Re: [ATM] More Mirror Disasters!

[Gary Fuchs <gary@rcn.com>]

I don't mean any disrespect, but it doesn't sound right to bury 
something that might scratch in the pitch. At some point, if your 
polishing/figuring continues, that contaminant is going to become 
exposed - isn't it? Shouldn't you rather remove the contaminant, or 
remake the lap with clean pitch?

Gary Fuchs

Thomas wrote:

>Flash heat the surface of your polishing lap so that the contaminant is
>*in* the pitch and not on it. 
>
>To do this wash of all the cerium oxide etc, and then with a heat gun or
>small gas flame work the whole lap over real quick so just the top 1/16"
>or so melts. This way the contaminant will be locked away in the pitch,
>but you will have to re-press the lap to get good contact again...
>
>On the other hand you could just ignor them scratches, remember the
>spider vanes for your secondary support act like four huge
>scratches.....
>
>Clear skies,
>Thomas Janstrom
>http://www.tjanstrom.com
>http://www.norsewines.com.au
>"Your nobody until you've been ignored by your seventh cranio-facial
>nerve."
>
>
>-----Original Message-----
>From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
>Of David Grennan
>Sent: Wednesday, 8 June 2005 5:04 AM
>To: atm@atmlist.net
>Subject: [ATM] More Mirror Disasters!
>
>
>Hi all,
>
>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.
>
>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.
>
>So in summary my questions are.
>
>1)  How (if at all) should I best deal with these fine sctratches.
>2)  How could the classic parabolizing stroke bring my mirror back to a
>sphere?
>
>Confused :-0
>
>Dave
>
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