[ATM] Astigmatism and Ronchi questions

[vorblesnak@peak.org]

So here is the deal ..

I have Da Vinci days coming up and hope to have a pie plate scope to show
the masses.  I was trying to decide whether to grind a new one or not as
the first one had a nasty figure when I looked at it last in May.  This
got me to thinking about how bad was it, could I track the errors, blah
blah blah .. etc.

One problem with the scope as it sat was the secondary was one of those
ones I tried to make out of front surface plate glass mirrors from copy
machines, and I was not sure of the pedigree.  So I got the scope down and
took the spider out, (kind of a pun there as there was a large spider
living in the tube when I looked in it), and RTV'ed a new secondary on it.
 While the tube was down I got to wondering how the curve etc looked.  Now
for the rest of the story.

With my trusty homemade 50 lines per inch ronchi grid I looked in the tube
and found nice straight lines and not a hint of astigmatism.  One question
had been, was it a positional astigmatism due to the thinness of the
glass.  It appeared so as not trace could be found with the mirror
horizontal or vertical.  This is odd as the image at the eyepiece was
passable at low power on big bright objects and a disaster at higher
power.  Ted can verify that.  I took some shots and sent them to Mel for
his opinion.  He said it looked good and suggested cooling problems
exacerbated by the thin glass.  You know me, I can't leave well enough
alone.

I pulled out an 85 line tester and dood it again.  AH HA!  As I passed
through the ROC, (also known as the zone of confusion since the lines all
disappear), I caught a flicker of that old stig twist.  Slowly now,
through the zone and I had a classic astigmatism swirl to the lines..  90
degree twist to the orientation .. yeah, there was astigmatism.  BUT! on
both sides of the zone the lines were perfect and kept the same
orientation.  Now this is odd as the orientation should jump some if not
fully 90 degrees when there is astigmatism.  I verified this with index
marks on the tube edge and the lines were spot on.  Two notes.  The lines
inside ROC were evident but fuzzy, the lines outside ROC were dark and
sharp. They were markedly different.   Starting and ending points were 1/4
inch inside and outside the ROC for a total draw of ½ inch.  The figure
was more sphere than anything else.

Hmm, I pulled out ATM 1 and N.E. Howards book and reread what I could find
on astigmatism and testing in general.  This brought me to the idea of
tracing where the twist started and ended and comparing it to the sagitta
via Foucault zonal testing.  Using r squared over R, (classic calculation
for the distance of draw between the center and the edge zones),  I found
a total draw of 6+  32nds of an inch,  about 3/16ths.  Paper on the table
and start tracing the starting and ending twist zones.  The average was
about 1/4 inch.  So the astigmatised zone was between the boundaries of a
zonal testing scheme.  The surface was astigmatised, no question there,
but lots of questions to answer.

1.)  Why does the ronchi reveal good lines, smooth curve with no
orientation flicker?

2.)  Empirically,  how severe is the astigmatism?

3.)  The correction for astigmatism is to drop back to fine grind and fix
the curve, then repolish.  Could I polish out the  astigmatism if I could
locate the zone of error?

4.)  Would a Foucault shadow reveal the zone / zones in error and give me
some idea of the severity of the deformation?

5.)  Could this just be a sag / mounting issue?

6.)  Why is the error only seen with finer grids?

Add your own questions and comments but give me a shot on what you think
is happening here.


David Davis
Toledo, OR
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