Re: [ATM] 12.5 inches BVC mirror polishing and figuring difficulties

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

First of all, remember that if you always use the same
stroke and technique, you will always get the same
result, so you probably should change something there.


I don't think you are pressing long enough; I would
press for about 1/2 hour. I would also put some
netting (the kind that onions and citrus fruit is sold
in) in-between the lap and the mirror after you have
heated them both up, and then push down very hard for
about 10 seconds to create little indentation lines on
the lap (mini-facets). Then remove the netting and
press with your polishing agent for about 30 minutes.

Are you doing mirror on top (MOT) or tool on top
(TOT)? There are those who claim that MOT causes TDE
(turned down edge). How long does it take for the
grooves or channels to close up? If it takes a long
time, as in many, many hours, then it might be too
hard to figure with easily. Why is the central part of
your lap a different color from the rest? What sort of
pitch is that anyway?

What I would do first is to try to tackle the
turned-down edge, because it is the hardest thing to
eliminate (IMO). A relatively foolproof and simple
technique there is to take VERY short
center-over-center strokes: maybe 1 inch overhang in
front and then 1 inch overhang in back. This takes a
while, however; do it for about 1/2 hour, after having
done the other steps I recommended above. You can do
this MOT if you like. This will raise the center
somewhat, creating a central hill, but that's
relatively easy to eliminate later. This technique
does not work quickly, but it does work on getting rid
of TDE.

(There are other techniques for eliminating TDE, but
they are trickier to implement, so I won't go into
them now.)

After you eliminate the TDE, then you will change to
an entirely different stroke, long ones with plenty of
side-to-side wiggle, for the purpose of eliminating
the central hill and then of producing a parabola.

What texts are you using to guide yourself? Texereau?
The LeCleire book? Sam Brown? Instructions from Andre
Taillon?

By the way, that mirror looks very nice - from the
photos, I don't see scratches, and it looks nicely
polished, so you are doing something right. I would
not give up now. Figuring is the hardest part and can
be frustrating for sure!

Pads are good for beginning to polish, but they are
absolutely worthless for figuring. IMO.

Congratulations on doing so much so far by yourself,
essentially without guidance. If you feel like driving
down to Chicago or Washington DC there are active
mirror/telescope making classes. Your knife-edge and
ronchi photos and annotations were very informative
indeed, and could be used as models to show other
folks what information should be provided to others if
you want to get help over the internet.

You should also consult the ATM archives and some of
the resources listed on my webpage, which will lead
you to many, many other sources of information.

Guy

--- RM L <atm@axeinformatique.com> wrote:

> Hello,
> 
> Please take a look at my BVC 12.5 in. F/4.8 mirror :
>
http://www.axeinformatique.com/temp/Mirror/BVC_Mirror.html
> 
> and please advise me if I should :
> 
> 1- Sell the thing to a more experienced polisher,
> and buy a completed mirror (It's my first mirror).
> 2- Persevere and do what ? How much energy still
> needed ?
> (I don't want to make another lap tool or anything
> like that. If it's too much work...)
> 
> It's been a while... more than two years since I
> began working on my BVC 12.5 inches Mirror. Worked a
> little bit on it last summer. I was discouraged.
> 
> Initially, everything went pretty well, I think for
> a rookie like me.
> After a while of polishing (I was using polishing
> pads) some pads came loose from the tool and I
> rearranged the pads.
> It was a mistake because all areas were not well
> covered and  the uneven polishing made a central
> crater. (in just a few minutes)
> 
> Dumped the pads, I started polishing with my lap
> tool.
> Some of the crater went away after a long time (it's
> much less apparent). Some rolled edge appeared.
> 
> Anyways I'm stuck, and I don't know what to do. I'm
> discouraged,
> and I may sell the mirror and the tool to someone
> more experienced,
> and buy one that is ready made.
> 
> My polishing tool seems to work very slowly. Or
> maybe I don't use the right stroke, or oxide
> mixture. Sometimes the mirror "squeeks" while I
> stroke.
> 
> I recently recreated the grooves on the lap..
> Today I polished for about 1/2 hour and I saw no
> change in the figure.
> My lap could be too hard... I always warm my mirror
> and tool in hot water, the press them for 5-10
> minutes, then start working.
> 
> Please have a look at my webpage
>
http://www.axeinformatique.com/temp/Mirror/BVC_Mirror.html
> 
> And tell what I should do. How much would someone be
> willing to pay for my mirror and tool ?
> 
> Thanks all,
> 
> B.T.W. I'm near Montreal, anyone close that could
> help ?
> 
> RML
> _______________________________________________
> 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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