silver coating on mirrors

Has anyone ever had any problems with a silver + overcoated mirror?
Am thinking of getting my 12" done plus secondary. Is it worth the extra
costs over Al/Si coatings?
TIA

Jim

Hi Jim,
I do my own coatings with our vacuum systems.

I have a couple mirrors that are over 3 years old and the silver is
still perfect. I give it a multi-overcoat of ZrO2 and SiO2 for better
protection. For the most part, the differnce will not be easily
noticed over Aluminum. Planetary and double stars observations won't
be aided. DSOs may be better. Overall transmission with silver will be
about 94% and with aluminum about 83%.


Al M


Jim Barclay wrote in message ...
Has anyone ever had any problems with a silver + overcoated mirror?
Am thinking of getting my 12" done plus secondary. Is it worth the extra
costs over Al/Si coatings?
TIA

Jim

Jim Barclay wrote:
Has anyone ever had any problems with a silver + overcoated mirror?
Am thinking of getting my 12" done plus secondary. Is it worth the extra
costs over Al/Si coatings?
TIA


Hi Jim,

I have several of my own telescope mirrors (primary and secondary) with
enhanced silver coatings. They use TiO2 and SiO2 over silver. I am
strongly considering gradually changing over the coatings of our secondary
mirror line to enhanced silver. The only hesitation I have is due to
marketing reasons; not their performance. Back in the 70's and 80's, some
silver coatings with poorly applied overcoats were tried out by a couple of
vendors. Some people (understandably) have long memories.

Silver and Aluminum are approximately equally reactive (i.e., oxidize with
about the same tendency). However, aluminum turns a translucent whitish in
color after oxidizing, while silver tarnishes to a dark color. An aluminum
coating has to get very bad indeed before you'll see a problem with your
eye, whereas a bad silver coating will get no mercy. To me this is a plus,
but it obviously puts a high priority on applying dense dielectric
overcoating layers.

Silver does produce a modestly higher peak reflectivity of around 98%, but
to me a more important advantage is the broader reflectivity over the whole
visible spectrum. It maintains reflectivities in the mid to high 90% range
through all of the visible spectrum. (Enhanced aluminum drops down into the
80% range at both ends of the visible spectrum.)

As to your specific question, I can only say that the quality of the
overcoat has everything to do with the durability of your silver coating.
TiO2 and SiO2 -- when properly applied -- is a very effective protection
layer for the metal under it. These materials are very hard, but the SiO2
in particular can be easily applied wrong. SiO is easier to deposit, but
isn't as durable.

Sincerely,
Bryan Greer
Worthington, OH

On Wed, 23 Jul 2003 16:36:38 GMT, "Bryan Greer"
wrote:


Hi Bryan,

As to your specific question, I can only say that the quality of the
overcoat has everything to do with the durability of your silver coating.
TiO2 and SiO2 -- when properly applied -- is a very effective protection
layer for the metal under it. These materials are very hard, but the SiO2
in particular can be easily applied wrong. SiO is easier to deposit, but
isn't as durable.

I've always wondered if the scatter would be the same for either
metal. Obviously if one chemically silvers their own mirror and then
burnishes it as I used to do, the scatter will be higher than Al, but
evaporating eliminates that step and would seem to produce
scatter not worse than Al.

Dan

Dan Chaffee wrote:
I've always wondered if the scatter would be the same for either
metal. Obviously if one chemically silvers their own mirror and then
burnishes it as I used to do, the scatter will be higher than Al, but
evaporating eliminates that step and would seem to produce
scatter not worse than Al.

Dan


I've wondered this too, because silver has a higher melting temperature (960
C) than aluminum (660 C). At least for the dielectric materials, the
melting point has much to do with how easily it forms a smooth, dense, film.
However, according to Ronald Willey (prof at Univ. of Rochester) on page 167
of his book "Practical Design and Production of Optical Thin Films":

"...silver, like aluminum, gives a better reflectance if deposited as
rapidly as possible, and on substrates that are not hot. (This can be
attributed) to greater agglomeration at high temperatures and slow rates
which in turn causes greater absorption."

Agglomeration is the clumpiness that sometimes happens during deposition of
metals. Obviously, it is bad.

In the same section of the book, he makes reference to three Applied Optics
journal articles by J.M. Bennett and Chang Hwangbo in regards to scattering
and microstructure of both aluminum and silver coatings with IAD overcoats.
I need to order those papers. It might save me a lot of time and money.

Sincerely,
Bryan Greer
Worthington, OH

I wrote:
Agglomeration is the clumpiness that sometimes happens during
deposition of metals. Obviously, it is bad.


