RE: [ATM] Another Astigmatism question, but at the sphere stage

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

All Right. I have looked enough to say that I do see astigmatism on the
first mirror up loaded. That is the test017 images. The aspect ratio of the
image is not quite right on my monitor but I don't think it interferes much.
The image displays just slightly longer in the vertical than horizontal. 

Now let me explain the signs of astigmatism a little.

First for the shadows on the right side (just so we are all looking at the
same thing) in the test017_0_054.jpg which I will hereafter call 054 look
out at the width of the shadow inside the nice edge (most of the way around
anyway) and see that at the upper right quadrant the shadow is wider than in
the lower right quadrant. Similar on the left side for "bright width" from
edge to shadow difference top to bottom.

In the same image look at the loss of diffraction ring from high noon to
almost 2 o'clock and 4:30 to 6 o'clock. There is also increased width of
shadow at those locations. There is a darkening of shadow with a somewhat
straight edge to the shadow. This is a local bend of the surface to a longer
radius. On the bottom it looks like maybe the bend is close to level and
goes across to the left a bit also.

I said earlier there was a hexagon crater with a bump in the middle but when
I really look close to all the images I see that local figuring was done on
each zone. In the very center it looks like a thumb or small lap was used to
reduce the bump and the rim of the crater. I guess it is possible that it
was accented pressure. I think it was a small tool because I've had this
kind of stuff using small laps or a thumb and I have an easier time blending
zones with a full size lap and accented pressure. for It looks like 3 steps
around the mirror on the bump and 4 steps out side of that then 5 and the
increasing steps at each rev around the mirror. It is that 6 is at the rim
of the crater and easiest to see. The same farther out. In some of the pics
you can see small individual W's on a zone at one step around the mirror and
then another. In image 006 you can easily see 4 areas of work around the
inside of the crater. Two on the vertical and two on the horizontal. And I
think I see three inside of that on the bump. These irregularities from
local figuring have to be small in magnitude and behind the diagonal for the
most part but that is astigmatism.  

It looks like the conic is not centered on the disk. The slight lean to the
right of the images and the elongation on the vertical axis might be
combining to cause and illusion. But I think it is off center.


I actually think that either this is the same mirror that James has labled
Mirror C. or the same person made it by their "usual technique". There is a
difference in contrast and resolution in the images but they show a
remarkably simlar surface. Maybe Dale used a wider slit than did James. I
see the same zones.

I will look at the others a little later tonight and keep the description
shorter. 

Jerry



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