[ATM] Bubble Wrap mirror Cell.

[Ross Sackett]

> From: "Marco" <marco.miglionico at btinternet.com>
>....> > 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 think that is the big snag with bubble wrap,
astroturf, thick felt, etc.  While the padding might
help bridge over small irregularities in the backing
plate, it will still shape itself to the overall
contours of the plate and present less force to the
mirror in those areas where the mirror and plate
surfaces are further apart, and more pressure where
they are closer together.  The result is that both the
mirror and the backing plate will tend to warp towards
a similar contour.  I don't think the bubblewrap will
take up *any* of the sag across the face of the plate,
just accomodate the irregularities smaller than, say,
half the contact surface of each air cell.  

The only way I see this working is to have a backing
plate much stiffer than the mirror (=very heavy) and
both the plate and mirror back ground to precisely
similar contours.  For example, plywood would be a bad
choice for a backing plate since it is much more
flexible across the grain than along it, nearly
guaranteeing 'stig in the mirror.  If it takes a 3"
thick steel backing plate ground flat to 1/4 wave
across its face to support my 18" mirror, I don't see
any advantage to using a bubble wrap pad over a much
lighter and cheaper flotation cell that works
adequately well even though machined to much looser
tolerances.

Of course, all this only makes a difference in larger
mirrors.  Standard thickness 6" and 8" mirrors are
nearly bombproof to mount, as long as you don't pinch
them.  I guess you could mount these on bubblewrap
pads over plywood if you felt the need, but on the
other hand supporting them on three equidistant points
at whatever radius you like will give figure errors
less than you will likely see under most conditions,
and will be easier to ventillate.

Ross








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