Space imagery provides a unique resource for addressing environmental challenges associated with land cover change, land use, disaster relief, deforestation, regional planning and global change research. At Ball Aerospace, we are developing the Compact Hyperspectral Prism Spectrometer (CHPS) as a candidate imaging spectrometer technology for insertion into future Sustainable Land Imaging missions. The 2013 NRC report Landsat and Beyond: Sustaining and Enhancing the Nations Land Imaging Program recommended that the nation should “maintain a sustained, space-based, land-imaging program, while ensuring the continuity of 42-years of multispectral information.” In support of this, NASA’s Sustainable Land Imaging-Technology (SLI-T) program aims to develop technology for a new generation of smaller, more capable, less costly payloads that meet or exceed current Landsat imaging capabilities. CHPS is designed to meet these objectives, providing high-fidelity visible-to-shortwave spectroscopic information. CHPS supports continuity of legacy Landsat data products, but also, provides a path to enhanced capabilities in support of land, inland waters, and coastal waters science. CHPS features full aperture full optical path calibration, extremely low straylight, and low polarization sensitivity; all crucial performance parameters for achieving the demanding SLI measurement objectives. In support of our space-borne instrument development, we have developed an airborne instrument to provide representative spectroscopic data and data products. Now in the final year of this 3-year development program, we have completed our initial engineering airborne flights and are beginning science flights. We present initial results from laboratory characterization and calibration and from our engineering flights and close with an overview of instrument 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.
The newly launched Operational Land Imager (OLI) aboard the LDCM satellite has stringent prescription on the levels of ghosting and diffuse stray-light in the reflective bands in order to preserve the mission radiometric requirements. The LDCM project science team and instrument teams wrote the requirements such that they were image based, inclusive of all effects that appear to be ghosts or stray-light, and consequently more directly testable. The OLI Instrument Developer, Ball Aerospace Technology Corporation (BATC), working closely with experts from aerospace, academia, and the NASA/USGS LDCM project were able to identify and mitigate the various contributors to ghosting and stray-light, resulting in outstanding imagery for the wide field-of-view push-broom imaging sensor.
We will describe the ghosting and stray-light requirements and some of the contributing effects such as the leaky pixels that were seen on the EO-1/ALI. We will also highlight some of the technical challenges encountered and the solutions resulting in the substantial reduction of ghosting and stray-light which were verified by ground test. We will compare these ground measurements and analytic predictions with Lunar scan data to, potentially, resolve the question of whether the source of some of the performance outliers was the instrument or the test equipment.
Non-sequential ray tracing for stray light analyses have demonstrated value, but are over-constrained when high
sampling and speed are both needed. In cases where real geometry and mechanical surface properties are critical, such
analyses are certainly required. But the goal of these analyses is often to attempt to approach the performance that would
be achieved if only the optics contributed scatter and only through the sequential optical path. In other words, optical
element scatter is the limiting case for system performance. An analysis technique is therefore presented that enables
approximate but rapid sequential stray light estimates through deterministic modeling. Results of correlation to nonsequential
analyses demonstrate the large range of applicability of this approach. Examples of parametric studies show
the value of rapid paraxial estimates for understanding system performance sensitivities.
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.
The standard use of the parameter σ for the rms surface roughness of optics has obscured the fact that the effective
surface roughness is a function of both the measurement wavelength and bandwidth. A more appropriate method for the
flowdown of surface specifications from stray light requirements is presented. Acceptance test methods for validating
surface properties of optics using a Zygo NewView Profilometer are also discussed.
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.