I just finished up some hypothetical maths, as I saw yesterday that someone else on the internets repeated my repeated assertion that a really big telescope lens would collapse into a black hole. I don't think that's correct. Now, the original author of the claim, claims to be an Astrophysicist PhD student, so I'm hesitant to correct his maths, as I am notoriously bad at maths. But we did come up with the same number for a "spherical" silica telescope: 248,544,369,352 m Om's Schwarzschild radius 252,000,000,000 m quarksandcoffee radius My problem was, that telescope lenses are disks, and not spheres. Given that I no longer know how to do calculus, I did some very basic maths on the gravitational pull at the edge of a very large and flattish type mirror, and kept coming up with diminishing values for "g" at the edge of my extraordinarily large mirror: g = 0.0000000005 m/s^2 given: radius = 130 billion meters thickness = 0.001 meters The above radius was based on us extracting all of the silica from the earth's crust (1.4E23 kg of SiO2), as, it seems people like to make mirrors out of glass, even though the working parts of the mirrors are made of aluminium. ps. According to my calculations, it would take a quadrillion earth crust's worth of silica to reach the Schwarzschild radius.