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Re: [ATM] Bubble Wrap mirror Cell.
I have seen mirrors on bubble wrap several times. Twice I have seen (in
scopes several years old) that no air at all was present in the bubbles any
more and in fact the mirror was laying on a wooden disk. A couple of times
some I saw some of the bubbles had lost most air while others did not. And,
for new bubble wrap, you can't be sure all bubbles are on the right spot
under the mirror and you can't be sure all push against the mirror with the
same force. You already mentioned the insulation problem . It's not very
difficult to calculate and build a good floatation cell, so I can't hink of
a good reason to use bubble wrap.
Jan
http://home.wanadoo.nl/jhm.vangastel/Astronomy/
----- Original Message -----
From: "Marco" <marco.miglionico@btinternet.com>
To: "AMT LIST" <atm@atmlist.net>
Sent: Sunday, September 24, 2006 9:46 PM
Subject: [ATM] Bubble Wrap mirror Cell.
> I have trawled the archives and the web for an answer on this and there is
a
> brief discussion
> in 2003: about using bubble wrap to float the mirror.
>
> However the responses are inconclusive and anecdotal at best: as to
whether
> it gives good
> results or not. It seems so simple an answer and yet so unaddresssed a
> solution.
>
> I have a feeling that ATM'ers perception of the idea being ridiculously
> simple is ironically what
> is stopping serious research into the idea.
>
> As it stands, there is no testimony either way as to its effectiveness as
a
> mirror floatation system.
> So let me voice my thoughts, and please join in for criticism.
>
> The ideal solution is to apply equal pressure to all areas of the rear of
> the mirror surface, and also
> to minimise 'distortion' of the mirror at low telescope attitudes (The
sling
> doing a reasonable job
> of supporting the weight in this condition.
>
> There are 3 main areas of concern:
>
> 1) Bubbles insulate the mirror and therefore increase time to reach
> temperature equilibrium.
>
> 2) Bubbles are not uniform in size and do not all 'Push equally on mirror
> surface'.
>
> 3) Bubbles expand/contract with temperature change. Causing mirror shift.
>
> I think I can address all above problems/(mis?)conceptions.
>
> 1) Threre was written a few years back a good article in improving laminar
> airflow over the face of
> the mirror in order too improve seeing - blowing air paralell to the
> surface of the mirror. This would
> also have the effect of cooling the mirror more rapidly.
>
> 2) If my manufacturing knowledge is correct the jig to mould the bubbles
> will be accurately machine die cut
> and I would expect the roll of bubble wrap to be very uniform. This is
> conjecture however. I will try and
> contact a manufacturer and try to get some info on it's uniformity.
> Secondly I think
> if that there are and non-uniformities from bubble to bubble the
> flattening effect will spread the area of the bubble
> with more pressure, therefore spreading the load more so that pressure
> is the same per bubble. I could spray some
> dye on a bit of bubble wrap and see what pattern ends up on the back
of
> my 8in and 12in mirrors.
>
> 3) The mirror would only change the focus by at most a few mm. I don't
> believe the collimation will be changed as the mirror
> should only move down parallell to the optical axis. I can hear the
> retort, so let me pre-emp: The bubbles around the outside
> edge may cool more quickly. If point 1 is addressed and the mirror
face
> is fan cooled, the bubbles will contract uniformly
> as the mirror should cool unifirmly. If not cooled, the center will
> remain with more supprt than the outside egde; but the bubbles
> may take up the slack by some unknown amount. Again, simple
> experimantation with some dye may shed some light on the
> matter.
>
> Finally I would add that the rear of the cell (under the bubble wrap)
would
> have to be quite rigid so the three collimation bolts
> didn't just lift localised areas. Remember the bubble wrap will take up
some
> sag.
>
> I will be pleased to hear what everybodys thought are.
>
> Marco Miglionico.
>
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