Re-reading my last post, I'm not sure I made it clear the point of this
citation. The point was that (apparently) the prescription for aluminum and
silver deposition is the same. This at least suggests they have similar
properties. Again, I need to dig into those other citations to learn more.

Sincerely,
Bryan Greer
Worthington, OH

Hi,
ZrO2 is rarely used by coaters. It takes quite a bit of power to get
it going and its decomposition in an E-beam is on par with SiO2. I
usually run the coatings very slowly and after pumping down to better
than 6X10-6 Torr. Our thickness measurements are in the accuracy of
1-2%.

When using silver, I feel that a multilayer overcoat is critical for
protection.

It took us about a year to get our mirror coatings down. There's no
money in small mirror coating operations, though we've done several
hundred mirrors for one manufacturer.

We may be able to acquire a Balzers 48" Box coater in the near future.
This unit has optical monitoring and ion-assist with 3 E-beam sources.

Al M


(Dan Chaffee) wrote in message ...
On Wed, 23 Jul 2003 16:36:38 GMT, "Bryan Greer"
wrote:


Hi Bryan,

As to your specific question, I can only say that the quality of the
overcoat has everything to do with the durability of your silver coating.
TiO2 and SiO2 -- when properly applied -- is a very effective protection
layer for the metal under it. These materials are very hard, but the SiO2
in particular can be easily applied wrong. SiO is easier to deposit, but
isn't as durable.

I've always wondered if the scatter would be the same for either
metal. Obviously if one chemically silvers their own mirror and then
burnishes it as I used to do, the scatter will be higher than Al, but
evaporating eliminates that step and would seem to produce
scatter not worse than Al.

Dan

Bryan:

This is an awesome topic as I have wondered about the scatter
characteristics of silver for quite a while. I did find a paper that
somewhat addressed this -
"O-1 Technology Demonstration of Silicon Carbide Polishing."

September 2001
Contract #1205103
SSG Precision Optronics
65 Jonspin Road
Wilmington, MA 01810

Basically the article demonstrates that in NASA-produced
high-precision space-based optics, bare SiC flats polished using
optimized polishing processes resulted in a 10x improvement in many
parameters including reductions in stray light from scatter. As
support, BRDF measurements made before and after Denton FSS-99 silver
coating on the optimized flats demonstrated that there was no
significant degradation in stray light performance for either bare RB
SiC or Si clad RB SiC optics from the Denton coating.

So I guess to summarize the summary - the coating didn't really
contribute to scattering.

- Shannon Mann

Shannon Mann wrote:
This is an awesome topic as I have wondered about the scatter
characteristics of silver for quite a while. I did find a paper that
somewhat addressed this -
"O-1 Technology Demonstration of Silicon Carbide Polishing."

September 2001
Contract #1205103
SSG Precision Optronics
65 Jonspin Road
Wilmington, MA 01810


Do you know what journal this was in, or is it just an internal paper by
SSG?


Basically the article demonstrates that in NASA-produced
high-precision space-based optics, bare SiC flats polished using
optimized polishing processes resulted in a 10x improvement in many
parameters including reductions in stray light from scatter. As
support, BRDF measurements made before and after Denton FSS-99 silver
coating on the optimized flats demonstrated that there was no
significant degradation in stray light performance for either bare RB
SiC or Si clad RB SiC optics from the Denton coating.

So I guess to summarize the summary - the coating didn't really
contribute to scattering.


I'd like to read this to see exactly what they were trying to achieve. What
would be considered objectionable scatter for one application might not be
the same for another. Also, the final roughness of the deposited metal
layer is at least partially determined by how smooth the bare substrate is
(one big reason quartz makes a good substrate for laser applications). The
metal layer will exaggerate any surface roughness on the substrate's
surface. Since they are applying these coatings to super-smooth SiC, I'm
not sure how applicable it is to other substrate materials like Pyrex, etc.
which presumably are rougher. In any case, it would be interesting to see
what they learned. Thanks for the reference.

Sincerely,
Bryan Greer
Worthington, OH

We may be able to acquire a Balzers 48" Box coater in the near future.
This unit has optical monitoring and ion-assist with 3 E-beam sources.

Just stay away from the nasty capaictors in the HV-unit on our Balzer's,
during a service I got zapped by an un-discharged 10kv electrolytic ,
the arc jumped into my hand - out thru the wrist and into the ESD cleanroom
coat and down thru the ESD floor. Felt like a horse kick to the head and
it was lights out and a trip to the hospital :-)

//Greg