I recently received a spider and diagonal from Bryan at
Protostar. Initially I thought I might glue my secondary to the holder,
rather than use the shroud. Bryan recommends GE Silicone II, Dow Corning
733, or Devcon Silicone Rubber. He warns to used a silicone-based adhesive
only, and to use a fresh tube, noting there is a catalytic component that
evaporates with time. He also made the point that the mirror should not be
glued directly to the holder, but a small gap should be maintained between
the holder's face and the back of the mirror. He has some good pictures of
how to proceed at his website: http://www.fpi-protostar.com/ftp/instman13.pdf
I have since decided not to glue my secondary, but this is handy
information for the future.
Jane
At 10:41 AM 3/2/2005, you 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 when 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 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 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 have
> > 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
> > > > 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 14:16:36 2005