From: "James Lerch" 
To: "steve desfosses" , "liste
atm" 
Reply-To: "James Lerch" 


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

>
> hi everyone,



> i was able to find a spot that has lost its aluminum bubble and light it up
> from behind the mirror. but what i saw was not a point of light but
> hundreds. lighting up the mirror from behind was like looking in the night
> sky in a very dark site.what is causing this? it is surely not a lack of
> polishing , this mirror recieved around 30 hours of polishing....if somebody
> can give me an answer on this,  i would be very pleased !
> thanks

Steve,

Trust me, your not going to be very pleased with this answer!

For your emotional health and over all well being, I suggest you just put
the mirror in the OTA and do your best to put this out of your mind!

If you really want to know read on....

.
.
.
.
.
.
.
.
.
.
.
.
.
.

Your "Starry Night" pattern is caused by micro fractures of the
substrate where the hot aluminum splatter landed on the mirror.

The aluminum when it landed on your mirror had a temperature somewhere
between the melting point and the boiling point of aluminum (660 - 2519C). 
Your mirror was at room temperature (27C).

Since everything is in a vacuum, asides from radiating some of the heat into the
chamber, most of this heat was conducted into your mirror at the point of contact.

This rapid temperature change caused a rapid thermal expansion of the substrate.
I don't believe any glass substrate will survive this rapid thermal
expansion with out micro fracturing. (but I could be wrong..)

My one and only personal experience (arghhhhh)  shows the amount of micro
fracturing is related to the size of the bubble of aluminum that landed on
the mirror.  While I'm not certain, there may be a critical size, below
which no thermal damage is done the optic, but I would have to conduct
experiments to know for certain.  BTW, I have no plans to do such an
experiment!

I might also add, that when you wiped the surface with a facial tissue, it
probably wasn't the bit of Al that caused the scratches, but little shards
of glass. (sorry!)

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"

--- BBBS/NT v4.01 Flag-5