RE: [ATM] quality suffering

["Jerry" <wa4guu@verizon.net>]

If it were #5 it would be a more pervasive problem in ATM.

Sounds much like the turn down around a bubble perimeter or deep scratch
("deep" enough to get a turned edge) that I was describing in a post in
response to a question posed about three weeks ago on causes of turn down
edge.

Since it is about 15 or 20 times as long as it is wide I would say a
scratch. But I guess there is such a thing as an elongated bubble. 

Are the six legs radiating from the same point on the length of the defect?
It may be radial pattern from an impact that fractured the surface.

Look at it with a magnifier. A bubble would usually have a concave bottom. A
scratch a V bottom.

I would say #4 or #6. If #6 you didn't see it until now because you didn't
polish into it until now.

I have polished into bubbles, seen much the same as what you describe,
continue polishing and then see it disappear. If you don't fill it with
something it will fill eventually with pitch and rouge on its own and then
the turn down will go away and you won't see it in the Foucault or Ronchi.
The mirror will be too far from your eye. You see it now because of the turn
down causing the defect to be wider an also throwing light away from the
knife edge. With the turn down gone it is like looking for a fine strand
spider web or maybe the period at the end of this sentence from the distance
of the radius of curvature. Most ATM mirrors will have a radius long enough
to make that impossible.
	I have also polished all the way through such a defect and the bb
sized bubble that caused it. Doesn't take as long as you would think by
reading the ATM books. I "may" have been able to get it out faster by going
back to grinding. After the turn down went away I really wasn't trying to
get the bubble hole out. It just took me that long to learn to figure good.

If it has already filled or is filling with pitch and polishing agent don't
clean it out until the mirror is finished. I don't mean don't wash the
mirror to test and so forth. Unless you really work at it you probably won't
keep it from filling in anyway.


Jerry 


-----Original Message-----
Guy Brandenburg
Subject: RE: [ATM] quality suffering

Last Friday one of the mirrors being figured in our
class had the most amazing little flaw near one edge.
With the naked eye you could see a little groove about
3 or 4 mm long and maybe 0.2 mm wide, and it also
appeared to have 6 'legs' coming out of it, sort of
like water-strider insects on a pond. The flaw was
very clearly visible in the Ronchi/Foucault tester,
also, as a fairly large and obviously wrongly-figured
area of the lap (say, about a 2 cm diameter area). We
were wondering if a flaw in the blank had suddenly
worked its way up to the surface; but that is not very
likely, since he is merely figuring at this stage.
None of us had noticed anything like this before hand.
The blank comes from United Lens. Sorry, I forgot to
look at the deformity with a magnifying lens. There
are no bubbles anywhere in the mirror blank that I can
recall.

A few possibilities occur to me:

(1) it really is the fossilized imprint of a water
strider, which was, most likely, out for a leisurely
water-stride on the vat of molten borosilicate
mixture, and fell in, and this person merely uncovered
its remains on Friday evening.

(2) The deformity it had been there all along [for
whatever reason] and we just didn't notice it until
Friday. (sort of like #1)

(3) the Pyrex in that part of the blank is stressed,
possibly because of poor mixing and annealing, and the
grinding and polishing and figuring process all of a
'sudden' released the stress in that area and caused
the deformity we saw.

(4) Somehow a large piece of rough grit fell on his
lap and made an insect-body type of outline just in
one small corner of the lap, without any scratches
being evident anywhere -- AND the deformities in that
area do not appear to be scratches to the naked eye. 

(5) We eat crabs and shrimps, which IS an abomination
unto the Lord, and he is wreaking His vengeance upon
us. (Plus, I 'knew' my wife before we were lawfully
wedded, which is another abomination.)

(6) It's a bubble that we never saw until now.

Any thoughts?

Guy Brandenburg


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