Remote Ultra-Low Light Imaging detectors are photon limited detectors developed at Los Alamos National
Laboratories. RULLI detectors provide a very high degree of temporal resolution for the arrival times of detected photoevents,
but saturate at a photo-detection rate of about 106 photo-events per second. Rather than recording a conventional
image, such as output by a charged coupled device (CCD) camera, the RULLI detector outputs a data stream consisting
of the two-dimensional location, and time of arrival of each detected photo-electron. Hence, there is no need to select a
specific exposure time to accumulate photo-events prior to the data collection with a RULLI detector - this quantity can
be optimized in post processing. RULLI detectors have lower peak quantum efficiency (from as low as 5% to perhaps as
much as 40% with modern photocathode technology) than back-illuminated CCD's (80% or higher). As a result of these
factors, and the associated analyses of signal and noise, we have found that RULLI detectors can play two key new roles
in SSA: passive imaging of exceedingly dim objects, and three-dimensional imaging of objects illuminated with an
appropriate pulsed laser. In this paper we describe the RULLI detection model, compare it to a conventional CCD
detection model, and present analytic and simulation results to show the limits of performance of RULLI detectors used
for SSA applications at AMOS field site.
Our numerical simulations use relatively complex models of the atmosphere to investigate both Kolmogorov and non-Kolmogorov models. We find that the measurements of Bester et al. for light passing through the upper atmosphere are within the limits of behavior for Kolmogorov models, but often only if the outer scale of turbulent fluctuations is between 15 to 100 m. The possibility that the measured behavior might be non-Kolmogorov is not excluded. We also examine measurements made along short paths in the surface boundary layer, where some measurements of Bester el al. showed variations in the atmospheric fluctuations with seeing conditions which appeared to be non-Kolmogorov. These variations can perhaps be explained by standard models, but require that seeing improve with increasing wind speed in the surface layer. We discuss some other measurements which lend some support to that idea. However, we cannot exclude non-Kolmogorov behavior. We find that meteorological data is needed concurrent with astronomical observations, to help constrain the models. The size of the outer scale, the wind velocity profile and the turbulence spectrum are important to the ultimate capabilities of interferometers and other systems with adaptive optics.
Bester et al. report measurements of atmospheric fluctuations made with the Infrared Spatial Interferometer, which indicated behavior not in accord with the standard Kolmogorov model with only a single constant wind velocity. Our numerical simulations use relatively complex models of the atmosphere to investigate both Kolmogorov and non-Kolmogorov models. We find that the measurements of Bester et al. for light passing through the upper atmosphere are within the limits of behavior for Kolmogorov models, but often only if the outer scale of turbulent fluctuations is between 15 to 100 meters. The possibility that the measured behavior might be non-Kolmogorov is not excluded. We also examine measurements made along short paths in the surface boundary layer, where some measurements of Bester et al. showed variations in the atmospheric fluctuations with seeing conditions which appeared to be non-Kolmogorov. These variations can be perhaps be explained by standard models, but require that seeing improve with increasing wind speed in the surface layer. We discuss some other measurements which lend some support to that idea. However, we cannot exclude non-Kolmogorov behavior. We find that meteorological data is needed concurrent with astronomical observations, to help constrain the models. The size of the outer scale, the wind velocity profile and the turbulence spectrum are important to the ultimate capabilities of interferometers and other systems with adaptive optics.
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