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

["Jerry Reddell" <jerry.p.reddell63@sbcglobal.net>]

"Dielectric "coatings can be used in optics to do at least two things. One,
it provides protection for the lense and or mirror "aluminum". Second, and
most important, properly designed dielectric coatings of specific thickness
can be used to reduce the reflection from lenses or increase the reflection
of mirrors, which increases the light reaching the observer's eye.

Sounds like the secondary has been contaminated, not by dust but by
molecules of 'stuff' in the air. The stuff, most likely low levels of  "acid
rain" or "acid moisture" in the air.

Jerry P. Reddell

-----Original Message-----
from Bruce MacDonald

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?

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.

What should I be looking for in a secondary?  Who are the good suppliers?

Regards,
Bruce MacDonald


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