Question regarding reflective coatings.

Neat looking toy, eh? =) Does anyone know how the general process works?

It's the same process that is used to deposit various layers on a
microprocessor chip.

Roland Christen

Jim Jones wrote:
As pretty much everyone here knows, the Astro-Physics MaxBright and
TeleVue
Everbrite diagonals utilise dielectric coatings. According to the A-P
site,
"The coatings are deposited by an electron beam evaporator at a high
temperature."


.....and

Additionally, the Starmaster site states that "Primary mirror coatings
utilize Ion-Assisted Deposition (IAD) and quartz overcoat." Is this
simply
different wording for the same process, or are we talking apples and
oranges?


These two references are describing different parts of the coating process,
Jim. The electron beam evaporator is used to simply heat up the source
material (i.e., whatever you are trying to make the thin film out of). All
modern coating chambers use e-beams, though some materials like metals
(aluminum, silver, etc.) can be evaporated just fine in a tungsten boat.
E-beams are typically used on materials with high vaporization temperatures
like the dielectric films.

Ion-assisted deposition is somewhat of a catch-all phrase that describes a
whole family of processes. Techically, any process that accelerates the
migrating ions through an electric field could be called IAD. The details
are important, though (things like what is being accelerated, by how much,
and when it takes place in the coating process). All of the IAD coatings I
am familiar with in the amateur astronomy market are oxygen IAD processes.


In short, I'm wondering if the dielectric coatings are scalable to the
primary and secondary surfaces of reflectors and catadioptrics. If the
process is scalable, then presumably the cost is prohibitive for larger
surfaces, or else I'm sure that everyone and their uncle would be hawking
99% reflective optics by now, eh? Or perhaps people are reluctant to
trust
in the durability of these coatings versus the ease of re-aluminising
every
few years?


The durability you could trust, as the pure dielectric stack coatings are
tough to damage. They are essentially a lifetime coating.

Sincerely,
Bryan Greer
Worthington, OH

"Chris1011" wrote in message
...
[...]

Multi-layer (i.e. greater than 50 layers) dielectric coatings are very
durable,
but on a large curved surface like a Newt mirror, you will get a definite
change of figure which will indeed ruin it. You cannot then rmove the
coatings

_will get_ or _probably will get_ ? I suspect you are correct in your
statement in general, but there isn't anything in the process that
prohibites applying the coating succsessfully is there? That there might be
difficulties to do this on a repetitive and controlled basis I can
understand.

Clear Skies,
Magnus

because they have tightly bonded to the glass. You must re-grind,
re-polish and
most of all re-figure in order to get back to square 1.

These coatings are useful in star diagonals only, where the actual surface
used
by each individual star is a small percentage of the overall surface. In
that
case, they work very well, they are cleanable and will last essentially
forever.

Roland Christen

Hey Roland, How about a 1.25 version of your Maxi?

Baader is making one for his Binoviewer, and it is excellent. I will find out
if he can add a 1.25" eyepiece barrel to it.

Roland Christen

"Chris1011" wrote in message
...

_will get_ or _probably will get_ ? I suspect you are correct in your
statement in general, but there isn't anything in the process that
prohibites applying the coating succsessfully is there? That there might
be
difficulties to do this on a repetitive and controlled basis I can
understand.

You WILL get a distorted surface, I can just about guarantee it.

What causes that ? Limitation in the current technology, or something
inherent ?

Clear Skies,
Magnus


Roland Christen

For thick EB deposited coatings the miss-match of thermal expansion and
lattice parameters between the layers creates stresses that will distort the
substrate. As the layers have to be of different materials it is
unavoidable with that process. I am sure the degree of distortion can be
influenced by selection of parameters and coating details but if it were
viable for large precision surfaces it would be an available option.


What causes that ? Limitation in the current technology, or something
inherent ?

Clear Skies,
Magnus

"Enyo" wrote in message
...
For thick EB deposited coatings the miss-match of thermal expansion and
lattice parameters between the layers creates stresses that will distort
the
substrate. As the layers have to be of different materials it is
unavoidable with that process. I am sure the degree of distortion can be
influenced by selection of parameters and coating details but if it were
viable for large precision surfaces it would be an available option.

Ok, so it is mostly a substrate problem - one that cannot easily be solved
(though perhaps improved little by little). Thanks!

Clear Skies,
Magnus



What causes that ? Limitation in the current technology, or something
inherent ?

Clear Skies,
Magnus

"Magnus Nyborg" wrote in message
...
"Chris1011" wrote in message
...

You WILL get a distorted surface, I can just about guarantee it.

What causes that ? Limitation in the current technology, or something
inherent ?

The degree of inherent stress in the film can be reduced with changes to the
parameters but only at the expense of film density and thereby durabilty
and/or optical characteristics.

Dan McShane


Clear Skies,
Magnus


Roland Christen




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Magnus Nyborg wrote:
For thick EB deposited coatings the miss-match of thermal expansion and
lattice parameters between the layers creates stresses that will distort
the
substrate. As the layers have to be of different materials it is
unavoidable with that process. I am sure the degree of distortion can
be
influenced by selection of parameters and coating details but if it were
viable for large precision surfaces it would be an available option.


Ok, so it is mostly a substrate problem - one that cannot easily be solved
(though perhaps improved little by little). Thanks!

Clear Skies,
Magnus


Pure dielectric stacks can be applied to primary mirrors, but you'll need to
sell your car to pay for them. I've seen pictures of paraboloids with pure
dielectrics in past optical magazines, but they were for expensive
government jobs. High energy laser applications require pure dielectric
coatings, since they would destroy metallic coatings pretty quickly. These
were undoubtedly applied with a cold process involving some pretty heavy ion
peening (probably argon). This type of IAD isn't cheap. I also don't know
what surface accuracy they were needing to maintain, so Roland's comments
are still valid. Telescopes mirrors (at visible wavelengths) require some
of the best surfaces routinely created.

In short, my guess is that someone could successfully apply a pure
dielectric stack to a primary with an unlimited budget, but there just isn't
that much to be gained by it. In fact, you might even lose some of the
color at the ends of the spectrum. These types of coatings drop off
precipitously outside of their design. Metallic coatings tail off much more
gradually.

Sincerely,
Bryan Greer
Worthington, OH

I seem to remember you saying once that the edge warp of the mirror
while baking on the dielectric coating made it difficult to get
accurate mirrors in the 1.25 format. Has the coating/cooking process
improved?

The coating is not "baked on" after application. Normally, coatings are applied
to glass substrates at high temperatures to give the surface molecules the
mobility to adhere to the incoming coating particles. New methods like ion
assist now allow this to be done at low temperatures. However, this does not
solve the problem of inter-layer strain. Baader has perfected a method of
applying the layers so that no strain is transmitted to the substrate. His
method makes possible an accurate small diagonal using these type of coatings.

Roland Christen