What happened to the Hubble mirror?
The mirror of the Hubble space telescope was ground with the aid of a flawed measuring device. Although the mirror was perfectly ground, it was given the wrong curvature and thus did not form a clear image at its focus. Light from one star that hit different points on the mirror did not converge to a single point on the imaging chip. To correct for this problem, the astronauts inserted a corrective optic into the path of the light. This refractive lens compensates for the incorrect convergence of the light so that it reaches a single point on the imaging chip. However, because it is a refractive optic, it cannot pass all wavelengths of light. Any light that is absorbed by the refractive optic is no longer measurable with the telescope.