COMPARATIVE VIEW OF A STAR BEFORE AND AFTER THE INSTALLATION 
      OF THE CORRECTIVE OPTICS SPACE TELESCOPE AXIAL REPLACEMENT 
                               (COSTAR)
 
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.
 
[left]
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).
 
[right]
Following the installation, deployment, and alignment of COSTAR,
the FOC met its 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 1/10th 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. 


PHOTO RELEASE NO:  STScI-PR94-08