From: "James Lerch" 
To: "ATM List" 
Reply-To: "James Lerch" 


----- Original Message -----
From: "steve desfosses" 

>hi everyone,
>today i received my 12.5" mirror from the coater (moonward vaccum
coating) >a
mirror that i have finished a month ago.so it is not a recoating of an old
>one.when i was doing my little inspection of the coating i noticed a lot of
>"bubble-like" spots on the surface.Ididn't count,but there is
a dozen, maybe
>more of them. the bigest ones seems nearly 1mm across.

Steve,

Do these "bubble-like" spots have a height component?  If so,
then these are little blobs of molten aluminum that landed on your mirror
during coating.  In all likely hood, these will not have a visible effect
at the eyepiece.

However, if you have the mirror stripped and re-coated, you will probably
find that for each 'Bubble' you now have an equal and opposite sized
"pit".

I may be wrong, you may not get a "pit" for each
"bubble", it might depend on the size of the "bubble."

I can say with certainty that if a blob of molten aluminum 5mm in diameter,
and about a 2 mm high, lands on a pyrex surface you do indeed get an equal
and opposite size "pit"!  Fortunately myself and a colleague were
able to "erase" my
mistake with the application of 80 grit silicon-carbide abrasives (long
story short, the optic was for one of our club's raffle scopes, and we had
time to grind, polish, re-figure and fix my coating system to prevent
future mistakes...)

If you "bubbles" don't have a height component, and only
discoloration, it might
just be water marks left over from cleaning.  Again, something that
probably won't be visible in the eyepiece.

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"

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