Few know that two primary mirrors were made for NASA's Hubble Space Telescope. The first mirror, the "flight" mirror, is currently aboard the telescope. ITT made the second primary mirror, to be used as a "back-up," in case the flight mirror was damaged prior to the telescope's launch in April 1990.
Measuring 97.5in. (2.5m) in diameter, and weighing 1700 lbs. (773 kg), Space System Divisions's Primary Mirror #2 is currently on display at the Smithsonian National Air and Space Museum. It is identical in design to the flight mirror currently in space. However, this ITT-manufactured mirror does not suffer from the spherical aberration discovered in the prime mirror after launch.
ITT's tasks were to grind, aspherize, polish and test the backup primary mirror. The project's exact requirements (dimensional, figure, and surface quality) involved 16 months of effort in 1979-80, with follow-up evaluation in a simulated gravity-free condition.
In 1979-80, state-of-the-art optics fabrication still relied on highly skilled opticians using manually controlled tooling and delicate hand polishing techniques. Today, using sophisticated computer numerical control tooling and equipment, such as waterjet cutting and ion figuring processes, ITT fabricates similar mirrors in just a few weeks.
Shaping the Hubble Mirror
Corning Glass Works fabricated a 13in. thick (0.3m thick) blank for the Hubble primary mirror. It consisted of a glass honeycomb core fused between 1 in. thick (25mm thick) glass front and back plates. Although five times lighter than a solid blank, the honeycomb structure weighed 2400 lbs. (1090 kg.)
At ITT, opticians shaped the backup mirror blank on a large Blanchard grinder equipped with a special diamond-cutting tool. They cut the front and back plates to a spherical shape, and rounded, beveled and polished the inner and outer diameter edges. The process reduced plate thickness from 1.5 to 1.0 inch, lowering the mirror's finished weight to 1700 pounds (773 kg.).
The mirror was then fine ground and aspherized on a large, loose-abrasive fine grinder. Removal of approximately 0.030 inch of material left the mirror surface to within 2 waves (1.3 microns) of the required surface figure. The mirror plates were then shined to a specular condition prior to testing. To work both sides, opticians used a special vacuum lifting device to flip the mirror, and move it from one processing machine to another.
Polish, Test, Evaluate
The polish, test and evaluation phase reduced the initial figure error of 2 waves (1.3 microns) to just 0.1 waves (0.06) peak-to-valley across the entire mirror surface.
A large Cartesian coordinate (X-Y) polishing machine polished the mirror. Then it was moved to the base of a rigid test tower where it was tested by an interferometer located 40 feet above the mirror. Convergence on the precise figure required 18 polish and test cycles, during which the mirror was supported on an inflatable air bag device. The air bag deflated for polishing, and inflated for testing to simulate the gravity-free condition of an orbiting space telescope.
To view the mirror's figure error, opticians recorded interferograms on film. These interferograms were analyzed to create a topologic map, where the surface imperfections could be measured in millionths of an inch. Computer programs assisted the polishing team to design the optimum polishing tool patterns to maximize the rate of convergence to the required figure.
The average time for each polish/test cycle was six days. Total accumulated polishing time was just 74 hours.
The ITT-built backup primary mirror for the Hubble Space Telescope does not suffer from the spherical aberration discovered in the prime mirror after launch.
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