In April of 1990 the American space shuttle Discovery launched up from the
earth into a higher orbit than ever before to deploy a special payload, the
Hubble Space Telescope above even the thinnest parts of the atmosphere Hubble
would be able to see far-off stars and galaxies without distortion and expand
our understanding of the distant past or at least that was the plan but by June
it was clear there was something a very wrong. The telescope with one of the most
perfectly ground mirrors couldn't focus properly, apparently the first great
space observatory was short sighted. Hubble's two and a half meter or 8'2"
foot mirror was rendered incapable by an error in its shapes less
than one fiftieth of the width of a human hair. The primary mirror suffered
from what is known as spherical aberration more often found in sloppily
made amateur telescope projects not multi-million dollar space telescopes.
However this was a problem almost a decade in the making
NASA had contracted the private firm Perkin-Elmer to make Hubble's optical
assembly and the fine guidance system. Construction of the primary mirror began
in 1979 and was completed in 1981 but during these years the whole project was
under threat, costs were rising and the scheduled to deliver on time was
slipping. NASA put the heat on the management of Perkin-Elmer pushing them to
keep up with the schedule or loose for contract. Behind-the-scenes the polishing
and testing of the primary mirror was underway. Because Hubble's mirror was an
aspherical design it required special apparatus to check its quality known as
a null corrector. Perkin-Elmer even designed their own high specification
reflective null corrector or RNC to test the exact standard of one micrometer or
one thousandth of a millimeter as required for the Space Telescope.
The RNC was a cylinder 76 centimeters two and a half feet high and about half
a meter or one and a half feet wide inside two mirrors and a lens were
arranged to create a wavefront on the Hubble primary mirror below. If the
mirror was fabricated perfectly this wavefront would show no contour lines
hence the a null in the instruments name. The apparatus was approved and set up on a
tower in a dedicated room, a process that took two weeks. The position of the RNC's
lens was measured by an Invar Rod, a precision made and measured rod of
nickel iron alloy named for its invariability, specifically low expansion
under different temperatures. If the length of this rod varied by even a
fraction of a millimeter the test measurements would be wrong and the
mirror would be ground to a wrong shape. As the technicians lowered the lens of
the RNC into position they noticed they couldn't get it all to fit together. the
Invar Rod stuck out by just over a millimeter. The ends of the invar rod
were rounded and polished because the positioning system used a light
interferometer rather than a mechanical measurement. As part of this process a
focused beam of light was reflected off the end of the invar rod. To make sure
that it was focused exactly on the center of a rod end, field caps were
fitted to hold the rods in position laterally, these caps had a hole in the
center to allow the focus beams hit the rod end and a non reflective surface
coating around it. At some point a small section of this non reflective coating
on one of the field caps was chipped off around the center hole. When the
technician was setting up the focused beam, he got a reflection from the
chipped area of the endcap instead of the rod end
which caused a 1.3 millimeter misplacing of the invar rod. Rather than wasting
time investigating why the assembly didn't match the Invar Rod the crew found
three metal spacers and squeezed them into the bracket holding the lens so now
it agreed with the measuring rod. Then the RNC was left in place so as not to
disturb its supposedly fine calibration. The result of this oversight in the
Assembly of the RNC would lead to the mirror being precisely polished but with
an error ten times greater than the specified tolerance. Every day for nine
months the crew polished the primary mirror periodically moving it on its
tracks to the null corrector to see how much closer they were to the final shape.
At the end of the long process the RNC gave a Null result, at last the
so-called perfect mirror was finished. To verify the mirror shape another test
was carried out using a cruder refractive Null corrector but this test
gave a different result this time it appears as though but Hubble's mirror
had a flaw but as the refractive null corrector had not been certified for
testing against spherical aberration the result was discounted. The fault had been
found but it had been ignored and moreover the discrepancy between the two
tests had not been investigated further and the written documentation of the fix
with the spacers was also misplaced. Over the four years that the mirror was
stored due to delays because of the Challenger shuttle disaster several
universities requested permission to test the mirror and all were turned down
because the manufacturers were confident about their own testing procedures. A
decade later the one and a half billion dollar Space Telescope was high above
the earth with a flawed mirror. There was a perfect backup mirror that
had been made by Kodak but it would have been impossible to replace the mirror
once in orbit and too time-consuming and expensive to bring the Hubble back to
earth of refitting. So in 1993 and ambitious and expensive shuttle mission
with five spacewalks managed to install corrective optics on the secondary
mirror to fix the problem. When the cause of Hubble's trouble was eventually found
Perkin-Elmer agreed to pay a fine $15 million dollars thereby avoiding a
lawsuit by the government. Both Perkin-Elmer
and NASA were heavily criticised for not picking up on the quality control issues
and relying upon a single test instrument. Hubble is still in operation
today and has been updated over the years and
it's expected that it will be in operation until between 2028 and 2040
when the natural decay its orbits will bring it back to earth if it is not
reboosted into a higher orbit. Until then it will continue to provide mankind with
some of the most scientifically useful and inspirational views of our universe
to date. So thanks for watching and I just like to say that this episode shirt
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