Re: [ATM] 20incher with segmented and ribbed mirror at 12Kg

["Andreas Derwahl" <andreas@duda-derwahl.de>]

Hello Ric

> It might be a new idea over in Europe,but there are
several of us here in the US and Canada that have been
doing fused ribbed glass for decades now(!).

I wasn't suggesting fused ribbed mirrors were new, and yes, I am aware there
have been successful projects (Richard Schwartz comes to my mind, tried
unsuccessfully to to contact him, is he still on the list?).
I have not seen somebody trying to make the front surface of the mirror out
of different segments, though.

> Strange how each new generation needs to learn that gluing
doesn't work .Had he Frit bonded the pieces he might
have had a chance.

Of course first thing that came to my mind when I saw Willems project was:
it cannot work! but I was very surprised to see the foucault picture:
http://www.astrotreff.de/upload/CVerstuyft/20060602/fouc.jpg
which in my opinion shows a decent near spherical shape - yes, it's with
segments and printthrough but for low power work it might work good enough
if you can get it stable and parabolic.
I'm not aware if glueing techniques have produced a decent mirror yet - I
know none -, but I still wouldn't count it out categorically.

> Some mirror coaters give a mirror a
slight pre-heat before coating the mirror and that may
have distorted the glass back to a sphere,plus if the
annealing was not done properly that too would have
added a factor in the shape change.

I would expect the changes to occur along the glue lines, giving a checkered
pattern, but not a change from paraboloid to spheric. But Willem hasn't
posted new Foucault pics after coating, so I'm just guessing from his
comments on astrotreff that he doesn't see checkered.

> if the annealing was not done properly that too would have
added a factor in the shape change.

The slumping/annealing cycle he posted was:

1: 60 min 20 degC > 640 degC
2: 15 min 640 degC
3: 30 min 640 degC > 575 degC
4: 900 min 575 degC > 375 degC
5: 720 min 375 degC > 20 degC

which sounds a bit on the short side to me, but hey, it's only 4 mm glass...

> Look at:  http://wwww.geocities.com/radicell2
for another layout scheme that does work in many ways
critical for success.

Sorry, your link doesn't work. Is it corrupt or is the error at my end?

Thanks for your comments,

Andreas

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