The Libera instrument is being developed as part of a NASA Earth Venture Continuity mission for extending Earth radiation budget (ERB) measurements by the currently operational Clouds and the Earth’s Radiant Energy System (CERES) instruments into the future. Libera will be launched on NOAA’s JPSS-4 satellite. Libera introduces several new technologies, including advanced VACNT detectors, a split-shortwave channel to quantify shortwave near-IR and visible radiation, and a wide field of view camera (WFC) that advance the state-of-the-art in Earth radiation budget measurements. The WFC is a monochromatic wide field of view camera operating at 555nm over a 123-degree field of view that will continuously observe the full Earth disk from low-earth orbit. The WFC provides a unique capability for scene identification and Angular Distribution Model (ADM) generation that complements similar measurements from the VIIRS instrument that will fly on JPSS-4 with Libera. By demonstrating that Libera’s WFC provides the data required for ADM development, a path forward for future free-flier ERB measurements will be explored. We focus on the development of the WFC, its science objectives, unique design features, its current state of development, and how it could help to enable a constellation of smaller, more cost-effective ERB instruments for the future.
Commissioning the Webb telescope to realize the observatory’s full capability necessitated the development of robust wavefront sensing and control processes. These processes rely on techniques that were adapted or newly innovated for the mission, and further adaptation of these techniques may be expected for future segmented telescopes. Over the course of mission development, these techniques were refined to form a baseline wavefront commissioning plan that assumes several conditions and performance requirements are met. Herein we present efforts carried out to define and develop contingency concepts of operation for Webb telescope commissioning, and the mission-level approach to managing the response to deviations from the baseline plan in the event of significant off-nominal or anomaly scenarios encountered by the wavefront team. An overview of selected contingencies is presented along with more detailed example model cases and instances of interest encountered in flight.
The James Webb Space Telescope (JWST) is a segmented deployable telescope, currently operating at L2. The telescope utilizes 6 degrees of freedom for adjustment of the Secondary Mirror (SM) and 7 degrees of freedom for adjustment of each of its 18 segments in the Primary Mirror (PM). After deployment, the PM segments and the SM arrived in their correct optical positions to within a ~1 mm, with accordingly large wavefront errors. A Wavefront Sensing and Controls (WFSC) process was executed to adjust each of these optical elements in order to correct the deployment errors and produce diffraction-limited images across the entire science field. This paper summarizes the application of the WFSC process.
Segmented-mirror telescopes such as JWST and Keck provide a particular challenge when first pointed to the sky: to access a suitably isolated star with which to align the mirror segments, one must first determine the sky location to which the telescope is pointed. Prior to stacking, the primary mirror segments each produce a separate image of the sky; the expected result is a confusing image in which the star field is convolved with the randomly pointed segments so that each star appears multiple times. To establish the initial sky pointing of JWST, we have developed a pair of novel and complementary approaches for identifying the field. The first approach uses image pairs in which a single primary mirror segment is tilted from its initial pointing by a small amount. This motion of the segment produces a corresponding motion of the stellar images from that segment, allowing us to resolve the ambiguity between the array of stellar images and the array of segment images. The second performs a pattern match within a single image to identify the repeating pattern of the segment array (i.e., the star pattern in the field of view) which can then be matched against an astrometric catalog. Both algorithms produce a resulting array of star positions from which the astrometry.net engine can identify the sky location. We describe the application of these algorithms to both simulated JWST NIRCam images and actual images acquired with MOSFIRE on Keck I, explain how we employed these approaches during the initial stage of JWST primary mirror commissioning, and speculate on future applications for mirrors with more segments.
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