NEO Surveyor is a NASA Planetary Defense Coordination Office mission designed to detect and track >2/3 of potentially hazardous asteroids >140 m in diameter during its 5-year prime mission. NEO Surveyor entered Phase B in June 2021 and is scheduled to launch in 2026 to survey the sky in two infrared bands. The infrared detectors are a key technology for the mission and have been the subject of focused development for more than a decade. In this paper, we report test results for recently produced detectors and describe design elements of the focal plane module relevant to operations for NEO Surveyor.
Near-Earth Object (NEO) Surveyor, a NASA planetary defense space mission, is currently in Phase B with a launch date in 2026. NEO Surveyor is an infrared telescope designed to detect and characterize Potentially Hazardous Asteroids (PHAs). The required sensors leverage the space flight heritage and further development over the last 15 years of HgCdTe arrays to detect infrared light spanning from 4 to 10 μm. NEO Surveyor will employ eight passively cooled HgCdTe Sensor Chip Assemblies (SCAs) across two bands, each band consisting of a 1x4 SCA mosaic to cover a wide field of view. Four of these SCAs have a >5.5 μm cutoff wavelength and cover the shorter 4-5.2 μm (NC1) band, while four SCAs will have a >10.5 μm cutoff wavelength and span the longer 6-10 μm (NC2) band. We present calibration and performance results from two recently produced pathfinder SCAs, one for each band, manufactured by Teledyne Imaging Sensors with development guidance from the University of Arizona, the University of Rochester, and JPL. Both devices demonstrate the requisite low dark current, high well depth, and high quantum efficiency, exceeding mission requirements.
The modulation transfer function (MTF) is a useful measure in image quality analysis and performance budget determination. Sensitive long wavelength infrared (LWIR) detectors for astronomical space telescopes require slight modifications to the existing MTF measurement methods due to the increased prevalence of high dark current pixels. Presented here are the specifics of a modified slanted edge method to determine the MTF in λc > 10 μm HgCdTe detectors to be used with the planned Near-Earth Object Surveyor Mission. The measured MTF at Nyquist using 6 μm light is 0.22 ± 0.02 and is 0.25 ± 0.02 using 10 μm light for both 250 and 350 mV of applied reverse bias. These measurements are from edge spread functions with median signal values around 50% of the well depth, as the MTF is expected to change with signal value due to two brighter-fatter type effects. The expected trends caused by the influences of these two effects and the expected trends with wavelength of absorbed photons are all observed.
For space optical systems that image extended scenes such as earth-viewing systems, modulation transfer function (MTF) test data is directly applicable to system optical resolution. For many missions, it is the most direct metric for establishing the best focus of the instrument. Additionally, MTF test products can be combined to predict overall imaging performance. For fixed focus instruments, finding the best focus during ground testing is critical to achieving good imaging performance. The ground testing should account for the full-imaging system, operational parameters, and operational environment. Testing the full-imaging system removes uncertainty caused by breaking configurations and the combination of multiple subassembly test results. For earth viewing, the imaging system needs to be tested at infinite conjugate. Operational environment test conditions should include temperature and vacuum. Optical MTF testing in the presence of operational vibration and gravity release is less straightforward and may not be possible on the ground. Gravity effects are mitigated by testing in multiple orientations. Many space telescope systems are designed and built to have optimum performance in a gravity-free environment. These systems can have imaging performance that is dominated by aberration including astigmatism. This paper discusses how the slanted edge MTF test is applied to determine the best focus of a space optical telescope in ground testing accounting for gravity sag effects. Actual optical system test results and conclusions are presented.
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