Friday, July 11, 2014

140702 TimC: From the Workshop...#7

From the Workshop...#7 (from SBAU July newsletter):
by Tim C
    I believe we are at a point where we talk about polishing and testing. Before we do, I am going to take a brief aside and tell a good story - the story of the Hubble Space Telescope and what happened to it. I will not go into specifics of how the testing
was done. Instead, I'm going to tell you what happened and how a "shower" solved a giant problem. The Hubble telescope became a political football for a time as the huge cost was viewed as wasted money because the main mirror was flawed, and seemingly beyond correction. 
    The Hubble mirror was finished and figured on a ground floor room. Above this was a small, rather awkwardly situated room. This small upper room housed the testing equipment. This design made it possible to test the mirror in place, so there was no need to pick up the heavy mirror and put it on a testing rack. Instead, they only needed to open access to the test apparatus. The “Null Corrector" was an arrangement of mirrors and lenses held to each other with specific-length rods. The only problem was the "field caps" covering the ends of the rods. Reflections from these field caps introduced a 1.3 mm error into the optics. This error was overlooked due to pressures to finish the mirror. The damage was done. The outer edges of the mirror were over-corrected or too flat. 
    So, when the Hubble Space Telescope saw “first light” there was much consternation as to what happened. The views were terrible. How do you go about fixing something like this? Replace the main objective, or replace the corrector mirror, or use a series of corrective mirrors with the instruments on board? Any of these techniques should work but there was no way to get it done. Think of the spacesuits of the astronauts with their big bulky gloves. In principle there was no way to delicately replace the mirrors.
    In Europe a Strategy Panel met to address the different ways of fixing the problem. Jim Crocker, an engineer by training, thought if they could fix the cameras it would solve the problem. Maybe they could place corrective mirrors in front of the camera? But there were several instruments on board. One mirror would not fit all of the instruments’ needs. He went back to his room one night thinking hard on the subject and went to take a shower. The European shower system was unlike those found in America. A rod holds a showerhead in place. The showerhead can slide up or down to adjust to the height of the person in the shower.  During the day, the maids slid the showerhead all the way to the bottom of the rod. As Crocker slid the shower head up and pivoted it outward to be over his head, he had a vision of several different
“showerheads” swinging into position with their corrective mirrors in the light path between Hubble’s secondary and the science instruments. If they sacrificed one of the first generation instruments in place and plugged in this corrective mirror unit, it would fit the simple kind of maintenance the Hubble team had planned for decades. It would work. In fact, and as you may recall, it worked perfectly. Moral of this story: you just never know when or where a solution to a problem will present itself!
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