Re: [ATM] My first sphere

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Hi, Darran,

Congratulations on the progress and for sticking with
the project. It is very hard to make a mirror for the
first time by yourself.

I have a hard time comparing the smoothness of the
first (yellowish) ronchigram with the subsequent two
(red-blackm) ones, but that doesn't matter a whole
lot. The more you polish, the smoother it will get, in
genearl. If you use rouge or zirconium oxide, as
opposed to cerium oxide, then it will get smoother
still. I don't recall what you said you were using,
but it looks like cerium oxide. 

The situation as I see it is that you have a narrow
turned-down edge (TDE), but it looks smaller in the
last 2 photos than in the first one. You also have a
large, or should I say, broad central hill. The TDE is
harder to eliminate than the hill. In fact, a hill is
easy to get rid of, and a TDE is a lot of work to
eliminate.

In my experience, to get rid of a TDE requires a
couple of things:

(1) make sure that the lap is the same size, or
perhaps a tad smaller, than the mirror. Forget the
soldering iron for this; simply take a single-edge
razor blade, or else a utility knife, or a chisel you
don't need any more, and use it to bevel the edges of
your pitch lap. While you are at it, I think some of
the channels are closing up between your pitch facets;
use the blade to carve out the channels' edges. See
Texereau for technique. If the channels close, then
the lap can no longer conform to the mirror. The
micro-faceting looks fine as is.

(2) use a real short stroke (only about 1 inch, or 2.5
cm, or even slightly less, overhang forward + back)
while rotating everything, and no side-to-side
swinging motion (or V or W). Press well beforehand,
and do this short stroke for about 30 - 50 minutes. If
your pitch is not too soft, then this should eliminate
the TDE in less than an hour.

That being said, there are other strokes that will
work to remove a TDE, but they are more complicated to
describe. You can look them up in an old copy of one
of Sam Brown's books on making telescopes (copright
sometime in the 1960's), or in the new one by Karine
LeCleire and her husband (copyright 2004 IIRC),
published by Willmann-Bell. These TDE-removing strokes
do work, but they are also hard to do.

It seems to me that having pitch that is too soft is
probably the main culprit in TDE. the LeCleire book
and Texereau describe tests for that. Hardness is
obviously temperature-dependent. To make pitch harder,
you generally just boil it for a while to drive off
the more volatile elements. Obviously you would want
to scrape it all off the lap before doing this, and
strain out any little bits of plaster or whatever,
while molten, before pouring a new lap!

Then, after the TDE is taken care of, you should then
switch to one of the long parabolization strokes with
a lot of side-to-side W motion, first to bring the
center of the surface to a sphere, then later to make
it into a paraboloid.

Oh, another thing - if you can figure out how to
attach a band of copper about 1 cm wide and say 5 cm
wide, in a loop, around and firmly attached to the end
of your soldering iron, you will get something that
will act much more nicely in carving channels. It is
sort of a cross between a hot knife and a hot scoop.
Depending on your type of soldering iron, you might
need to drill a hole into the hot end of the soldering
iron, and perhaps to tap (thread) it, so that you can
insert a small machine screw to hold the scoop-loop in
place. We made a couple of these, and it makes this
method much easier. This method works well if you have
one of those rather crude, inexpensive soldering irons
with a somewhat obelisk-shaped end; you just remove
the obelisk and drill and tap into the body (avoiding
the heating element!). If you have one of the more
fancy soldering guns with a trigger and a sort of a
V-shaped heating element, then I bet that fastening a
short loop-scoop onto the heating element, with a nut
and bolt holding it fast, would not be hard at all.
Having the scoop makes it easy to remove unwanted
pitch without making big piles of pitch all over your
facets.

All this being said, using a pitch lap mold is much
easier in my opinion. I don't know how much it would
cost to mail one from the company over to Yorkshire,
however.

There are quite a few good web pages on interpreting
ronchi images. Look for 'matching ronchi test' using
any search engine. The best ones are by Mel Bartels
and Peter John Smith, IIRC. You can even read what
Leon Foucault had to say about checkerboard-type
Ronchi gratings long before Vasco Ronchi was even
born...

Keep up the good work, and be patient.

Guy Brandenburg

--- Darran Summerfield
<darran.summerfield@ntlworld.com> wrote:
> Hello everyone,
>  
> I would appreciate opinions and advice regarding my
> first mirror. It's a
> 10inch f6.4 that's fully polished, but now I want to
> get it to a sphere.
>  
> I've posed my Ronchi test pictures at this web
> address.
>  
>
http://homepage.ntlworld.com/purple_jenny/ATM10NewtPolish.html
>  
> I'd appreciate any advice.
>  
> Cheers, Darran
> West Yorkshire, UK


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