Re: [ATM] Hoping to Grind a Mirror

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

But if he makes a refractor, he'll need to grind four (4) surfaces, 3 of which will need to be convex, AND he'll need to get a crown and a flint. That's not easy at all.
Guy

Kevin MIchael Zabbo <chaosopher23@yahoo.com> wrote:
Reflectors do have drawbacks, and that silver coating
is the weakest link. I don't know how polluted Uganda
air might be, (sulfurous emissions from old cars,
coal-burning power plants, etc) but if you consider
that the thickest glass is 1.2cm, consider a refractor
of 120mm diameter rather than the Dob. You'll see
about as much and you will never need to re-coat.

As for abrasives, I don't know what to use. Fine sand
will take a lot longer than abrasives. You may need
to import the stuff.

Kevin of Eastern Iowa
Seeker of the Darkness

--- Kampala Uganda 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/
> 


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