ATM Re: [obsessionusers] astigmatism with a sling?

["Frédéric Géa" <geaf@unionfinancieredefrance.fr>]

Hi,

For thick blank, the sling is not a bad choice but when the blank is thin
and large,
problems are possible. One of my friend got a 30 inc and often experment
astigmatism because of the stress of the sling after some collimation
change, and most of the time, he has to sligtly move the glass in the sling
to destress it. More and more people want to use large thin blank. They have
intresting feature. They cool faster and more important, track the
temperature of the air better, so images are quite often better thanks to a
better local seeing. For transport, they have intresting point to. The
weight of the scope is reduced and if one wants to transport the optic in a
seperate box, that's enable to have a larger blank for a given weight. The
problem is how to maintain their good figure. The sling is intresting but
can give some problem on large thin optic. A combo of astatic cell and piano
wire lateral support is really efficient. For thick blank, the sling is far
less a problem thanks to the higher rigidity but I have nonetheless use this
piano wire system for my  22inc, 2 inc thick, so on a thick mirror, and I
have found other benefit. A mirror can have stress in many diffrents ways
and point that support it musn't "glue" it at all. A pefectly polish back
help a lots but when the scope is used on an equatorial platform, the tilt
of the platform can easily move the blank and "kill" the collimation. I have
experimented the new cell during a week trip. With the former cell, I had to
check collimation during the night but with this new system, I havn't touch
it for few night of observation in a row .



Frédéric Géa
http://www.astrosurf.com/altaz
-----Message d'origine-----
De : Jan van Gastel <jhm.vangastel@wanadoo.nl>
À : ATM <atm@shore.net>; obsession <obsessionusers@yahoogroups.com>
Date : samedi 24 mars 2001 19:37
Objet : [obsessionusers] astigmatism with a sling?


>I just read the (very interesting) article at
>http://www.astrosurf.com/altaz/lateralsupport.htm about the lateral support
>of a mirror. I just made a sling because I didn't trust the silicone I had
>used to attach my 20" f/5 mirror to its 18 pts cell.  I have never read
>before that I sling causes astigmatism. Have other sling users experienced
>this astimatism?
>
>Jan
>http://www.geocities.com/janvangastel/
>
>
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