Re: [ATM] The 16 inch Ultrathin ronchi pics

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

It is under-corrected.

Setting aside the difficulties of thin and just looking at the curve, the
center could be deeper. Looking at it that way it is a hump. There is
another parabola that would make the outer zones high. 


Looking at the last Ronchigram in the last group.
Starting in the center 30 % of the curve:

It looks like it is "more" under-corrected on a diameter at a 45 angle
bottom left to upper right. Looking at the inside edge of the first bands
from center, the bottom left bend inward and the top right does also making
the light between them lean to the left. If the upper the curve of the bands
were "pushed" outward there it would tend to right the lean. And of course
the same for the bottom left. 

Then just outside of the center 30% looks to be oriented on the other 45
angle. 

If I were working that mirror, I would have to be out of ideas on any other
way but (for lack of a better term) "spot figuring" to remove the figure  of
revolution errors. I would hope that when I got the process right, they
would fix themselves. When I ran out of ideas I would have to have an
interferometer. I wouldn't try spot figuring with a Ronchi. That shape is
just too complex for me to trust a thing I've said about what I think I see
on it in Ronchi.

But I suppose that as you go thinner you reach a point where there may be no
other way. 

Of course, I don't know Diddly about thin mirrors. Every thing I know about
it has been learned from your example and the list discussion of it.

I don't think it changed shape. It will work just as well in a scope as
before. At some power it will give satisfying views. As you increase power
you will increase awareness of defects.  I've looked through some pretty bad
optics that gave pretty good views. 

Jerry










-----Original Message-----
From: vorblesnak@peak.org
 
I got the mirror back today and had a half day of vacation left so I
stayed home and twiddled a bit.  I built a video ronchi device with a 1/4
x 20 TPI adjustment.  The final video was about 64 meg, or 6000+ images in
AVI, so I pulled some shots for discussion.

These first one is near the ROC, that was my index point.  Then I moved
one rotation or 1 20th of an inch and paused.

http://www.atmlist.net/contrib/vorblesnak-at-peak-dot-org/Ronchi%20Pictures/
ronc001%20ROC2.jpg

This one seems to show that there is a hump in the middle ..

http://www.atmlist.net/contrib/vorblesnak-at-peak-dot-org/Ronchi%20Pictures/
ronc007%20Hump.jpg

This is just inside the ROC one or two 20ths ..

http://www.atmlist.net/contrib/vorblesnak-at-peak-dot-org/Ronchi%20Pictures/
ronc008%20one%2020th.jpg

In you look closly this one seems to show a bit of twist in the line ..

http://www.atmlist.net/contrib/vorblesnak-at-peak-dot-org/Ronchi%20Pictures/
ronc012%20Astig.jpg

This is a stacked shot and if you look you can see that the center line
shits a bit between the two images.  Look at where it exits the edge of
the mirror ..

http://www.atmlist.net/contrib/vorblesnak-at-peak-dot-org/Ronchi%20Pictures/
ronchi%20compare.jpg

Finally a line of four posing the question, what is in the middle?  I say
a hump ..

http://www.atmlist.net/contrib/vorblesnak-at-peak-dot-org/Ronchi%20Pictures/
What%20is%20in%20the%20middle.jpg

The mirror did not ship well and the coating has gotten somewhat damaged. 
There are little abrasions here and there all over it.  BUT!  There is
plenty enough left to snag some moon shots when the new moon come out.  I
will also try for a nebula or two.

Finally, if anyone wants to see the entire ronchi movie let me know and i
will put it on a CD and mail it to you.

David Davis
Toledo, OR 97391

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