Saturday, September 30, 2023

The Incredible Feat That Was Putting the “Glasses” on the Hubble Telescope

The Hubble Space Telescope is one of the most valuable scientific instruments ever built by humanity. It has taken many inspiring and historic photographs and made discoveries in many different areas of astrophysics. Since its launch 33 years ago, it has explored the Solar System, our Milky Way and even the early universe. However, when he started sending his first images we learned that his main mirror had a serious flaw. The mirror, about 2 meters and 40 centimeters in diameter, was polished to a very precise shape in order to focus light from objects light-years away to a very tiny point. During the polishing and manufacturing process, a spherical aberration was apparently introduced into the mirror, causing the images sent to us by the telescope to be slightly distorted and largely out of focus.

This meant that the billions of dollars that had been invested in the design, construction and launch of the telescope would be wasted unless a solution to the problem was found. Although some of the images taken at the time surpassed those taken by ground-based telescopes, they were not of sufficient quality for a telescope of that cost.

The problem was found that the outer periphery of the mirror was very flat and the light reflected from that part of the mirror was focused at a different point than the light reflected from the central part. It wasn’t particularly bad for brighter objects, but the loss of light from that turning meant that dim objects or images requiring high contrast could not be photographed. This made, for example, a complete program of cosmological observations of the ancient universe infeasible. So if we want to start using Hubble telescope then the mirror has to be fixed or replaced.

Comparison of pictures taken by Hubble before and after the repair. Photo: NASA, Hubble ST

Space missions are always complex, especially when they require extraordinary activities. A mission like this, which involved repairing one of the most sophisticated pieces of equipment ever made by man, would be a huge challenge. It was decided that replacing the mirror with a correctly sized part would be too costly and cumbersome for the astronauts involved in such a mission, and a corrective system was eventually chosen. A system was created that compensated for the original polishing error with a similar but opposite defect, which would be installed during a Space Shuttle Endeavor mission in December 1993. The mission lasted 10 days, during which six NASA astronauts and one ESA astronaut worked to repair the Hubble telescope. During the mission, he carried out five additional maneuvers lasting several hours, during which he not only installed the corrective system, but also made other electronic and computer improvements. This corrective system will work like glasses.

Our eyes, if they function correctly, are able to deform the crystalline lens to focus light coming from various objects onto the retina. However, sometimes it happens that this focus fails and they are not able to produce sharp images, so a lens is required which distorts the light that reaches us in a precise way so that our The imperfect eye can redirect light to a precise point. , That is to say, glasses introduce a distortion that compensates for the distortion that may be present in whatever part of our eye it manages to resolve. The system installed on the Hubble telescope did just that. The system was named COSTAR for its abbreviation in English.

After the repair was completed, the installed system was able to correct defects in the main mirror and the quality of images increased significantly. Since then the Hubble telescope has been visited on 4 more occasions by various missions of the Space Shuttle program, the last in May 2009. This telescope was designed with these missions in mind and its ongoing maintenance and best of all it is the system that has allowed it to continue to give us stunning images of the universe 33 years after its launch.

Telescopes such as the James Webb could not achieve such improvements, partly because they were not designed with that possibility in mind and partly because their orbits were made more difficult to reach. The Hubble Space Telescope orbits directly around the Earth at an altitude of about 540 kilometers above the surface, while the James Webb orbits around the L2 Lagrange point of the Earth-Sun system, thus about one and a half million kilometers from our planet. Is located at.


  • Goodwin, Irwin; Cioffi, Dennis F. (1994). “Hubble repair improves sight and helps restore NASA’s image”. Physics Today. 47(3), doi:10.1063/1.2808434
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