Re: [ATM] More Mirror Disasters!

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Check your water supply; practice cleanliness; try mixing up a slurrry of the polishing agent, then let some of it settle for a few minutes, then decant the top part and only use that.
I suspect these really fine, almost imperceptible scratches will polish out within an hour, but you will have to polish for a while for them to disappear. Only if they are really deep will you have to go back to fine grinding.
Guy

Woodchuck <djv@bedford.net> wrote:
On Tue, 7 Jun 2005, David Grennan wrote:

> 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.

They'll probably plague your conscience for a decade if you ignore
them, since you'll always know you could have gotten rid of them.
Whether they'll affect the image can't be judged from here. They'll
scatter some light, reducing contrast. Your friends will taunt you.
They'll look worse after aluminizing. Ask, "What would Ritchey do?"

If they're the same depth as the scratches from your fine grinding,
they'll polish out in finite time. If they're deeper, they'll
polish out to dotted lines connecting craters. Chances are you're
going to have to go back to fine grinding. Even if you have a long
time and a machine, polishing them out may well result in a bizarre
figure on the mirror.

Polish for an hour. Judge if they're going away. Inspect them
with a strong magnifier (10x loupe or linen tester) or a low-power
microscope. Progress? Likely to get them out in another hour?
If it will take in your judgment (judge the depth and width of
the scratches) longer to polish them out then you've already spent
polishing, go back to fine grinding. Be pessimistic.

You did save the grinding tool, right? :-) [You might get away,
if the scratches are fine enough (without deep pits involved), with
using the polishing lap and 600 or so carbo or alox instead of the
polishing cpd. Needless to say, such a lap must be remade and the
pitch not recycled.]

Whatever caused the scratches is now very likely embedded in your
polishing lap. You may wish to remake it. Sometimes one can get
away by melting the surface (hair drier) and praying that the
scratchers sink in the lap and won't surface again later. I am not
sure which god or demon properly gets the prayers and sacrifices.
How expensive is pitch? (rhetorical question). Maybe scrape off
the top 0.030" or so of the lap using a single edged razor blade
held perpendicular to the surface, like a cabinet scraper. Or paint
on a layer of new pitch. Warm and cold pressing and channel renewal
follow, etc etc.

If a contaminated lap or polishing cpd is the problem, you'll
polish them out maybe, but they'll keep dancing around. You mark
one scratch somehow, polish, it goes away, new ones appear.

Improve cleanliness of polishing room. Wash the ceiling maybe.
Stuff can fall from above. You can tack plastic sheeting over
the polishing area if this is a problem. If feasible, you can wet
down the floor prior to work. If you suspect that your polishing
agent is the source of the scratches, (boulders in it), then wash
it and elutriate it. If the polishing agent is contaminated with
grit or filth, make metal polish from it, or throw it in the
yard.

Sometimes scratches were there all along and are only revealed
as polishing proceeds.

Dave "Limited experience, but I've been in this position."

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Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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