[ATM] Scratches

[Mike Lockwood <melockwo@uiuc.edu>]

Dave,

I changed the subject heading since this isn't really a disaster....

David Grennan wrote:
> Last evening while working at figuring my 8 inch f6.3 I managed to get
> something between the lap and the mirror which resulted in lots of tiny
> (almost imperceptable) scratches all over my mirror.  Even though I tried
> scrupulously to clean up every single grain from previous operations.  Hence
> my first question.  Would you guys tolerate these tiny scratches and just
> move on, should I go back to fine grinding, or is there a chance I can
> polish these scratches out.

Sounds like sleeks.  Could be cause by not-so-great Cerium.  This 
happenned to me with Willmann-Bell Cerium.

Where did your Cerium Oxide come from?

If the Cerium is not of great quality, try mixing it with warm water 
instead of cold, which may help break up clumps (that can cause 
sleeks).  You can also decant it - mix it up, shake it well, and then 
let it settle until you see large particles settling on the bottom. 
The pour off the top part and use that.

> Now my second question.  After a period of using the classic parabolizing
> stroke.  I was able to observe my mirror moving toward a parabola.  (I was
> continuing with this stroke when I got all the little scratches).  After my
> second session of parabolizing I fully expected to find my mirror closer to
> the required section.  I thought if I was close enough I'd just forget about
> the scratches and get the job finished!  However to my amazement the
> foucault test shows me I'm back at a sphere again!!  I have no idea how this
> occured.  I cold pressed for 1 hour before my second session of
> parabolizing.  <aybe this wasnt enough.
> 1)  How (if at all) should I best deal with these fine sctratches.

Depends on their severity, and how much pride you have.... ;)
Can you see the scratches when you do the Foucault test?  (They would 
look like very bright lines.)  If not, and this is your first or 
second mirror, I wouldn't worry about them too much.

They may actually polish out during figuring.

> 2)  How could the classic parabolizing stroke bring my mirror back to a
> sphere?

If you actually had an oblate sphere before, then applying 
parabolizing strokes will first get you back to a sphere, and then on 
to a parabola.  Are you sure you didn't misread the shadows?

I guess a parabolizing stroke could also somewhat spherize if the 
strokes were too short, but I've never had this happen.

	Mike Lockwood

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