RE: [ATM] 12.5 inches BVC mirror - more details

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

It is a crater. Figure A is the profile. 

You would not really have to say which side of the center of curve you are
with the grating. Your TDE shows that. It will be a cold day way down south
before you ever get a TUE to look like that if you were on the other side of
the COC. You can follow the slope changes inward from there and you are in a
crater in the center. I'm not saying your TDE is bad. You will have to look
a whole lot of mirrors to find one that won't give you that clue as to which
side of the COC the grating is on. 

But its nice to say so anyway and how far in or out and with foucault images
to say which side the knife comes from and if the tester lamp or
grating/knife both move or if one is fixed.

Now I would call those first three ronchigrams to be with the grating AT the
COC. That is why it is mimicking a foucault knife edge test. The first image
is using the left side of a line of the grating for a knife edge and the
next two images use the right side of a grating line. That is why the
perception of up and down slope reverses. Those first three images are more
foucault than ronchi.

Now image number four has enough lines to see the contour in ronchi terms.
That image shows the contour nicely and if I were doing this I would like to
see in addition one with only four or five lines. Not necessary at this
stage but a little closer it helps see shallower slope changes. In fact it
helps to look at a few positions with the central-most grating line moved
slightly off center more in each. Visually you just sweep the grating slowly
across the slit. Its all part of visualizing the shape on the surface. The
better you can visualize what you want and what you have the better you can
get at going in the right direction. You will need some method for
masurement of degree.

Now the thing about looking at one line.... I made a typo and that was not
clear.  In the fourth ronchi image that has several lines, pick one line
off-center but close to center. As an example now take the first to right of
center and visualize that as the profile of your mirror. The only thing is
you have to picture it in relation to what you would see if you had the
parabola. The parabola would have the lines bowing outward and if you know
the shape well you know what you would do to the surface to bend those band
toward the right shape. You would push the rim of that crater down and you
would push a down some but less from there out to around 85% or so and blend
with that bend of the turn-down. The trick is doing that without increasing
the turn down. 

In general the classic stroke MOT parabolizing turns the sphere into a
parabola by shortening the radius of curve just inside the edge compared to
the edge and then again inside of there progressively on toward the center.
If you have a turn down and all of your work shortens the radius of some
zone closer to the center you can't make the edge better. If it changes it
can only get worse. That is if you only use the classic big W stroke. It
will take you to a parabola but it won't make the edge better.

You have to do something different to wear the edge to make it better. But
then wearing away out there can lengthen the radius of the edge relative to
the zone just inside and then ... well what is that?  More turned down edge.


Hey!!! I'll bet that is why people have so much trouble with TDE! It seem
that no matter what you do it don't get better. If anything it gets worse!!

So the trick must be to wear some zone inside from the turn-down to a longer
radius quicker than the ROC of the edge is lengthened. Well it turns out
there is nothing to it.

I think I agree with Mel that the parabolizing could begin. That turn down
is not so bad. Since you don't want to make another lap 10 minutes MOT would
show you how your lap is working. I would never say impossible but it
wouldn't be likely you would mess it up and it might make it look better.
Just getting some variable stroke will help remove the zony look. But maybe
somebody else might have better ideas.

There is nothing wrong with the pitch transparency. It looks like typical
tree based stuff. It doesn't have to be black. I don't like the black stuff.
Not the right smell for me.

Looks like George is might get with you.  Nothing like live help.


Not to diminish anyone's web stuff but I still think a book is better. I
have my favorites by some of those old times or even dead people. But the
times they are achangin.

Well if you got to bevel it you got to bevel it.  Since I wouldn't do it I
won't make any suggestions except one... be careful.



Jerry





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