The 12" Meniscus Mirror
My third telescope project pushes the boundaries of telescope technology, using kiln draping techniques to create a thin yet constant thickness meniscus-shaped mirror that will give bright views for cheap. This mirror will be 12" f/3.3, but only 0.5" thick. Only a few people in the world have tried making mirrors this thin and this fast.
Making mirrors this thin is a bit heretical for amateur telescope making. Telescope mirrors must hold the correct shape to within a fraction of a wavelength of light (500 nanometers). Glass may look rigid, but on small scales, glass is floppy and will happily bend under its own weight like a piece of paper. To resist bending, traditional mirrors are almost an inch thick (or often more).
However, large volumes of glass may take a long time - hours, even - to cool down when brought from warm indoors to cool outdoors. Because materials expand and contract with temperature, the difference in thickness between a larger edge and smaller center will change the mirror’s shape during those hours of cooling down. Traditional mirror owners must wait many hours for their mirror to cool down to ambient temperatures before the stars will look pinpoint. A meniscus mirror is thin - only 0.5" thick - and its curved shape gives it a constant thickness the entire way through, reducing the problems of cooling.
My 0.5" thick meniscus mirror with 0.25" sagitta, bending-wise, should like an 0.75" thick traditional mirror with a thickness of 0.5" at its center. Less glass means it is easier to carry and cools down faster in the cold night air.
Also, buying thick glass cylinders costs hundreds of dollars. I bought a glass countertop from an used furniture store for $20 and cut a hole out of it.