The Hubble Space Telescope on its 35th birthday

April 24, 1990 was the launch of the Hubble Space Telescope (HST), one of the satellites most credited with new scientific discoveries.  

"Hubble is more scientifically productive now than ever before, which is kind of mind-blowing," Jennifer Wiseman, the Senior Project Scientist for Hubble at NASA's Goddard Space Flight Center, told Space.com. 

For years, astronomers had wanted to get an "observatory class" telescope into space; there had been smaller instruments but nothing in the size class as HST, a 2.4 meter (94") aperture Ritchey Chrétien type of Cassegrain reflecting telescope.  It was certified for operation on May 20.  RCs have become the preferred optical configuration in high end research observatories because they provide a wide photographic field with less optical distortion off axis than others. There are a few types of optical Cassegrain reflectors, which have a concave primary (big) mirror and a convex secondary mirror that typically is around 25% to 30% of the diameter of the primary.  In the RC, both mirrors are hyperbolic (a hyperboloid of revolution).  The primary is concave and the secondary convex, as shown here.

Convex mirrors of all sorts are more difficult to test than concave mirrors, and require another calibrated mirror to test against.  It has become common practice in testing optical systems to develop a test system with its own optical surfaces that need to be certified correct and use that to test the system being built.  According to a report in New Scientist in 1990, the issue with the HST was because the test system was built incorrectly and yet certified. The problem?  One of the mirrors in the test system was positioned 1 mm off its intended position.  The first images were glaringly bad due to this.

My fellow metal workers will instantly think of how absolutely huge that is.  The 1mm error is just over .0394 inch and "that's bigger than 1/32" so even woodworkers can work to that!"  (Sorry).   The surface is being judged compared to wavelengths of light, around 20 millionths of an inch.   Because of this error, Hubble's primary was made flawlessly to the wrong shape.  It has been said that an amateur with a light bulb and a straight edge - a Foucault tester - could have told them their mirror was wrong.  And that amateur would have been ignored because their instrument wasn't certified for something that exacting.

I've mentioned a couple of times that I've made a few telescope mirrors grinding glass and then building the telescope.  I've done a lot of Foucault tests.  I think even I could have told them their mirror was wrong.

The fix, called COSTAR, the Corrective Optics Space Telescope Axial Replacement, flew up in 1993, and was removed in 2009.  Wait!  Removed?  Didn't I say it had bad images without COSTAR?  While Kodak had ground a back-up mirror for Hubble, it would have been impossible to replace the mirror in orbit, and too expensive and time-consuming to bring the telescope back to Earth for a refit.  Instead, the fact that the mirror had been ground so precisely to the wrong shape led to the design of new optical components with exactly the same error but in the opposite sense, to be added to the telescope at the servicing mission, effectively acting as the COSTAR to correct the spherical aberration.

In overview, the secret to the 35 year old telescope being more productive than ever before is things just like this: very clever people on the ground looking at something that needs to be fixed and figuring out a way to make it better.  For example, after that 2009 flight that removed the COSTAR fix, the coming end of the Space Shuttle missions meant losing the ability to reach the height at which HST orbits, and they identified the next most important system to address. 

Its longevity from thereon was thought to depend upon how long its gyroscopes, vital for accurately pointing the telescope, could last. Hubble was installed with five gyroscopes; the received wisdom was that it needed at least three to operate correctly.

By 2024, only three gyroscopes were left functioning. Then, one of them started malfunctioning.

"It became very noisy and difficult to work with, and ultimately was disruptive to Hubble's observations," said Wiseman. It seemed like perhaps time had finally caught up with Hubble, but others on the ground thought otherwise.

"Our brilliant technical team came up with an ingenious way of honing Hubble's point-and-control system so that it only needs one gyroscope," said Wiseman. "This one-gyro mode is now working very well."

And so, Hubble survived.

For lots more, go read the post that prompted this.