This pair of images of a single star, taken with the European Space Agency's Faint Object Camera (FOC), demonstrate that NASA's Hubble Space Telescope has been restored fully to its planned optical performance.
The COSTAR mirrors remove the effect of spherical aberration in the HST's primary mirror. The FOC will now be able to observe extremely faint celestial objects with a clarity and sensitivity unmatched by ground-based telescopes.
An FOC image a star taken prior to the sts-61 space shuttle HST servicing mission that installed COSTAR. The broad halo (one arc second diameter) around the star is caused by scattered unfocussed starlight. Because of aberration, only a small fraction of the light is concentrated in the star's pinpoint image (.1 arc second diameter).
Following the installation, deployment, and alignment of COSTAR, the FOC met it's pre-launch specifications. Most of the starlight is concentrated into a .1 arc second circle, and the blurry "skirt" of light is completely gone. By comparison, large ground based telescopes can concentrate I/I 0th of starlight into an area smaller than one arc second, even under optimum observing condition. This clearly shows that the effects of spherical aberration have been successfully removed from the FOC.