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COE Alum receives a NASA Distinguished Public Service Medal
On April 16, 2009, Lester M. Cohen was awarded the NASA Distinguished Public Service medal "...For your crucial and hugely significant role in the development of the lightweight telescope structure and mirrors for the James Webb Space Telescope and the highly successful Chandra Observatory..."
Lester M. Cohen came to the CfA in October, 1978, as an engineer to work on AXAF (later renamed the Chandra X-ray Observatory). His mission was to gain as much insight into what the prime contractors were doing, verify their calculations, and if possible to make sure that small problems did not become big ones. That meant doing everything that the prime contractor teams were doing, and just as fast, but with a much smaller group. Just as important a goal was to ensure that the scientific instruments (telescope, camera, etc.) performed in orbit at least as well as the minimum specs, so that all of the scientific goals could be met.
Cohen's areas of expertise include structures, structural mechanics, and mounting and fabrication of optics -- areas that basically cover a telescope from end to end. Since space telescopes are by necessity complex, the computer models used to simulate and predict ground performance (that is, the fabrication and testing) and the in-orbit performance are also complex. Meticulous care must be used in creating the numerical models (he uses the Finite Element Method, and in particular the �ANSYS Program�) that simulate what is physically going on.
The James Webb Space Telescope mirrors are made from a very pure form of beryllium called O-30. This beryllium is not perfectly elastic. Computer models must simulate the non-linear behavior, and the models that Cohen has created do so quite well. Moreover, during the fabrication of the mirrors stresses can develop, and if they are not eliminated the mirror will gradually change its shape-- a very bad thing. Cohen and his team developed the creep equations for O-30 beryllium (with data provided by Draper Labs) and now use it to accurately predict how well residual stress must be removed during fabrication.
Cohen and his team have also spent considerable time optimizing the grinding and polishing tools that are used to fabricate the optics. This requires making numerical models of grinding and polishing tools, putting these numerical polishing/grinding tools onto a numerical model of the mirror, and then accounting for the interaction between them. The idea is to make the grinding and polishing tools do what the optician wants (i.e., to remove material accurately and quickly).
Two other examples of Cohen's expertise illustrate how simple things can work even better than planned with smart engineering. The aspect camera on the Chandra X-ray Observatory includes a periscope that directs starlight from the center of the X-ray mirror assembly to a camera that is off-center. The designer of the periscope had originally proposed a very complicated mount for the very simple reflecting mirror, a mount that included many graphite-composite parts (which have a complicated 3D expansion coefficient). The design barely met the pointing requirements. A simple change from the composite material to a low thermal expansion form of steel called Invar improved the performance by a factor of ten. Cohen also introduced Invar into the design of the JWST wavefront sensing system. The glass optics had originally been supported only by a titanium frame and could fail at very cold temperatures (~30K) because of overstressed conditions. Cohen created instead a design that used a thin invar spacer to reduce the stress in the glass by a factor of three.
Cohen emphasizes that building good telescopes and scientific instruments is a joint effort. He acknowledges with appreciation all of the engineers and scientists with whom he has worked over last 30 years. He especially acknowledges Leon Van Speybroeck, one of his important mentors, who passed away a few years ago, for the intellectual discussions/arguments that they had.