Chuck, my email server caused some problems this morning and I lost most of the earlier messages regarding your undertaking. I've given a lot of thought (but neither time nor $$$ yet) to upgrading my 18-year-old 10" alt-az, the design of which is based largely on Richard Berry's book, Build Your Own Telescope. I followed Berry's instructions for attaching the mirror to the plywood mirror cell, with silicone glue. I rarely need to re-collimate; usually I only have to tweak the collimation once or twice per year. Near-perfect out-of focus star images show that it seems to hold the collimation from observing trip to observing trip, and I have never noticed any mechanical stresses on the mirror. BTW, my mirror is a high-quality, full-thickness Parks mirror. Do you or does anyone else have experience using silicone glue on larger and/or thinner mirrors? What kind of mirror cell are you contemplating? Have you considered any alternatives to both solid-tube designs and truss-tube designs? If so, I'd be interested to know your thoughts if you're willing to share. Kim -----Original Message----- Michael, now you have me re-thinking some aspects of my design, due mostly to the scale of the project. Rich and I discussed this briefly a few weeks ago. A 17.5" mirror is considerably more massive than a 10" mirror! I think I'll re-examine some of the cell details with this in mind. Somewhere, years ago, I remember seeing photos of two approaches to remotely operating primary collimation screws, but both required a fairly loose setup, so large operating torques would not be required. Unfortunately this also meant that gross collimation was required almost every time.