When the Hubble Space Telescope launched in April 1990, it was an instrument decades in the making. Originally called the Large Space Telescope, Hubble was the most advanced telescope ever deployed at the time and promised to give us a view of the universe unlike anything we'd ever seen before.
Almost as soon as we started taking pictures with Hubble, though, it was painfully clear that something was wrong. A “spherical aberration” was blurring the images, distorting the details, and making faint structures in the universe difficult hard to detect.
It was discovered that a slight flaw in the machining process of Hubble's primary mirror produced an error of just one-fiftieth the size of a human hair, but it was enough to reduce the quality of the photos considerably.
Corrective optics were installed on Hubble in a 1993 servicing mission that acted like a pair of corrective glasses to counter the effect of the aberration and Hubble went on to return truly awe-inspiring images for nearly three decades thereafter, but it was a hard lesson to learn that almost ended in disaster.
But NASA was determined to learn it before launching the James Webb Space Telescope, one of the highest-stakes deployments NASA has ever undertaken.
What the heck happened with Hubble anyway?
Hubble's primary mirror was fabricated by a company called Perkin-Elmer Corporation, beginning in 1979. The 7.9-foot/2.4-meter surface was ground with incredible precision, but when dealing with the incredibly faint light of distant stars, nebulae, and galaxies, incredibly precise sometimes isn't enough.
During the grinding process, miscalibrated equipment introduced a defect in the curvature of the mirror of just two microns, which is enough to blur the images Hubble takes.
NASA Jet Propulsion Laboratory director Lew Allen headed up the investigation into the defect and found that a reflective null corrector hadn't been measured properly and it directed a polishing machine to shape the wrong curvature into the otherwise perfectly smooth surface.
In fact, the resulting defect exactly matched the flaw in the offending null corrector, and a second reflective null corrector had actually identified the mistake. However, technicians rejected the test results showing the error and assumed a test showing the mirror to be properly shaped was correct.
Allen blasted both the contractor for its lack of quality control safeguards as well as NASA for not catching the error before launching Hubble. Both these conclusions left a lasting impression on the agency, and when work began on the James Webb Space Telescope just over a decade later, the conclusions of the Allen report would not be forgotten.
Preventing another Hubble debacle
The initial blurriness of Hubble's photos proven to be a major embarrassment for the agency, Conrad Wells, a Senior Optical Systems Engineer at L3Harris and a lead engineer on Webb's optical telescope element, told TechRadar on Monday. As a result, the agency took the matter of quality control very seriously this time around.
“When the Hubble Space Telescope went up, it had a little bit of a problem,” Wells said. “It turns out they didn't do that full, system-level test on the Hubble Space Telescope. They tested the primary mirror, they tested the secondary mirror, they tested all the systems in it, but they never took light from one side of it all the way to the other.”
That kind of full-system testing matters, since it is what most closely replicates the actual operation of the telescope in practice. Fortunately, Hubble was able to be repaired and the defect corrected, and those repairs and what engineers learned from that process lives on in Webb in a couple of ways.
First: test, test, test. In order to carry out the kinds of tests that would instill confidence in the James Webb Space Telescope, L3Harris engineers used the second largest vacuum chamber in the world and brought it down to less than 25 degrees Kelvin using compressed liquid helium to reproduce the operating conditions Webb will be working under at L2.
They then went through every step in the process of aligning the 18 panels that make up the primary mirror of the telescope just as engineers are set to do in the next couple of weeks, down to simulating starlight with fiber optics and ensuring that light accurately reaches all four of Webb's main instruments.
They also made sure that those instruments are all confocal, so that they are all able to focus on the same object properly. All while operating the optics at temperatures approaching -400 degrees Fahrenheit in a vacuum.
“All of us have a high degree of confidence that the optical system is going to work well,” Wells said. “This is the second time that we’ve done it, it’s not the first time. It’s going to work. NASA’s insistence, and our execution of that system level test, is going to make this mission a success.”
Another important legacy of that initial Hubble aberration is in the alignment and focusing of Webb's primary mirror, which is directly derived from the technology and techniques used to correct the original Hubble defect.
“The technology that they used to test [the corrective optics] in space,” Wells explained, “imaging starlight through it, and looking at that blurred image as you go to focus, that's actually how the James Webb Space Telescope is going to align itself on orbit.”
That process is now underway and will take several weeks at least, but Wells isn't nervous about how Webb's first light images are going to turn out. After all, he tested the alignment of the mirrors himself.
“It's going to work.”
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