Simulating a Telescope
This photo was captured before the flight telescope element of the James Webb Space Telescope
was mated to the sunshield/observatory element of the spacecraft. This telescope element is also known as OTIS - a nested acronym for "Optical Telescope Element (OTE) + Integrated Science Instrument Module." Essentially it means the optics and instruments.
A simulator for the OTIS was used during a phase of testing in 2019. The simulator is the silver frame-like structure. The purple foil covered object is the flight sunshield.
What is the OTIS simulator's actual purpose?
The OTIS simulator emulates the main mass properties of the actual flight OTIS, meaning it has essentially the same mass, center of mass, and moment of inertia of the flight OTIS and similar stiffness at key mechanical interfaces. The OTIS simulator also approximates thermal conductance of the OTIS where the spacecraft element and the OTIS connect together.
This matters because the spacecraft element and OTIS are structurally tied together in the stowed-for-launch configuration to experience and withstand the rigors of launch and the temperatures of space. The complicated paths for mechanical and thermal loads though the spacecraft element during launch and before deployments are not possible to test without either OTIS itself present or something that emulates it from a thermo-mechanical standpoint.
We did not use the real OTIS for spacecraft element environmental qualification because we had already qualified OTIS and tested it optically end-to-end, and to subjected it to qualification-level environments again would have require another optical end-to-end test campaign. However, testing OTIS again optically while integrated with the spacecraft element is infeasible because it’s not possible to duplicate the necessary thermal boundary conditions in a test chamber on the ground at the observatory level of assembly. This is why we qualified Webb as two halves—OTIS and the spacecraft element—separately rather than as one complete observatory.
While the OTIS simulator looks a lot simpler, and is a lot simpler, than the real OTIS, it was still an engineering challenge to design and built and verify an OTIS thermal-mechanical simulator that would emulate OTIS during spacecraft element environmental qualification.
Image credit: NASA/Chris Gunn
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