Re: [ATM] ATM: Slit equipped and slitless testers-

["David Harbour" <stainless_steel@suddenlink.net>]

Mark, all:

I didn't get the explanation right, either. The blunder was large, however, 
the result is the same!!!

The first part of the aerial image of the slit that the knife encounters is 
not its own edge (its own lower edge, as one of the jaws of the slit) but 
the other edge- the opposite edge, first. This is the edge of the other 
blade; not the blade that is extended up from the other jaw- I forgot my own 
rule!!! The image of the slit is reversed, so of course the knife-edge 
encounters the opposite jaw first- even so, after the edge of this jaw is 
covered up completely, it cannot possibly serve any useful function 
whatsoever; it is not visible. It is not part of the image any longer, or if 
we consider that it is, it is completely occluded by the knife edge. Then, 
as the knife edge is continued inwards, laterally, it crosses the "virtual" 
gap of the slit (image of the slit), getting closer to the image of its own 
edge, progressively making the gap narrower than it actually is. So the gap 
of the actual slit does not count; the tester is making it's own "virtual" 
slit, by approaching the mirror reversed aerial image of its own edge. So if 
we both have a gap of .010", it begins to get smaller; we stop the advance 
of the knife edge laterally, when the "self-made" slit is at a value that is 
productive, and yet still providing enough light to see what we need to see 
with regard to mirror defects.

And so, although the result is the same- one edge of the slit serves no 
purpose, it is completely occluded, obstructed by the knife edge totally, 
and only the other edge remains visible for any useful purpose, unless we 
continue the advance of the knife edge all the way in, and obstruct the 
entire aerial image of the slit.

I cannot believe that I made so COMPLETELY a blunder. I expected to be 
banned from the list this morning, after enunciating the theory incorrectly. 
But, this is how it works; now I understand why the visibility of surface 
defects INCREASED as I advanced the knife in further- I was making the 
"virtual slit" narrower. In retrospect, I see that the other side of the 
slit/knife edge is still useless, as soon as it is covered completely by the 
knife edge; one may do, as Ralph Dakin did, and completely eliminate it; it 
is as he said- the knife edge makes its own opposing slit jaw, after getting 
close to the aerial image of its own edge.

For me to "reason" my way to such a blunder, I should have lost more face, 
more of you guys should have chastized me. Probably one of you should have 
banned me from the list for this blunder. I thought I had it all figured 
out; I guess NOT! I guess I did by the time I went to bed, but the head was 
willing to get up and fire up and post a correction (self correction) but 
the bones were not!

All of you guys should have been laughing at me by this morning- you should 
have been laughing at the old guy, who got it backwards- or I should have 
been chaastized, rebuked.

Thank you all for letting me "discover" my own mistake, and posting my own 
revision, and revealing my own wooden headed-ness!

The important thing is that Ralph was right; none of us needs an opposite 
jaw, just the one razor blade. However, he made a blunder too: instead of 
epoxying his assembly on the back side of a bar, with a generous sized hole 
to view past the razor blade, he just left the razor blade exposed, and 
badly lacerated his hand on it. He was lucky- what if he had lacerated his 
eyeball on it, his cornea?

Me? I only damaged my brain, I guess, trying to figure the whole thing out 
(why my extra large gap worked so well: it worked so well, because as I 
advanced the knife in, I closed the gap).

Thank you all, including you, Mark, for not stomping me flat, and letting me 
discover my own error. It is all the worse because I knew the mechanism of 
how a slitless tester works, long ago. I guess I retained a slit out of what 
one might call, a "superstition". The other jaw never did anything useful; I 
never needed it.

R-101

----- Original Message ----- 
From: "Mark Cowan" <toolontop@yahoo.com>
To: <atm@atmlist.net>
Sent: Sunday, May 18, 2008 2:53 AM
Subject: Re: [ATM] ATM: Slit equipped and slitless testers-


> Quite true, the slitless and slitted tester operate identically when the 
> KE
> closes down enough to show shadow detail.  I've taken hi-res foucaultgrams 
> that
> illustrate that very fact - and using a very narrow slit doesn't improve
> detail, merely adds diffraction artifacts.  If anybody's interested in 
> seeing
> them I'll send you a copy.  BTW I now use a 200 micron slit on my tester - 
> same
> as you.
>
> I just didn't get what the original question was about...
>
> Best,
> Mark
>
>
> --- David Harbour <stainless_steel@suddenlink.net> wrote:
>
>> Gary, Roman-
>>
>> Only two thought it worth while to check in. You both voted with me. So,
>> here it is for you guys: (not that it matters; anyone who needs math may
>> skip this windy dissertation)-
>>
>> If, when one arranges the knife edge on his or her tester, so that it is
>> simultaneously one side of the jaw of the slit (this is easily 
>> accomplished
>> by merely staggering two razor blades, lengthwise, vertically, by half 
>> their
>> length, each one) then, even though one has a slit behind which to place 
>> his
>> or her favorite kind of illumation source, the slit equipped tester also
>> operates, automatically, every time the KE is advanced in from the side,
>> laterally, as a SLITLESS tester.
>>
> <snip>
>
>
>
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