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.
As the next generation of Earth science programs demand more spectral bands, larger fields of view, faster speeds and reduced size, the optical designer will need to adapt to these new requirements. With the advent of manufacturable freeform optical surfaces, compact high-performance optical systems utilizing these surfaces are becoming practical. Freeform optics provide additional degrees of freedom for the optical designer which allow for more compact optical systems of equal performance, potentially operating at faster speeds or over wider fields of view. While numerous design studies on freeform systems have been published, little has been presented in the open literature on as built freeform systems. In this paper we describe the successful outcome of a hardware development program where we designed, built, aligned, and tested a compact WFOV three-mirror telescope with freeform surfaces. It is important that in addition to good optical performance, excellent stray light control is required in Earth remote sensing systems to minimum calibration errors across spectral bands. While compact size is often emphasized in the design of freeform systems, this needs to be balanced against the requirement for good stray light control. As such, the telescope presented in this paper balances the desire for small size with good stray light control. We present the results of the computer-aided alignment of the telescope along with measured stray light performance.
The Operational Landsat Imager (OLI) stray light performance was tested in 2010 in Ball’s stray light test facility. After the launch of OLI in 2013, measurements of on-orbit stray light performance confirmed the excellent pre-launch results. Ball is currently building OLI-2 for launch in 2020 and stray light testing was performed on the instrument in March 2018. This paper compares these measurements to OLI stray light test results and shows how they provide high confidence that OLI-2 will also provide excellent on-orbit stray light performance. Stray light performance of the two near identical builds is quite similar. This demonstrates the consistency of the assembly process and the repeatability of the testing performed in the Ball stray light test facility.
KEYWORDS: Stray light, Telescopes, Data modeling, Mirrors, Calibration, Space telescopes, Systems modeling, Sensors, Photodiodes, Bidirectional reflectance transmission function
Ball Aerospace & Technologies Corp. (BATC) developed motion control systems to move the NASA LDCM
Operational Land Imager (OLI) relative to the source in the stray light test facility. Stray light tests were performed on
both the imaging and calibration apertures over a wide range of illumination angles. Test results will be shown that
demonstrate that the stray light performance of both the telescope and the test facility are excellent. Model predictions
are also compared to the test results.
BATC has developed a new stray light test facility (SLTF) and performed initial tests demonstrating its capabilities. The facility interior is nearly all black and is a Class 5 cleanroom. Coupled with a double cylindrical chamber that reflects the specular light away from the instrument under test, the stray light control in the facility is excellent. The facility was designed to be able to test a wide variety of instruments at a range of source angles from in-field to large off-axis angles. Test results have demonstrated PST performance below 1E-9.
SNAP is a proposed space-based experiment designed to quantify dark energy by measuring the redshift-magnitude
diagram of supernovae and to quantify the growth of structure in the universe by measuring weak gravitational lensing
over cosmological distances. The baseline SNAP telescope is an ambient temperature three-mirror anastigmat (TMA).
The goal of the stray light design is to ensure that stray light in the 0.4 to 1.7 micron wavelength range does not exceed a
small fraction of Zodiacal radiation within the mission's target field near the North ecliptic pole. At visible wavelengths,
we expect the primary source of stray light will be starlight scattered by the primary mirror. In our longest wavelength
NIR band we expect thermal emission from the mirrors and structure will dominate. Scattered stray light is mitigated by
an internal field stop, and a cold (140K) internal aperture stop. Stray light scattered by mirror roughness and particulate
contamination, as well as scattering from the telescope baffles are modeled and quantified. The baseline design and
analyses contained herein ensure that stray light will be less than 10% of Zodiacal in all bands.
There has been a general awareness for several years that the IEST-STD-CC1246 standard particle distribution with a slope of -0.926 does not reasonably represent the contamination on optics that have not been recently cleaned. As a result, the CL (Cleanliness Level) nomenclature actually counters effective communication and modeling of particulate contamination scatter. An analysis method and communication standard centered on Percent Areal Coverage (PAC) and particle distribution slope is presented that improves the ability of Contamination Engineering and Stray Light Engineering to tackle ever more difficult instrument stray light requirements in the most cost-effective manner. Modeling the expected particle distributions for multiple contamination species improves accuracy and reduces costly overdesign.
Ball Aerospace is building the Cryogenic Telescope Assembly (CTA) for the Space IR Telescope Facility (SIRTF) project and stray light control is a critical part of this effort. The heliocentric orbit where the Sun is never allowed to illuminate the CTA and the Earth is relatively far away is a key feature for stray light control. Mirror and paint BRDFs were measured at various SIRTF wavelength sin order to provide inputs for the stray light models. Stray light from telescope self emission, a distant point source, and the galaxy were analyzed. The design ensures that stray light will be less than 21 percent of the celestial background at all SIRTF wavelengths.
A new coating, whose specular reflectance at far-infrared wavelengths is considerably less than that of either Ames 24E2 or Martin Infrablack and whose diffuse IR reflectance is less than that of Ames 24E2, has been developed through cooperation of NASA-Ames Research Center and Ball Aerospace & Technologies Corporation. The new coating is called Ball IR Black. This new coating is harder and more robust than Ames 24E2 and has a lower tendency to shed particles. It is also rougher and thicker than Ames 24E2. At least two lines of evidence point toward multiple scatter among the huge surface facets of this coating as the principal mechanism behind its lower IR reflectance. A special technique to permit spray application of the coating (which contains SiC grinding compound) has been developed at Ball ATC and may be responsible for the unusually large surface facets.
Ball Aerospace is currently designing and building several instruments with strict stray light requirements in the ultraviolet (UV) region. Available BRDF data in this spectral region is very limited, so Ball modified a UV monochrometer to measure the BRDF of critical surfaces. Measurements with the monochrometer were also possible in the visible, so results in the visible were compared to results using a TMA scatterometer. Results from the two instruments compared reasonably well and were used to generate correction factors for the UV BRDF data. Surfaces measured included graphite fiber reinforced cyanate (GFRC), Desoto black paint on GFRC, spectralon, and an aluminum diffuser.
As part of a study being performed for the U.S.A.F. Phillips Laboratory, Ball Aerospace has extensively tested a relay mirror that flew on the Hubble Space Telescope (HST) as part of the University of Wisconsin's high speed photometer (HSP). In this paper, a review of the history of the HSP mirror is presented, including: (1) pre-flight and post- flight optical testing, (2) bi-directional reflectance distribution function (BRDF) performance before and after the post-flight removal of contaminants, and (3) chemical fingerprints of the contaminants. The BRDF data from the HST/HSP relay mirror are summarized in this paper and are also made available on DOS diskette.
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