[ATM] Bubble Wrap mirror Cell.

["Marco" <marco.miglionico@btinternet.com>]

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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