RE: [ATM] mirror damage

["Scott Milligan" <starzkey@charter.net>]

Chips are effectively prevented from further propagation by grinding the
affected area using a small abrasive honing or dressing stick.  A #150 grit
white aluminum oxide stick is what I use for this purpose.  Essentially, you
are locally enlarging the bevel, in the area where the chip occurred only.

Scott Milligan

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
James Lerch
Sent: Monday, June 06, 2005 8:18 AM
To: ATM
Subject: Re: [ATM] mirror damage

----- Original Message ----- 
From: "Cec & Cris" <ccabar@bigpond.com>

> I was grinding with the 240 grit (mirror on top) when a chip came off the
> side (edge). It only slightly affected the circular shape < 1 mm. The
problem
> is that it is a rather large chip and it has not left much thickness at
the 
> top
> edge about 1mm. Can someone advise me on what I should do?

A picture would be helpful,  the list has the ability to host pictures from
a 
list managed web-site, or you can email me one and I'll host it, just don't 
email it to the list directly!

The most practical thing to do is make sure there is plenty of edge bevel on
the 
face of the optic near the chip.  The worst thing that can happen, from this

point on, is you accidentally bump the face of the mirror near the chip, and

remove a large clam shell shaped piece of the glass that extends into the
face 
of the mirror.

At the moment, consider it this way, you have conveniently built a permanent

indexing mark on the optic,  which will be useful while testing the mirror
once 
it is polished out.  (Gotta think positive!)

> I was thinking of filling it  up with fibreglass filler (builders bog) to 
> strengthen
>the edge a bit so that I can continue grinding. Can I do this or will this
have
>an affect on the illuminisation?

Not recommended, as most aluminizing companies won't put unknown materials
into 
their vacuum chambers in fear of contaminating the environment.   In
addition, 
odds are that you would not be able to find a compound that would bond to
the 
glass with sufficient strength to provide any useful support.

> Is the illuminisation done with heat or not?

Most all overcoated aluminized surfaces are done at room temperature, with
the 
amount of heat applied to the mirror being similar to shinning a 600+ watt 
halogen flood light on the optic for a few 10s of seconds.  In other words,
heat 
is used to raise the material to be evaporated, but the amount of heat
actually 
finding its way to the mirror is minimal.

Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction,testing, and coating
site)

"Anything that can happen, will happen" -Stephen Pollock from:
"Particle Physics for Non-Physicists: A Tour of the Microcosmos"

" Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent. "
                                                           Calvin Coolidge


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