The OTA for the Nancy Grace Roman Space Telescope includes the primary mirror, secondary mirror, and aft optics for guiding light into the Wide Field Instrument and the Coronagraph Instrument. The telescope is taking shape as the tested optical mirror assemblies are integrated. The assemblies have been thermal cycled to the cold temperatures for infrared operation, load tested to launch loads, vibration tested, and optically tested. Testing included launch-level vibration testing of the 2.4-meter light-weighted primary mirror assembly. In addition, the telescope control electronics (TCE) box has been fully assembled and the environmental testing of the TCE is progressing. Pictures and descriptions of the integration and test progress are provided, along with performance results measured at these levels of assemblies. Planning and test equipment preparation for the telescope thermal vacuum testing continues including plans to take advantage of the large dynamic range available with focus diversity phase retrieval and a Shack-Hartmann wavefront sensor for the gravity-sagged primary mirror.
On-orbit optical stability of the Nancy Grace Roman Space Telescope (RST) is a key requirement that enables multiple science objectives and drives multiple aspects of telescope design and analysis. Thermoelastic changes are typically large contributors to optical instability, and both extremely low CTE materials and extremely stable temperatures are needed to achieve the RST optical stability requirements. We will present the results from a test that demonstrated the L3Harris capability to sense and control temperatures to milli-kelvin levels of stability across a range of operating temperatures.
The Nancy Grace Roman Space Telescope (RST) is a Hubble-class telescope with a large field of view for large surveys of the sky, cold temperatures for enabling near infrared imaging, and controlled temperature stability for long exposures and coronagraphy. The OTA includes the primary mirror, secondary mirror, and aft optics for guiding light into the Wide Field Instrument and the Coronagraph Instrument. The testing of the optical assemblies and structures are nearly complete in preparation for telescope integration. Pictures and descriptions of the assemblies are provided, followed by performance results measured at these level of assemblies. The assemblies are nearly complete as they are tested through thermal cycling to cold temperatures for infrared operation, mechanical strength and vibration, and optically testing. Optical surface figure error results are shown for all the optical surfaces.
The Strategic Astrophysics Technology (SAT) program has recently identified several critical technology gaps related to future missions that have direct relevance to thermal control methods for optical payloads: (1) thermally stable telescopes, (2) sensing and control at the nanometer level or better, and (3) sensing and control at the picometer level [1]. Implementing very tight control stability on optical payloads in the space environment to achieve precise line of sight and wavefront control is more than just a thermal problem. It is a combination of system design challenges implementing thermal, electronics, and control methods. These challenges are further complicated by size, weight, and power (SWAP) constraints for large-scale optical platforms due to both quantity of sensors and physical separation between sensing and control electronics. For thermal hardware, control errors arise from sensors indirectly coupled to the controlling heat source that may result from installation constraints, sensor or heater attachment methods, or poor thermal diffusivity. For electronics, control errors arise due to system resolution limitations which are dependent on bit accuracy over the temperature range of interest, current and voltage source accuracy, sensor self-heating, noise sources, sampling rates, and circuit averaging methods. Errors arise from limitations in the control methods such as over- and under-shoot with bang-bang (on/off) or proportional-integral-derivative control (PID). Within the PID control method, there are many nuances to the implementation as well, such as the need to tune each control zone for optimal control. This paper presents a description of the challenges and opportunities that come with high precision space telescope thermal control for candidate future astrophysics missions like LUVOIR or HABEX and provides examples of thermal design and analysis methodologies underway for the WFIRST program.
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