[ATM] Ronchi quiz

[vorblesnak@peak.org]

Dale Eason wrote ..

> I thought ronchi users might like to take a stab at
> the this image.  It is from a 16 inch mirror in
> progress. It is polished out.  Can any tell me what
> they think is going on?  I know and will tell you in a
> few days.  I wonder if anyone has ever seen anything
> like it.  The position of the ronchi screen is very
> close to the ROC.

I suppose I should jump in here and claim something.  Cool!  Huh!

It is indeed the Ultrathin 15.5  What you see was brought about by two
things I think.  The first was hand heat while grinding.  The glass was
ground on a full size tool, bare handed.  Little did I suspect the
thermal expansion that would occur as I worked.  Mechanically .. enh!
Optically .. WOW!  The second cause was an initial hand polishing
done with a 12 inch lap, mirror on top.  I don't know why!  It looked
interesting!  If Mel were to join in here he might tell you what I did
to a 12 inch I was working on just to see what it might do.  "What did
you do to this poor mirror."  he sighed.

As bad as it looks, it is easy to correct in hind sight.  I would work
mirror on the bottom on a firm support.  The support would have to cradle
the glass and hold it.  I would start by dropping back to 500 grit and hog
that middle out a bit.  Sub-diameter tool.  Then polish the same way.  What
little distortion I might cause would be easy to correct compared to the
amazing mess caused by hand heat.

I don't know if Dale will send it back or keep it and bang his head
against it,  I have given him the choice.  I have a second one here
slumped from a microwave turntable, (Pyrex glass), same diameter, same
thickness.  I have some other projects to polish out first and then I
am looking at that project.  Probably April now.

Anyone got any other questions?

BTB:  Dale tells me the 16 is not as good as I thought it was.  How about
some ronchis of that one?  I honest thought I was too deep on that one.
Good turned edge though.  I think his astigmatism on the 16 is due to
support while testing.  I could be wrong though.  It wouldn't be the first
time.

David Davis
Toledo, OR 97391

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