Shoot, we don't care about our lungs... .it's that expensive piece of glass
that gets us worried ;)
Michelle
________________________________
From: sf-bay-tac-bounces@No-Spam
[mailto:sf-bay-tac-bounces@No-Spam] On Behalf Of Lynne Jolitz
Sent: Wednesday, March 02, 2005 11:36 AM
To: The Astronomy Connection
Subject: RE: [TAC] (OT) RTV and secondary mirror
Oh, gee, like it really fills up the room and suffocates everyone
and destroys lungs and stuff, so you have to use the more expensive product.
:-)
It's just sticky stuff. Anyone who tells you otherwise is selling
something. -l
----
We use SpamQuiz.
If your ISP didn't make the grade try http://lynne.telemuse.net
<http://lynne.telemuse.net/>
-----Original Message-----
From: sf-bay-tac-bounces@No-Spam
[mailto:sf-bay-tac-bounces@No-Spam]On Behalf Of Grimly Fiendish
Sent: Wednesday, March 02, 2005 11:25 AM
To: The Astronomy Connection
Subject: RE: [TAC] (OT) RTV and secondary mirror
Lynne,
Actually it is silicone, the MSDS lists it as RTV Silicone
Adhesive Sealant , and personally I would be reluctant to use it because it
out gasses. I would recommend the GE Silicone II that Michelle uses because
the only out gassing is a little bit of acidic acid which is not as
corrosive as the RTV materials.
Kevin
Lynne Jolitz <lynne@No-Spam> wrote:
Room tempature vulcanizing rubber (RTV) is also
called "silicone" rubber. You can buy it in hardware stores, but you've got
to read the ingedients labels carefully (easy to get fooled). You want to
use one that is an adhesive - RTV is used in many other apps, such as
caulking, but that's not what you want. Suggest clear (less filler).
the blot should be enough such that one can hold the
mirror comfortably. Usually a dab is good enough to hold quite a bit because
too much induces astigmatism with a thin mirror. More isn't necessarily
better. Thinner films of RTV work better than thicker films.
Clean off all of that old stuff (cork, glue, paint,
moths) - get rid of it all right down to the metal and glass, keep the RTV
as thin as possible, - and start fresh. The blobs should be squeezed down to
about 1/16th of an inch thick and no bigger than one inch in diameter w hen
you adhere the metal to the mirror, so you have about 3 square inches of
adhesive holding your 4.5in mirror.
It's a bit of an art. But if you find you do too
much, you can remove it carefully with the eucalyptus oil and an exacto
blade and try again. -l
----
We use SpamQuiz.
If your ISP didn't make the grade try
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> -----Original Message-----
> From: sf-bay-tac-bounces@No-Spam
[mailto:sf-bay-tac-bounces@No-Spam]On
> Behalf Of Mike J. Shade
> Sent: Wednesday, March 02, 2005 10:25 AM
> To: The Astronomy Connection
> Subject: RE: [TAC] (OT) RTV and secondary mirror
>
>
> You mean the RTV like the stuff for automotive
gaskets? They
> make blue and
> black...how big of a blot? I did get 100% silicone
or might this
> be a bad
> idea? You offered the use of cork...don't have
any, is this critical?
>
> At 10:27 AM 3/2/2005, you wrote:
> >RTV is a good choice if your surface is
well-prepared. We've used it.
> >
> >Don't do a bead (warping because of different
thermal expansion rates) -
> >do point contacts. For theory, google "mirror
cell plop" to
> calculate the
> >mechanical strain of a mirror against a plate.
Looks like for a .875in
> >thickness blank the max diameter is 8.7in. So a
4.5 is about .5in thick.
> >Try for 3 equidistant points - it would help to
have one in the center,
> >but when you try to remove it you're going to
have a difficult time, so
> >it's your call.
> >
> >Clips are a bad idea anyway - use the cork you
have if you can. But get
> >off that old cement completely - they're
incompatible glues. The cork
> >probably rotted because it was layered between
two nonporous
> surfaces, and
> >cork is very hydrophiliac. My father-in-law would
probably hav e
> put it in
> >a low vacuum chamber for a few weeks, and done
different pull tests to
> >make sure that any one of the pad areas was big
enough and strong enough
> >to hold the mirror, but he worked on satellites
at Ford
> Aerospace / Loral
> >(see http://jolitz.telemuse.net/wljolitz.) :-)
> >
> >An excellent solvent for RTV is eucalyptus oil,
but don't get
> too much of
> >it on your hands (poisonous in quantity) nor let
it near children / pets
> >(keep it locked up). You can get some cheaply at
the "Herb Room"
> in Santa
> >Cruz.
> >
> >If you don't clean well enough, RTV doesn't
adhere well and the
> weight of
> >the mirror will cause sagging or (worse yet) fall
off. RTV
> doesn't adhere
> >well to enamels, BTW, so go to the metal surface.
-l
> >
> >----
> >We use SpamQuiz.
> ; >If your ISP didn't make the grade try
http://lynne.telemuse.net
> >
> >
> > > -----Original Message-----
> > > From: sf-bay-tac-bounces@No-Spam
> [mailto:sf-bay-tac-bounces@No-Spam]On
> > > Behalf Of Mike J. Shade
> > > Sent: Wednesday, March 02, 2005 8:41 AM
> > > To: sf-bay-tac@No-Spam
> > > Subject: [TAC] (OT) RTV and secondary mirror
> > >
> > >
> > > I'm going to reattach the 4.5" secondary
mirror for the 12.5"
> > > Ritchey-Chretian. It looks like there were
three cork pads,
> > > about 1.5" and
> > > these were glued to the back plate and the
mirror was glued to
> > > these...I'm
> > > thinking that three buttons or RTV might be a
better choice,
> > > about the same
> > > spacing but perhaps not quite as
large...anyone have any thoughts on
> > &g t; this? Might it be better to put a thin
bead at the edge? More
> > > cork (which
> > > I suspect might have been stressing the mirror
a bit)?
> > >
> > > Clips are out of the question...
> > >
> > > Thanks...BTW. the original cork pads are
destroyed and it
> looked like the
> > > previous owner used windshield repair cement
(that yellow
> looking stuff
> > > that dries hard)...it doesn't look like RTV...
> > >
> > >
> > > Mike J. Shade: mshade@No-Spam
> > > Director, Sonoita Hills Observatory, Sonoita
Arizona
> > > www.sonoitaobservatories.org
> > >
> > > See work done at the observatory:
http://c3po.cochise.edu/astro
> > > under the "images & photos" panel on the left
of the page
> > >
> > > Fight wasteful and obtrusive outdoor nighttime
lighting:
> > > International Dark Sky Association:
www.darksky.org
> > >
> > > "I like the dark, it's cheap." Ebinezer
Scrooge
> > >
> > >
> > >
>
> Mike J. Shade: mshade@No-Spam
> Director, Sonoita Hills Observatory, Sonoita
Arizona
> www.sonoitaobservatories.org
>
> See work done at the observatory:
http://c3po.cochise.edu/astro
> under the "images & photos" panel on the left of
the page
>
> Fight wasteful and obtrusive outdoor nighttime
lighting:
> International Dark Sky Association:
www.darksky.org
>
> "I like the dark, it's cheap." Ebinezer Scrooge
>
>
>
Received on Wed Mar 2 15:16:03 2005