MOUNTING SMALL MIRRORS WITH GOOP
Actually, there really isn't much to this. Mostly this brief article is a response to my upset at seeing fairly large mirrors attached to particle board or plywood with three gobs of silicone caulking material or similar RTV type stuff (hereinafter to be referred to as goop). One of the amazing things about the times in which we live is the proliferation of inexpensive and readily available high-tech miracle adhesives that allow us to radically simplify and shorten construction time and even to totally redesign instruments. My own construction techniques are based entirely upon the availability of polyester resins, plywood, particle board and cardboard tubing. To many people these materials still conjure up a hamstrung amateur project, but to those of us who build for the love of it and derive no particular joy in taking things out of boxes, imaginative use of these materials can result in truly amazing and self-rewarding creations. I'm always inwardly amused when someone walks up to my 6" refractor and starts asking me what it's made of. "Is it metal?" "Nope." "Ehhh... plastic?" "No." "Well, what is it?" "Cardboard and wood." "..... oh."
I give you the preceding only to make it clear that I in no way deride the naively simple in favor of what is traditionally considered sophisticated by way of complexity. If someone finds a way to reduce the number of parts, all the better - as long as it's done well and does not result in reduced performance.
My concern is that I have seen primary telescope mirrors of 6" and even 8" directly mounted to particle board by means of only three dollops of goop. While people claim that this does not affect the performance of their optics, common sense tells me that this procedure cannot possibly be a good one. As I have stated in my article describing a simple mirror cell, optics, particularly larger ones, need to be able to move freely. In fact, they need to move completely freely. Anyone who has ever manufactured optics understands just how detrimental to good performance the improper holding of an optic can be. I for one, would love to see a series of tests run on mirrors stuck to particle board with goop. I have never seen such tests, nor have I heard of any, and I would welcome any concrete information anyone might have on the subject. But, I emphasize, I'm looking for information regarding the examination of high-quality optics under demanding circumstances. And, preferably, in controlled test environments. Two things brother me: the optic itself expands and contracts, and the particle board not only expands and contracts at a different rate than the optic, but particle board is a porous hygroscopic material that simply expands and contracts according to the amount of humidity in the air. So, as far as I can detect we have three different sources of motion, three of which are different from each other in terms of degree and two of which are different in terms of propagation.
Degooping a 16" thin mirror with a coarse bow saw. This mirror was seriously distorted in manufacture and will be remounted on a molded plaster backing or fixture and repolished.
See confessions of an ex-gooper below.
Given what I have just said, I think there is ample opportunity to successfully mount smaller mirrors using goop rather than complex and sophisticated metal cells. I would say that this would apply to the telescope primary mirrors 6" and smaller and diagonals 3" and smaller. My wrinkle is simply to install a barrier material of some kind between the optic and the particle board, or metal, or whenever the optic is to be mounted on. For this purpose I have successfully used 1/16" felt. I would imagine that other materials can be thought of that would work equally well. The caveat appears to be that the material has to be able to support the optic in such a way that it will not move enough to matter, yet provide complete freedom for expansion and contraction (or even actual distortion) of the surface upon which the optic is mounted. When spreading the goop onto the optic and mounting board one should be careful not to use more than just a smear. If the goop is too heavily applied, it will soak through the felt and once again create a solid and relatively unyielding mass. This will not be a problem if materials other than felt can be found that are sufficiently flexible but not so porous. I've had good luck using this technique in the mounting of diagonals though I have not applied it to primary mirrors. I would suggest, for a 6" primary mirror, cutting three circular pads approximately 2" in diameter and spacing them 120 degrees apart at a position approximately 1/2" in from the edge of the mirror. After the mirror is mounted, one should be able to jiggle it and feel it freely move ever so slightly. If the mirror were gooped to the backing such a jiggle would not be felt.
Any thoughts on this subject are welcome.
In response, here are the comments of an ex-gooper
I was interested to read your comments on attaching large mirrors to
particle board cells with goop. I own a Meade 10" Dob and about a year ago decided to replace the particle board cell with a
metal one from University Optics. I found when I came to try to remove the mirror from
the particle board that it was attached by three 3" diameter pads of goop. It took me close to a full day of very arduous
work to cut through all of that. Now that the mirror is mounted better, the optical
improvement is unbelievable. Not only have tube currents diminished to almost zero, but the image has improved in resolution
and contrast now that it is no longer being systematically distorted by the goop.
Meade's mirror is pretty thin (1") so the distortions were greatly magnified.