Angstrom: An Imaging Star Photometer

Ground-based sun photometers provide a vital consistent global long-term aerosol data record used to better understand aerosol impact on climate, improve aerosol transport models and bound lidar-derived aerosol products. Sun photometers only provide aerosol information during the day, and even though there is scientific and commercial interest, there are very few aerosol measurements made at night. Innovative Imaging and Research proposes Angstrom, an affordable, easily deployable multiband wide field of view (FOV) imaging star photometer that measures aerosol optical depth (AOD) and the Angstrom parameter across the night sky using stars. It can be used to augment traditional sun/lunar photometer networks and significantly improve atmospheric monitoring. Angstrom applies state-of-the-art image processing techniques to imaging systems that use emerging high quantum efficiency, low read noise CMOS sensors and high-quality machine vision optics. Early simulations and test data suggest these imaging systems can acquire dim star fields at a relatively high signal-to-noise ratio. Our goal is to achieve a comparable level of accuracy as gold-standard daytime sun photometers. Imaging star photometers acquire large sky regions measuring near-instantaneous spatial variability not possible with traditional narrow FOV photometers. By imaging multiple stars in a portion of sky covering a wide range of air mass or by continuously imaging stars moving through varying air mass, Angstrom can take advantage of traditional Langley calibration or multi-star methods. Angstrom tracks stars through image processing, eliminating complex precision moving mechanisms. It also uses the relative positions of stars to determine the camera’s orientation, reducing installation and maintenance costs. This allows it to be more easily deployed on ships, UAVs, and fixed terrestrial locations where it has been difficult to obtain measurements.

Angstrom supports atmospheric studies by providing additional nighttime aerosol measurements to atmospheric models. It also supports Decadal Survey recommended ACCP and TEMPO satellite missions and is directly relevant to numerous field campaigns measuring and monitoring aerosols. Combining Angstrom data with micropulse lidar can improve the accuracy of lidar aerosol retrievals. Angstrom data also helps scientists who require atmospherically corrected products from night imaging remote sensing instruments such as the VIIRS DNB.

Emerging remote sensing applications that require nighttime aerosol measurements include mapping artificial lights and estimating power usage, important economic measures. Angstrom can complement the Aeronet ground network of solar/lunar photometers to help fill current nighttime data gaps to support these new applications. It can also provide free-space laser communication atmospheric conditions.

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