You Can Only Build Telescopes for Questions You've Already Thought Of

The mirror was flawless. It was also the wrong shape.In 1990 we lifted a telescope above the atmosphere, which is the single greatest upgrade in the history of looking at things, and discovered that its primary mirror was ground to the wrong curve. Not wildly wrong. Wrong by roughly two micrometres at the edge, a deviation you could not detect by staring, licking, or praying, but more than enough to smear every point of light into a fuzzy halo. The error is not the interesting part. Manufacturing errors are boring. The interesting part is how the error passed inspection. The mirror's shape had been verified with a device called a null corrector, an instrument whose entire purpose in life is to say "yes, this is the shape you asked for." The null corrector had been assembled with one internal component sitting slightly out of position. So it did its job beautifully. It confirmed, with extraordinary precision, a shape nobody wanted. Simpler tests were available and reportedly hinted…