The 2. 4 metre primary mirror of the Hubble Space Telescope (HST) seen during manufacture. In this frame, the rough shape of the mirror is being ground. Later operations used finer and finer wheels and paste to produce the mirror surface. After the HST had been placed in orbit in 1990, it was discovered that the primary mirror was suffering from classical spherical aberration. This was due to a grinding error of just 2 microns in the edge of the mirror which went undetected due to a fault in one of the test instruments. A servicing mission is scheduled for late 1993 by the Space Shuttle, in which corrective optics will be fitted to the HST in orbit.