QUESTION:
How much better can the Hubble telescope see now, since the repairs were
made?

ANSWER from Chris Wilkinson on May 28, 1996:
The short answer is that the Hubble Space Telescope can see as well as was
originally designed.  The corrective mirrors were installed in 1993 and
work very well.

The long answer is:
Theoretically, when one observes a star, one sees only a point of light.
In reality, this is not the case, because all optical systems spread the
light in the image over a two dimensional area.  This is called the point
spread.

The Telescope's point spread function was designed so 70 percent of the
light would fall in a circle only 20 microns in diameter, with the rest of
the light spreading in a faint pattern of rings surrounding the bright star
core, like tree rings. Before the repair only about 15 percent of the light
fell within the 20 micron circle.  After the repair, the point spread was
restored to extremely close to the 70 percent it was designed to.  The
fantastic pictures that are released each month demonstrate the quality of
the improvement.

BACKGROUND INFORMATION:
You may wish to access this web site for further information:
http://www.stsci.edu/pubinfo/StarCatcher/1stServicing/NewImproved.html
http://www.noao.edu/hubble/wfpcII.press.html

Two months after Hubble was released into its 360-mile orbit, engineers
discovered the telescope's 2.4-meter (94 inch) main mirror was flawed. The
mirror was too flat near the edge by about 1/50th the width of a human
hair. The result: instead of light being focused to a sharp point, light
collected by the mirror spread into a fuzzy halo.

A new camera, called the Wide Field & Planetary Camera-2, had its
corrective optics built right in. To correct the telescope's other
instruments, engineers devised a special instrument, called the COSTAR
(Corrective Optics Space Telescope Axial Replacement) that would use
mechanical arms to place pairs of small mirrors in front of the openings of
the telescope's remaining three
instruments. Each pair of mirrors was shaped to properly refocus light from
the flawed main mirror.