ATM Re: silicone caulk adhesive strength

["Albert Highe" <ahighe@ix.netcom.com>]

> One of the problems with silicones are that they are poor adhesives, ....
> I can't imagine  using RTV to stick a mirror to metal
> over an expensive mirror.  It is just not the right use for silicones.


We need to put the discussion of the strength of silicone caulk into
perspective. One of the reasons silicone caulk is used is because it is a
weak adhesive. If the only criterion was adhesive strength, an epoxy would
make an excellent choice.

However, most mirrors need recoating from time-to-time, so another criterion
for mirror supports (unless you plan on throwing away your mirrors rather
than recoating them) is that the method should allow the mirror to be
removed at some later time. That's why mechanical means are typically used.
Screws are almost as easily to screw out as they are to screw in.

The use of adhesives evolved from the perceived needs to elliminate
secondary shrouds that mask part of the secondary, and to remove mirror
clips that mask part of the primary and, if installed improperly, can
distort the mirror. The use of adhesives can also elliminate the need to use
a sling.

But because silicone caulk is a weak adhesive, extra care must be taken to
ensure the surfaces are clean and have high enough surface area. It does not
tolerate misapplication as much as stronger adhesives, like epoxies, do.

Silicone caulk, if applied correctly, is plenty strong to hold primary and
secondary mirrors, and can be removed much easier than epoxy. However,
silicone caulk is  more difficult to remove than mirror clips. Both
mechanical supports and adhesive supports can introduce distortion into
mirrors if done incorrectly. Both mechanical supports and adhesive supports
can allow the mirrors to separate from their cells if installed incorrectly.

So its not an issue of whether the use of silicone adhesives is right or
wrong. Its even OK to deviate from accepted "standard" practice. However,
anyone who builds a telescope should attempt to understand the principles
behind the designs rather than just copy them. Failure to understand the
materials and design tradeoffs can often lead to disasterous results or
inferior-performing telescopes.

Finally, in a post to the ATM list, April 11, 1996, Dean Ketelsen reported
that "100% silicone caulk you can get in any hardware store for as little as
$2.is currently holding the [Steward Observatory Mirror Laboratory's22]
Magellan 6.5 meter vertically in full shear, and has been for the better
part of a year. Granted, it only weighs 12 tons and is supported by about 36
square feet of bond area, but there is a huge safety factor and no one here
is worried about it."

Albert