well) so
it's tough to do star sights. Does anyone know of a liquid (or
something to form a film on top of a liquid) that might do better than
a simple pool of water?

Indium/gallium/tin eutectic melts around 10.7C.
Probably not much use.

Yeah, I think mercury would be much better. On the other hand, I
wonder if one could obtain a plastic that is light enough to float on
water with perfectly parallel sides and just silver the top. That
might do the trick.

Polarising films might boost the reflection a bit, I suppose. (well,
cut down on irritating reflections.
Na/K will be easily molten enough.
You'd need to put it in a sealed vial, with an inert atmosphere, as otherwise
it'll burst into flame.
(Hmm, maybe self-illumination could be a feature.)

"Ed Keane III" wrote in message ...
"Gulliver" wrote in message
m...
What is the best way to clean dust from a telescope mirror, without
potentially introducing scratches into the aluminum or silvered
surface?

Buy a can of compressed air from a camera shop or Radio Shack.
Be careful to hold it upright because the propellant can mess up
an antireflective coating.

Since when does any telescope mirror have an antireflective coating,
and for what possible reason? Are you possibly confusing telescope
mirrors with camera lenses?

I normally use a soft camel hair brush to remove dust from my
telescope mirror, because a blast of compressed air tends to attract
more dust particles than it removes. For more serious cleaning of
surface films, I generally use a 50/50 mixture of CP ethanol and
distilled water, followed by a rinse with a freon based solvent.

Harry C.