Re: [ATM] What is the "dielectric" in dielectric coatings?

["James Mulherin" <jcmulherin@opticalmechanics.com>]

Bruce:

> I've been puzzled about this for a while.
> 
> Hitherto, I've known the word "dielectric" to be synonymous with
> "insulator", but in the telescope world, it seems to mean "more highly
> reflective than the other kind".
> 
> What is the significance of the dielectric?  Is it the coating
> material, how it is applied or something else?  What is the "other
> kind" and why is it inferior?

The dielectrics are non-conducting and transparent layers used to protect or
enhance the reflectivity of a metal base layer, typically aluminum. The most
common dielectrics used are: a single layer of silicone dioxide to protect
the aluminum, or multiple layers of dielectrics like magnesium fluoride,
titanium dioxide and silicone dioxide to enhance reflectivity. To enhance
reflectivity you use alternating 1/4 wave or 1/2 wave thick layers of high
and low refractive index dielectrics. 

Dielectrics are also used to create anti-reflective coatings on lenses.

If are designing a very narrow band pass high reflector, say for a laser,
you can make the coating purely from dielectrics but these coatings use up
to a hundred or so alternating layers.

> 
> Now to the reason this came up:
> 
> My secondary is discolouring.  It started out nice, but got some
> spidery coppery coloured lines around the edge after a year or so, now
> is it kinda amberish all over.  You really have to look to see it, but
> it is there.  I've not seen this on anyone else's scope.
> The secondary is otherwise pristine, and the primary is also 99
> 44/100% dust free, so I don't think that this is contamination, but
> some sort of manufacturing flaw.

The discoloration may be due to moisture absorption in one of the dielectric
layers in the coating. When a dielectric absorbs moisture it's index of
refraction changes. This may cause discoloration when viewed at an angle. If
you don't see discoloration at the angle of incidence for which the coating
was design the effects will probably be minimal. 

James

James Mulherin
Optical Mechanics, Inc.
jcmulherin@opticalmechanics.com
www.opticalmechanics.com
Tel: (319) 351-3960
Fax: (319) 351-3943


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