Re: [ATM] Hoping to Grind a Mirror

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Concerning the abrasives, you definitely can use sand,
but it will be slow, and you will have to learn how to
grade the sand by size, by a method known as
elutriation, which involves putting the sand into a
lot of water, mixing it up, and letting it settle out.
The larger particles settle to the bottom first,
followed by the slightly smaller particles, and so on.
The glass you have may work, but you need to find out
whether it hss stress or not. Crossed polarized film
will help you to find out if it has stress or not. If
you take 2 polarized sunglass lenses and hold one on
one side of the glass and one on the other side, with
the orientation of the polarized lenses at right
angles to each other, then you can see whether there
is strain or not by looking for uniformity.
Good luck.
Guy

--- Kampala Uganda <kampalaliving@gmail.com> wrote:

> Hello all,
>  I am hoping to make a 6" Dobsonian telescope, but I
> need some pointers and
> advice. I have a photocopy of the relevant chapters
> from the Texereau book,
> and I think I understand the (basic) steps required.
> However, I need to
> improvise on the materials needed, and I don't know
> where to start. So here
> we go:
>  1. Glass: The thickest glass I can find here is
> 1.2cm [BTW I am in Uganda,
> Africa, so life here requires improvisation]. The
> book suggests for 6inch
> glass that I want to build, I need about 1 inch
> thickness. Is this
> absolutely necessary? If so, what is the maximum
> diameter that I can have
> with 1.2cm thickness?
>  Furthermore, no one here can cut me a circle!
> However, I found on the
> internet some procedure for cutting the glass using
> a rotating can with a
> sharp edge, and lots of water with abrasives. Does
> this sound feasible?
>  2. Abrasives: I cannot find the carborundum
> abrasives. Even the eyeglass
> shops don't grind their lenses, they import them and
> just cut them to shape.
> I was thinking for the rough grinding, I could use
> fine sand (ditto for the
> cutting the circle above). Is this possible? For the
> fine grinding, I was
> thinking of a paste that is used for smooth grinding
> car engine pistons
> (when they have a carbon buildup). I have no idea
> what the size of the
> particles in the paste would be, and how hard they
> would be.
>  Is there any other sources of abrasives? There is a
> relatively well
> developed automotive maintenance industry here. Also
> there are knife
> sharpening stones, which may be carborundum. I will
> use them for bevelling
> the edges, but I don't know if there is any way I
> can get powder from it
> while controlling of the particle size (upto 2
> micrometer?).
>  3. Polishing: Are there more traditional methods to
> produce the polishing
> agent: rouge? Its chemical composition is Fe2O3, but
> surely the size of the
> particles also matters? The only place I have not
> investigated is jewellers
> (I am sure there are some).
>  There is an architectural glass shop here, but they
> said their cerium oxide
> is solid blocks that their machine takes, they don't
> have powder cerium
> oxide.
>  4. After this I have to start worrying about the
> reflective coating and the
> eyepiece, but to be frank, I will have my hands full
> figuring out (no pun
> intended) items 1-3. From the little I have
> researched on the net, I will be
> able to make the reflecting surface from silver
> nitride. However, with the
> eyepiece, would I be able to make that using the
> same process as the two
> mirrors?
>  Buying/receiving from outside the country is almost
> impossible, as the
> postal system is unreliable at best. In any case, it
> would be great if I can
> make it with local materials.
>  Thanks!
>  K.
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


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