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

["Jerry" <wa4guu@verizon.net>]

Well....  my advise is to finish your mirror yourself even if that means you
accept what you have as being the best it will get. 

Here are a list of don'ts 

Don't bevel again. You will risk damaging the polish for not much more edge
chipping resistance.

Don't sell the mirror. Aluminize it in whatever condition it is in when you
give up on improving the shape. Make the telescope. It will work as a
telescope as it is. And if you don't like the views you can then buy that
replacement mirror. I think of the two mirrors you will get the most thrill
from looking through the one you made. Actually I don't just think that, I
know.

Don't give up. You are almost done.



And now a few do's 

Resolve that you will finish the mirror no matter what. Do as good a job as
you can REASONABLY do.  You get to decide what is reasonable. When you have
reached the end of your desire to push it around, whether that is because
you have as good as you wished for, or you are just plain worn out, get it
aluminized and finish the telescope. You will be surprised how well it
works.

I think I have seen posts from ATMs in Montreal.  Remote help is difficult.
It can be done. But there is nothing like live realtime advise from someone
skilled. If you must you can do it on your own.

Do finish.


That settled now on to what to do to the mirror.  
A bit more information would be helpful. Are you using a machine or working
by hand?

It's helpful to know how far the grating is inside or outside or at least
how far it moved in which direction between images. It appears you are using
different sides of a single line for a knife in some of the Foucault like
images. That confuses interpretation.  You say the first two images are
inside but shadows are reversed from one to the other meaning one side of a
line was used for one and the other side of a line for the other. I would
recommend using a real knife so that there is no chance of confusion. The
grating could be part of the reason for the overall fuzzy appearance of the
images. But they are good enough.

Anyway that hole is insignificant now and I would say it doesn't matter now.
It would be removed by parabolizing. I would say that you have a zoned
sphere. You have a slightly raised zone out around 40 to 80% zone that rolls
to a turn down. Nobody likes a raised area there because it is hard to
remove without rolling the edge more. It is maybe easier to work if you
don't concentrate on it. 

Look at that Ronchi and lets look at a single line. Lets use the first to
the right of center. Picture that line is the curve on your mirror viewed
from the side with the glass to the right and surface to the left. You want
to work to the parabolic like shape that would bow to the right toward the
center.  That is your target shape. I'm sure you have seen it in books or on
a website somewhere. So you can see your raised area needs to be pushed to
the right to match the proper parabolic like bow. Looking at it this way you
can visualize where and how much glass you need to remove. Let me know if
this makes sense to you. 

What you would do now depends on how much work you are willing to do to get
the edge better. The edge is the hardest part to get right so if you think
it will discourage you if you must work a while on that and maybe it might
not in the end be better, you might just go on to parabolizing. Almost all,
all except for the very lucky go through the frustration you are having.
Only you know how much more work you are willing to put into it. Above all
though remember my very first advice. You can implement that idea at any
time. When you decide you cannot or do not have to make it better, declare
victory and call it done. Once you have seen the sky with your mirror you
will have renewed enthusiasm for mirror making and you can star mirror
number two and make that one perfect. The biggest thing with mirror number
one is to finish it. Don't let anyone tell you it will not be good enough.
It will be good enough for a first mirror. It will work.  

You did not say what stroke you have been using. More details please

Jerry 
PS  two years ain't nothing







 

-----Original Message-----TRIMMED
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
RM L
Sent: Saturday, July 09, 2005 5:28 PM
To: atm@atmlist.net
Subject: [ATM] 12.5 inches BVC mirror polishing and figuring difficulties

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.





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