RE: [ATM] slow polishiing

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

Why do you say it isn't polishing out. Do you see grayness on the surface or
are you taking the shape to mean it isn't polished? Is the Ronchigram with
the grating inside or outside of center of curve. I ASSUME it is inside and
that you have an oblate spheroid as is somewhat to be expected.

If you have not finished polishing the shape does not matter much. If you
are waiting for the shape to change as an indication that the polish is
finished, don't. The shape is a product of the strokes you are doing. Don't
worry about the shape until it is polished. 

The Foucault shows that you are polishing. It is just a mater of how long
you need to continue. Some people say 1 hour per inch of aperture. I don't
like that saying. Small can go faster than big but depends at least a little
on technique. I say polish until you are sure it is fully polished. If there
is still gray on the surface after 14 or 15 hours then you should begin to
think something wrong.

Looking at your Ronchigram and ASSUMING that the grating is inside the
center of curve you see you have the bands bunched in the center and fairly
straight. Straight (straight up and down)means spherical and lines close
means the grating is well inside the center of curve. Then the bands bend
outward and in this case still fairly straight but they are not parallel
with the lines of the grating. They are diverging meaning the radius is
getting shorter as you go farther from the center. Now that is as close as
you need to know about the shape.

But let's look closer just for the practice that will help you later. Just
look at the central part and ignore the outer part altogether. The bands
aren't really exactly straight. They bulge slightly in the center and come
closer together as you move away from the center. That area is slightly
towards a parabola. In fact on a 6 inch f7 the correction from sphere to
parabola is slight and it may well be a parabola or even a hyperbola in the
center. Can't tell as it depends on the lines per inch on the grating and
how far inside the center of curve it is. Your finished mirror Ronchigram
will look somewhat like the central part does now.

Working from the center outward, now look at the bands as they transition
from the central area just described to the outer area where they are
diverging. There is a short area where they are straight up and down. And
then diverging from there almost to the edge. The continued divergence means
continued shortening of the radius of curve. And then you have a short
length of the band that converges as it goes off the edge. A slight turn
down there. 

Now if that Ronchigram is outside of the center of curve then everything I
said is backwards except the spherical part. I won't go into that since I'm
quite sure I ASSUMED correctly the grating was inside the center of curve.

It looks to me that you are doing well. You could polish that thing in 7
hours but until you put twice that much time on it I wouldn't worry. I only
mention Ronchi analysis to help you interpret them when you are figuring. 

I'm sure some others will tell you how to with sunlight or lasers or
flashlights to see if you are polished so I won't.

I will recommend against pressing the mesh in for a half hour before
working. AFTER the mesh is removed the lap should be pressed well. If you
press the mesh in before work you are disturbing the close fit of the lap to
the mirror and not allowing time for it to restore.

Jerry 


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