From: "Jerry" 
To: "'Jerry B. Hillman'" ,
        
Reply-To: "Jerry" 




-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net] On Behalf Of Jerry B.
Hillman
Sent: Monday, April 28, 2003 7:49 PM To: atm{at}shore.net
Subject: ATM newest ronchigram


Hi all,
Please bear with me. I have posted several more Ronchigrams to my web site.
There is a number at the bottom right of each pic that matches the numbered
paragraphs. I would appreciate comments.

http://www.convertabarn.com/Figuring.htm

I am using a 10" subdiameter tool.  Working exclusively TOT.  Mirror
is F4.6.  16.5" x 3/4"

Clear skies, Jerry

BTW - Would it be easier to continue on to a parabola rather than try to
get back to a sphere first.  The mirror appears to be heading that way.


First I will ask are you trying to make a sphere or a parabola?

Then I will say that I think that the apparent astigmatism is probably not
really in the mirror but more likely that you have the grating or slit or
both not squared on to the mirror axis: a leaning from top to bottom either
forward or back and perhaps a little misalignment of the slit and the
grating lines. If it is astigmatism in the mirror it appears you are
placing the mirror on the stand in exactly the same orientation each time.
Make it a point not to do that at least once. Also make sure the grating
and returning beam of the slit are parallel and that they are squared to
the mirror and for the grating that means both top to bottom and left to
right.

My first suggestion on figuring would be to only work on one stroke a
session. Don't work a bit on one part and then another in the same session.
At least until you have a real good feel for what each stroke does. And
best not then either. Until you really know what a stroke does you can and
will be wrong about which stroke did what to the surface in some figuring
sessions. The logic of where to remove glass and how to remove it from
there is frequently counter intuitive... at least until you get the idea of
the relative difference between the shape you have and the shape you want.

Now as to the shape I see on your mirror...  pretty much what you see.  But
what I would do to it would depend on if I were trying to get it to a
sphere or a parabola.  At f4.6 looking at the outer 25 or 30% I would make
a guess that it is about 50% corrected toward the parabola out there. So a
bit of work to get some correction in the center to catch up with the outer
area would be in order if you are going for a parabola. And that outer part
looks pretty good to the edge. Ah... if the center were smoothed carry that
shape on to the center it would look nice.  In other words I would not try
to get a better sphere if I wanted a parabola.  But lay off that 5"
lap. The real small laps can make ugly zones. And you can see how it has
started to do that. At some point you may have a small area you want to
touch up with the small lap but you will find if you don't get it fixed
quick the small lap can make the mirror look real bad real quick.


Look at the last picture and the Ronchi line just to the left of center.
Picture in your mind that that line is the shape on your mirror and focus
of that curved line is to the right. It shows a bump in the central area.
You want to try to make that curve bow from edge to edge in a nice parabola
like shape. So you will have to wear the flat central part into the bow
that the outer part has. That will mean deepening gradually from the
beginning of the flat part to the center of the mirror.  There are some
aspects of visualizing the shape that way that can be tricky, but for the
most part you can use this way and do real well. It would take too many
words to elaborate on this and you would find the nuances just as fast on
your own as I could state them. But one or two are too important not to
state.
        The most important one is that while looking at the curve that way
keep in mind that what looks like a turn up at the edge is a turn down.
Read that last sentence again.
        The Idea is to get the curve into a smoothly parabola like bow. But
the steeper the bow the more correction.  If your center area were bowed
nicely and the outer part of the arc flattened you would look at the other
side of the Ronchi line image(the convex side) to see where to wear the
outer part of the mirror to make the arc continue to the edge.  I hope that
is clear. I hope some of the more concise writers will know what I'm saying
and word it better. If you don't understand I will make a drawing and scan
it and send it to you.

Damn!  What a wind-bag! I've run out of screen to type on.


Jerry

--- BBBS/NT v4.00 MP