ATM Re: coatings (was Dielectric Coatings)

[Mark Cowan <cowanm@open.org>]

LENSES@aol.com wrote:

> 
> As for the dielectric coating being "lumpy" give me a break! I just can't sit
> by and read this without commenting! In no way is a coating I don't care what
> kind is going to give you a Lumpy surface, IT WILL MIMICK THE SURFACE ITS
> APPLIED TO! Sorry for shouting.:( Unless we are talking about coating
> "spatter" which does happen but very rarely in professional coating houses,
> this will leave an outward bumb on the surface, and will usually pit into the
> glass.
> You are correct also in only wanting it on your secondary, the coating would 
> need to be ground off, something you wouldn't want to do if a problem arose 
> on a primary mirror :(


Hi Peter Nance of PAP:

What an interesting reply this is...  At least now I know where the pit
came from!
  
I got the diagonal in question out to have a close look at it.  Tiny
little lumps, average about .5mm in diameter, very shallow, about a
dozen all told on a 2" diagonal.  Most are gathered near a pit of the
same size, 2mm from the edge near one end. Also it looks like there's
these tiny traces meandering about the coating.  I'd be happy to send it
to you for a professional look--I'm not using it presently, and really
haven't decided if I'm going to use it at all.  It's kind of ugly.

Point being, it was a perfectly finished high quality diagonal with a
standard Alum coating when I sent it off to QSP for a 98% "upgrade." 
Can you say "oops?"  They couldn't.  :(
  
But can anybody add anything to my guess that the bumps, ugly as they
are, are going to do little more than diffuse about 1/1000th of the
light?  They're specular, not like dust.  Are such defects on a diagonal
of greater, less, or the same importance as defects in the primary? 
I've gone to a lot of trouble to build good primaries, and haven't yet
wanted to tackle the little flippy bits, although I know it can be
done.  I guess I'm glad I didn't make this one!    

QSP's reply about it being "something that just happens" still bugs me,
and if what you're saying is true, I think I'll just live with the Alum
coatings on diagonals--see, part of the deal about the dielectric is
that it's "made to last for the lifetime of your telescope" (from their
literature).  If what you say about having to grind it off is true, I
can see why.  There's no metal to corrode, nothing to oxidize, and it's
_hard_.  Seemed like a good thing to have out near the open end of the
tube.

BTW, Peter, I forgive your "shouting."  I had you do my first 8" mirror
with your 96% Titanium Oxide overcoated alum a few years ago, and it was
a beautiful job, even if it did take about 10 weeks.  Made that mirror
in one of John Dobson's classes, he OKed it and everything...but when I
went to put high power on it, serious disappointment.  Eventually built
a Focault tester, but thought the tester was no good at first when it
showed complete 100% extinction at the focal plane--no shadows, no
doughnuts, no nothing.  Yup, a totally spherical mirror...good to about
1/10th wave by my later experience.

Wish I still had that really nice sphere, but I polished off the coating
and embarked on a _real_ learning adventure on my way to finishing a
flawless mirror.  You weren't able to do the coating on that one (for
some reason I forget), and so I sent it to QSP as second choice.  They
did a fine job.  But I really like your philosophy of using low
temperature sputtering, which I chatted about with you on the phone
while your fell machinery was doing God knows what in the background.

Anyway, let this be a caveat...stress and mirrors are never a happy
combination--uh, unless it's one of those harness designs.


"Just think about it...if Moses could part the Red Sea, what could he
have done about Bad Seeing?"

Mark Cowan
Salem,OR
<cowanm@open.org>