ATM Mounting Large, Thin Mirrors

[Aplanatic@aol.com]

Hello Telescope Makers:

It’s been a while since I posted, other dragons to slay…  Anyway, greetings to all of my friends and acquaintances on the list.

I have been reading with interest the recent posts about mounting large, thin mirrors in alt-az telescopes.  The topic is timely because a group of us are about to make an OTA for a 25" f/8 Nasmyth Cassegrain.  

The significant problem with the standard floatation cell and sling is the transverse (compressional) forces applied to the mirror when the telescope is pointed toward the horizon.  All edge support methods seem to suffer from this.  On many occasions I have observed the astigmatism induced by the sling in a friend’s 25” Obsession when pointed near the horizon.  It was quite obvious.  In addition, no matter what we tried, we were unable to remove the side-to-side play of the mirror when the scope was tipped from zenith to horizon.  This motion was enough to cause collimation problems at some elevation angles.  In an attempt to rectify these problems, we RTV’ed the mirror to its 18-point floatation cell and dispensed with the sling.  This essentially eliminated both of the above problems, but created a new one.  On some nights a weak six-fold pattern could be seen in the star test and this almost certainly was due to strains induced through the RTV from the tr!
iangular floatation sections.  W
hether or not this was temperature related was never determined definitively.

So, I started thinking of various other methods to mount a large thin mirror.  The simplest mechanical configuration that seems to eliminate the sling-effect (at least on paper) is to take up the vertical (transverse) load due to gravity from a central support on the mirror.  One can either drill a hole in the center of the mirror and use a sliding tube or post through the mirror to take up this load, or one can glue a disk or annulus to the center of the mirror back and drill a close-fitting hole in a back plate in which this disk or annulus slides.  Surrounding this back plate would be a conventional floatation cell to take up and equalize the longitudinal forces of gravity.  If the mirror has a central hole in it already (because of forethought or because it’s a conventional Cassegrain primary) then the job is nearly done.  One simply adds a central, close-fitting tube or support post to the mirror cell and creates a conventional floatation cell around it.  If the mir!
ror does not have a hole in it (
and it’s far easier in my experience to polish and figure a large primary without the hole) then a Pyrex or plate glass or Aluminum disk is glued to the back of the mirror as the transverse load bearing element.  Ideally, the disk material should be identical to the glass used in the primary to eliminate TCE mismatch effects over temperature.

For a 25" Pyrex primary, I envision a 3" or 4" diameter Pyrex disk, one inch thick, glued to the center of the mirror’s back.  These small, molded Pyrex disks are readily available from several glass suppliers.  The disks that I have procured in the past are quite round.  This disk slides in a close-fitting hole in 0.25" thick Aluminum plate and the hole in the plate is made just large enough to account for the TCE mismatch, over temperature, between Pyrex and Aluminum.  For a 4" disk, the hole need only be 10 thousandths of an inch larger than the disk.

Several questions:

Have you seen such a mounting method?  If so, did it work well?

What type of glue would be used to bond the disk to the mirror?  RTV would be the obvious choice, but the normal, hardware-store type requires air to cure and is clearly not suitable for this application.  Are there two-part RTV-like bonding agents that do not require exposure to air to cure?

Do you see any difficulties with this method?

Thanks,
Dave Rowe