Design of a large lightweight space telescope optical system for the Next Generation Space Telescope

The Next Generation Space Telescope (NGST) program is currently conducting a pre-Phase A study to prove feasibility of design, show that the design can be implemented within cost guidelines, and meet the requirements of the science community. In an effort to achieve the science community's goals as outlined in the "HST and Beyond" report, the NGST team has developed a Government "yardstick" design of an 8-meter aperture segmented-mirror telescope which would be launched in an Atlas IIAS launch vehicle into the L2 (Lagrange Point) orbit. This paper will discuss the design of the Optical Telescope Assembly (OTA) and the various issues and complications of designing lightweight optics to be placed in the environment that the NGST will encounter both during launch and during its mission at L2. The OTA must be lightweight but also structurally able to withstand the launch environment and transfer to the L2 orbit. The OTA must be optically stable at L2 to provide high quality science at the cryogenic temperatures which it will experience. The OTA segmented petal mirror concept (Figure 1), is predicated on the development of lightweight optics which can either be manipulated to achieve and maintain a desired figure or be rigid enough so that change in shape is only necessary for phasing and alignment. Wavefront control is accomplished with a deformable quaternary mirror. The secondary mirror structural mast must also be rigid with frequency responses decoupled from spacecraft modes in order to maintain decenter and despace within several microns. The ultimate design of the OTA will eventually be provided by a contract team beginning sometime in the 2003/04 timeframe. The effort being accomplished today will serve as a stepping stone to proving OTA design feasibility and technology readiness to begin contractor design and development. This paper provides an overview of the NASA OTA design effort. It investigates material considerations for the OTA, structural design of the primary mirror and secondary support structure, thermal design, optical prescription/design, and the use of integrated optical modeling in development of the NASA design for the NGST OTA.