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Workers inspect Hubble’s primary mirror before installation.

Hopes were quickly dashed when Hubble began
returning data. Instead of crisp, point-like images

of stars, astronomers saw stars surrounded by
large, fuzzy halos of light. The problem was
spherical aberration; the edges of Hubble’s large,

primary mirror were ground too flat by just a
fraction of the width of a human hair. Although
perfectly smooth, the mirror could not focus light
to a single point. It had been ground to the wrong
shape because of a flaw introduced into the test

equipment used to evaluate the mirror’s curvature
prior to launch.

Although engineers designed Hubble with many replaceable components, the primary mirror was not one of them. However,
the ability for astronauts to upgrade the observatory in orbit ultimately led to a solution for this seemingly insurmountable
problem. Even before NASA launched Hubble, engineers were hard at work building an improved, second-generation camera.

Despite the promise of remarkable pictures due to its position above Earth’s atmosphere, Hubble’s operation started dismally. The image at left shows
a star field taken under ideal conditions from the ground. The center image shows the same view through Hubble’s initial camera, the Wide Field and
Planetary Camera (WFPC). While atmospheric blurring is gone and many more stars are visible, the effects of Hubble’s spherical aberration are also
seen in the halo surrounding the bright central star. The sharply focused image at right was taken with the Wide Field and Planetary Camera 2 (WFPC2),
installed during the first servicing mission with integrated corrective optics.

Ground-based image at 0 6-arcsec resolution WFPC image (before servicing) WFPC2 image (after servicing)

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