[ATM] tde hypothesis

[David Harbour]

Ted,

The entire subject is a deep mystery to me; I always stayed in each grade of 
carborundum long enough to get spherical, as measured by good methods of 
atm-ing (making penciled x's disappear at the same time) and I never had any 
larger pits left over; Ron Gafron has a 16" mirror that is siilky fine 
ground by me- I simply stayed in each grit an absurdly long time.

Only once did I get a turned down edge; after that, no matter what I did, I 
always got turned up edge; the late Bob Goff told me it was because the edge 
of the mirror, being exposed more often (for longer durations) during 
polishing, got less polishing action, and also, it was mostly because of the 
wetness of the edge, with the evaporative effect, pulling the edge down; he 
may have been right, because I worked in a very hot workshop (I only figured 
at night, though) and had a fan blowing on me; if that fan was blowing on 
me, it must have been blowing on that twelve and a half inch mirror, too, 
really shrinking the edge down.

I ground an f/2 12.5" mirror for a customer for his Schmidt camera, and I 
only just remembered, this was one mirror in which I did not get turned up 
edge- but I ground it without a fan blowing on me, polished it without that 
fan, and the edge came out perfect.

So, for me, the pencil X marks (test for them disappearing almost 
simultaneously) got me a near perfect sphere at the end of grinding; but I 
usually got turned up edge on polish out, due to that fan. I am a firm 
believer in staying in each grade of grit for a very long time, to remove 
all of the previous grade of pits; I believe I saw some advice about this in 
the old Ingalls book; I took it to heart, and, as I earlier said, I used 
W-B's very fine abrasives, and there was never a "rogue" grit to cause any 
trouble. Once those pencilled X's disappeared (as viewed through the back of 
the wetted mirror) at very close to the same time (i.e., nearly 
simultaneously) I was pretty sure that I was still spherical (at the level 
of the dimensions of that grade of grit)- it was almost always when I 
polished out, that the edge zone would pull away from the mirror, because of 
evaporative cooling, receiving less polishing action. That perfect f/2 
mirror, done in November, with that fan blowing on me, pretty much proves 
how I got turned up edge- it pretty much proved Bob Goff's assertions.

I only got turned down edge one time, with my second mirror; a huge majority 
of the others ended up with turned up edge. Everyone has told me that they 
found that curious. But I believe Bob Goff was right. No way for me to test 
his hypothesis, but that f/2 schmidt Camera mirror pretty much proved it. 
Turned out very well; made my tool just as on the site article, here:

http://www.atmpage.org/contrib/Harbour/Tool.html

It seems I made a lot of 12.5's, and they all turned out okay, except for 
the drudgery of repairing turnded up edge. Regarding your hypothesis for 
tde, it would seem that if you got it in the grinding stages-i.e., if it 
showed up latere in the polishing stages, that it would be of a very high 
order, if you got it in the grinding stages. So perhaps you can look for the 
cause in the polishing stages; I just got it only once, and it was not a 
sharp turned edge (it was not for a customer, but rather my second, for a 
planetary lunar 112.5" f/8; I confess to my fellow opticians, I just masked 
off this shallow tde, with a 5/16" ring, and that slightly less than 12.5 
inch mirror gave killer views of the planets. A sharply turned down edge 
will scatter the light long before it gets anywhere near the diagonal, and 
therefor will be invisible; but the portion that is only slightly turned, is 
the one that needs to be treated.

Since I have always (nearly) gotten the opposite of tde, I guess I don't 
have enough experience or knowledge to comment on it; but I have a feellling 
that too much extra long stroking when polishing causes it. Anyway, there is 
my two stupid cents worth of advice about edge defects. Try to stay in each 
grade a long time, Ted, and use those little graphite leaded pencil X's on 
each tile, and wet the back of the mirror; the edge ones should disappear as 
quickly as the center ones. If they do, you have been in that grade long 
enough (if you "take out the insurance" of the extra endurance with each 
grade, per the old Albert Ingalls book, that even showed some drawings about 
how many hours it takes to get some of the large pits out, if you did not 
stick with the next grade down long enough.

Good luck to you-

Davey/
R-101/

P.S.- I discovered an excellent article about making a tile tool, and using 
it, for hogging out a flat mirror blank, that I wrote, and it is here on the 
hard drive of muy computer, if anyone wants to ask for it. I explain the 
process of waxing the channels, and how to manage the exact depth of 
channel, when grinding- please, one and all, ask for it. I will send it to 
anyone who wants it, becuase the article at that link is for tools made to 
curve, and a lot of amateurs are starting from flat blanks; they need a 
flat, channeled tool; easy to make with the molithic tool they send with a 
kit. I have developed some useful adjuncts to using a channeled tile tool; 
they are recorded in the article. Please ask for it, and I will send it to 
any who ask for it.


----- Original Message ----- 
From: "Ted Cohen" <tcohen at blakeglobal.com>
To: <atm at atmlist.net>
Sent: Saturday, June 07, 2008 4:18 PM
Subject: [ATM] tde hypothesis



I have this hypothesis about a possible cause for tde, but not enough
experience to really know so am interested in to hear what the experienced
folk think of it.

Assertion #1) Mirror grinds out more slowly at the edge than at the center
Assertion #2) If two zones on the surface of the mirror, irrespective of
their relative location, are making equal contact with the tool at a given
point in time at any given grit, then the zone that is rougher (i.e. has
pits with a larger average diameter) will grind relatively faster than the
smoother zone.

Conclusion - Tde is to be expected at most stages during grinding and
polishing. It is only removed, at any given grit, when grinding/polishing is
extended for a sufficient period of time; and this extended period of time
begins only when "Assertion #1" has been thoroughly mitigated - i.e. the
surface roughness of the glass has become uniform out to the edge. Thus,
only when the surface roughness is uniform does preferential grinding of the
rougher perimeter zones cease, and attainment of a spherical surface become
possible.

Ted.

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