Re: [ATM] Bubble Wrap mirror Cell.

["Don D'Egidio" <djd521@comcast.net>]

Marco,

I seem to remember in all the previous discussions that bubble wrap did not have a consistency
between the bubbles.

Have you considered astro turf.

Don

----- Original Message ----- 
From: "Marco" <marco.miglionico@btinternet.com>
To: "AMT LIST" <atm@atmlist.net>
Sent: Sunday, September 24, 2006 15:46
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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> ATM mailing list http://www.atmlist.net/
>



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