Re: [ATM] Help with TDE

["RC Stargazer" <david@tabor.com>]

Hello, Hannes.

Sounds like your mirror is the twin to mine, as I also made a 203mm f6.2, 
and surpisingly enough, also had trouble with TDE.  I'm certainly no mirror 
guru, but I'll pass along what helped me fix my mirror.
The advice I was given at the time, was to return to 1/3 center over center 
stroke still using cerium-oxide, and just continue with that for a while, to 
return to spherical.  It took a while, but you should have it within a 
couple hours or so, depending on how long you went before you noticed that 
your figure was off.

I learned a couple things through that process, that depending on your 
circumstances, may or may not help you.

The first was that it's critical to make sure that you are polished all the 
way to the edge.  An unpolished edge can also look just like a TDE.  The 
remedy is still the same.  1/3 center over center until it's all polished. 
If you had a good foucault test at the start that didn't show an edge, then 
you may not have this issue, but it caused a little frustration on mine.

The second is not to let your pitch get too warm, like mine did.  Warm pitch 
can do strange things.  With mine, it prevented me from properly polishing 
to the edge (as the center of the pitch had expanded), and if trying to 
figure with warm pitch it would definitely throw your figure way off in no 
time at all.   Pitch that's too warm can also help create a TDE.  For 
example, in my house which is normally 22'C (72'F), I don't have to warm my 
pitch at all.  Just cold-press for 15-30 minutes and off I go.  As soon as I 
warm up the pitch, I start having trouble.

The third (ok... so I lied about it only being a couple) was that figuring 
is much more measuring time than pitch lap time.  Go around your mirror with 
W strokes a few times, and measure.  Sometimes it's no more than 2 or 3 
times around the mirror, and it's time to test again.  Measure very often.

Good luck, and keep plugging.  You'll get there, and you'll be much happier 
with a fixed mirror than a 'masked' mirror.
- Dave

PS:  Any ATM clubs in your area?  They can be VERY helpful in figuring your 
first mirror.  (Thanks again to the Springfield Telescope Makers for helping 
me!  www.stellafane.com)

PPS:  To those of you who ARE mirror guru's, please don't hesitate to 
correct me if needed or if there is a better way!



----- Original Message ----- 
From: "Hannes van Niekerk" <hannesvn@ananzi.co.za>
To: <atm@atmlist.net>
Sent: Wednesday, March 14, 2007 1:12 AM
Subject: [ATM] Help with TDE


>I am a novice in the ATM game, busy with my first project,
> a 203 mm f6.2 (at this stage) mirror. I went through the
> grinding and polishing stages alone, based on what I could
> learn on the internet. The first Foucault test was not too
> far off the mark, so I thought, well this is going to be
> plain sailing. Well I must have missed some vital piece of
> information, as I now have a mirror you can scare the kidds
> with. Can someone please point me to an article that will
> help me, step by step, to correct a horrible edge, even how
> to return the mirror to a sphere again and start the
> figuring all over.
>
> Hannes van Niekerk, South Africa
>
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