These values may be a better fit for what I am finding at the test stand and in the telescope. These values are similar in relationship to each other as above but are more modest in comparison to flat black mirrors. All dimensions are in inches.įor meniscus mirrors if we take the 'total thickness', that is, the edge plus the sagitta, then we get the following table and chart. If we look at relative bending for the edge using the empirically derived formulation R^4/e^2 where 'R' is the radius and 'e' is the thickness, then we get the following values. These 30 inch slumped to F2.8 mirrors are just under 3/4 inch thick (finished thickness is 0.64 inches ). The flexibility of glass increases exponentially with diameter so larger glass needs to be proportionally thicker to retain the same stiffness. And you may have to change your mirror making process from one designed to work with stiffer full thickness mirrors to one that takes advantage of more flexible thinner glass. However, thinner glass is harder to support during polishing and calls for a carefully designed mirror cell that supports the mirrors. Thinner glass means lighter weight tube assemblies and faster cooling to match the dropping night time temperature. Glass bends at the nanometer scale - the thinner the glass the more it bends and shears. The glass substrate is supported by a 'mirror cell' which holds the glass and is in turn held in position by the 'optical tube assembly' (which also supports the focuser and diagonal), which in turn is aimed at the sky by the telescope's mount. Glass became the most common substrate after Karl August von Steinheil and Leon Foucault invented the silver on glass process in 1857. Glass takes a beautiful polish and can be shaped accurately to nanometers - arguably the most precise surface made by man or machine. The metal coating must be held in position relative to itself across the mirror's face within a few dozen nanometers in order to sensibly perfectly focus the light. The 'mirror' is the reflective coating, a few nanometers thick. To slump the 30 inch and 42 inch glass blanks I need to make a refractory mold with a precision curve that the mirror will be slumped over. The point of slumping is to prevent the center from being ground through on large fast mirrors and eliminate the rough grinding stage. Sagitta: 0.67 inches focal length: 84 inches F2.81Ĭomments: David Davis, a pioneer in large thin slumped meniscus mirrors, offered to slump large thin blanks for me using his homemade kiln. Mold dimensions: 31.2" effective diameter (31 3/4" total dia), 11/16" sagitta, 89" focal length Mirror slumped face down over convex mold. The more precise the mold, the less glass I need to grind. Process: Use precision refractory mold at David Davis with his homebuilt kiln. Goal: using a kiln, slump the mirrors to the proper curvature and anneal the glass Step #2: slumping mirrors and bending glass However, tempered glass can be slumped and annealed in a kiln as the heating removes the tempering. Never attempt to grind tempered glass - it will explode on you. Goal: 30 inch diameter by 3/4 inch thick table top glassĬomments: bought two 30 inch and three 42 inch (one broke in the kiln with a loud shattering bang) glass table tops. ultra-thin mirror lessons learned from a year of observing. ![]()
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