Re: [ATM] slow polishiing

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Bill,
I would suggest that you take a look at three sources.
One of them is a print resource, 
http://willbell.com/tm/atmmanual/index.htm
(A Manual for Amateur Telescope Making
by Karine and Jean-MarcLecleire. Hardbound, 6 by
9-inches, 320 pages, $29.95)

and the other 2 are web resources. One is Mel Bartels'
many pages on "the Matching Ronchi Test" (look it up
on Google), and the other is Peter John Smith's many
web pages on ronchi testing.

One of the excellent points made by Smith is that a
single ronchigram, like the one you showed us, means
nothing unless we know several things:
(1) the ROC and diameter of the mirror (you told us
that);
(2) the number of lines per inch on the ronchi
grating;
(3) whether you are inside or outside of the Center of
Curvature;
(4) and by how much.
One could say similar things about the foucault
knife-edge image that you gave us. It is pretty, and
depending on where the knife-edge was in relation to
the COC of the rest of the mirror, it might show that
things are OK. But since the ronchigram has that
unusual center, there is definitely something amiss.

OK, please let us know: when you did the ronchi image,
were you inside or outside of the COC (i.e., where all
of the lines disappear)? By how far? And how many
lines per inch is your grating? Otherwise, we don't
know if you have essentially a hill in the center or a
depression, for example, or whether there are other
problems.

If we know those things, then we can give intelligent
advice. (By the way, if you keep on doing exactly the
same stroke with the same lap, it is unlikely that the
mirror will magically transform itself into a
paraboloid.)

The LeCleire book has many excellent tips on what to
do if you encounter various problems. Often, remaking
the lap will change things dramatically.
Guy

--- Bill Mitchell <astronutski@yahoo.com> wrote:

> I'm polishing a 6" f/7 Pyrex blank (my first mirror)
> and it seems to
> not be progressing at all.  I'm using CeO with a
> Burgundy pitch lap,
> everything from the WB kit.  After about three hours
> I took a peak at
> it in the Foucault and Ronchi and was told I have
> the classic sphere
> within a sphere and all that meant was that it wan't
> polished out
> yet.  I wasn't expecting to be at that point.  Now I
> have over 7
> hours in it and there has been no noticable change
> to that sphere
> within a sphere that I saw at 3 hours.  I am cold
> pressing with mesh
> for 1/2 hour before each hour polishing session.  I
> have had to
> re-channel twice which from what I've read sounds
> about right.  I
> alternate TOT, MOT at about 3:1 TOT.  Using 1/3 COC
> slow strokes and
> I can feel really good contact.  I'm using about a
> nickel to quarter
> sized CeO plop which is thinned about 4 water to 1
> CeO.  I usually go
> around the barrel three times to a wet, about 15
> minutes worth.
> I'm not afraid of the monotonous polishing but I'm
> not seeing any
> progress and it's getting frustrating.
> Any help would be very welcomed!
> 
>
http://www.atmlist.net/contrib/astronutski-at-yahoo-dot-com/n3068.jpg
>
http://www.atmlist.net/contrib/astronutski-at-yahoo-dot-com/n3078.jpg
> 
> Clear skies,
> Bill
> 

=====
Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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