Reflectance spectra of Earth orbiting satellites can be readily observed with small diameter telescopes (D < 1 m)
by utilizing a method known as slitless spectroscopy. Satellite spectra can be observed by simply placing a
transmission grating within the collimated optical path of the telescope without the need to image through a
slit. The simplicity of the slitless spectroscopy design makes it a promising alternative to spatially resolving
satellites with larger and more expensive diameter telescopes for applications of space situational awareness.
However, accurately observing satellite re
ectance spectra without imaging through a slit requires a dark and
homogeneous background. This requirement is frequently violated as background stars streak across the image
due to the slewing motion of the telescope during satellite tracking. Rather than throwing out all images with
noticeable stellar contamination, a principle component analysis of contaminated images from three geostationary
satellite observations showed that it may still be possible to assess and identify satellite characteristics depending
upon the amount of stellar contamination in the spectral region of interest. Additionally, a simple technique for
automatic removal of contaminated frames is proposed based on an outlier analysis using Gaussian statistics and
was found to successfully remove all signicantly contaminated frames.
Atmospheric oxygen absorption bands in observed spectra of boost phase missiles can be used to accurately
estimate range from sensor to target. One method is to compare observed values of band averaged absorption
to radiative transfer models. This is most effective using bands where there is a single absorbing species. This
work compares spectral attenuation of two oxygen absorption bands in the near-infrared (NIR) and visible (Vis)
spectrum, centered at 762 nm and 690 nm, to passively determine range. Spectra were observed from a static test
of a full-scale solid rocket motor at a 900m range. The NIR O2 band provided range estimates accurate to within
3%, while the Vis O2 band had a range error of 15%. A Falcon 9 rocket launch at an initial range of 13km
was also tracked and observed for 90 seconds after ignition. The NIR O2 band provided in-flight range estimates
accurate to within 2% error for the first 30 seconds of tracked observation. The Vis O2 band also provided
accurate range estimates with an error of approximately 4%. Rocket plumes are expected to be significantly
brighter at longer wavelengths, but absorption in the NIR band is nearly ten times stronger than the Vis band,
causing saturation at shorter path lengths. An atmospheric band is considered saturated when all the in-band
frequencies emitted from the rocket plume are absorbed before reaching the sensor.
Cramér-Rao lower bound (CRB) theory can be used to calculate algorithm-independent lower bounds to the variances of
parameter estimates. It is well known that the CRBs are achievable by algorithms only when the parameters can be
estimated with sufficiently-high signal-to-noise ratios (SNRs). Otherwise, the CRBs are still lower bounds, but there can
be a large gap between the CRBs and the variances that can be achieved by algorithms. We present results from our
initial investigations into the SNR dependence of the achievability of the CRBs by multi-frame blind deconvolution
(MFBD) algorithms for high-resolution imaging in the presence of atmospheric turbulence and sensor noise. With the
use of sample statistics, we give examples showing that the minimum SNR value for which the CRBs can be achieved
by our MFBD algorithm typically ranges between one and five, depending upon the strength of the prior knowledge used
in the algorithm and the SNRs in the measured data.
KEYWORDS: Actuators, Adaptive optics, Turbulence, Deformable mirrors, Control systems, Wavefront sensors, Signal to noise ratio, Wavefronts, Digital filtering, Cameras
The Air Force Research Laboratory's (AFRL) Sodium Guidestar Adaptive Optics for Space Situational Awareness
program (NGAS) sponsored research on spatially non-uniform gain for the servo-loop controller of an
adaptive optics (AO) system. The edge subapertures of a Shack-Hartmann wavefront sensor have lower signalto-
noise ratios and are more susceptible to measurement errors than fully illuminated center subapertures. These
measurement errors produce errant commands over the corresponding edge actuators and can induce instabilities
over these regions in strong turbulence conditions. The Non-uniform Gain Experiment (NUGE) concentrated
on the development and experimental analysis of spatially varying gain maps on the servo-loop controller of a
deformable mirror. The goal was to improve AO system performance and mitigate instabilities that can occur
over the edge actuators of a deformable mirror. A gain map with a ring of lower filter gains b over just the outer
actuators was experimentally shown to increase the overall Strehl ratio of the AO system in all of the tested
turbulence conditions. A Gaussian gain map was also shown to significantly reduce the overall residual phase
variance over the edge actuators thereby reducing the formation of the instabilities. Experiments were conducted
at the Starfire Optical Range (SOR), AFRL, Kirtland AFB.
This is the first of two papers discussing aspects of placing the deformable mirror in a location
not conjugate to the pupil plane of the telescope.
The Starfire Optical Range, Air Force Research Laboratory's Directed Energy Directorate
is in the process of developing a high efficiency AO system for its 3.5m optical telescope. The
objective is to achieve maximum diffraction limited performance, i.e., largest pupil diameter
possible, and maximum optical throughput. The later can be achieved by placing the deformable
mirror outside the pupil. However placing the DM in a location not conjugate to the pupil results
in a degradation in optical performance. This paper discusses experimental measurements of
the degradation.
In this paper we discuss the DM-not-in-pupil experimental testbed, the difficulties associated
with creating this type of testbed, and how these difficulties were overcome. We also present
results from the successful lab demonstration of closed loop performance with the DM placed out
of pupil. We experimentally measured the degradation in Strehl and implemented a mitigation
technique. Our experimental results indicate the mean degradation in Strehl as a result of placing
the DM out of pupil to be between 7% and 9 %. This result is comparable with wave optics
simulation and theoretical results which will be discussed in a companion paper, "Adaptive
optics with DM not in pupil - Part 2: Mitigation of Degradation".
A few years ago the Air Force Research Laboratory developed a Self-Referencing Interferometer (SRI) wavefront
sensor (WFS) that is able to accurately detect the magnitude and phase of propagating electromagnetic waves in
a strong scintillation environment. This proved to be very useful in applications of adaptive optics when detecting
light or transmitting laser beams through moderate to high turbulence. The SRI operates by interfering a beacon
beam, which has been aberrated by atmospheric turbulence, with a reference beam having a known phase and
detecting the intensity of the interference pattern. The phase of the beacon is then determined from those
interference patterns. At least three different phases of a reference beam are needed to accurately determine
the phase on the beacon beam, but four are preferable. These phases shift the reference beam by 0, π/2, π,
and 3π/2. In this paper we examine the effects of phase shift errors. Our method can be extrapolated to any
WFS utilizing the Carré algorithm with π/2 phase steps. These results show that the SRI is amazingly tolerable
to phase shifting errors, specifically that an adaptive optic loop still closes even with a phase error 'epsilon' of
nearly ±π/2. Even more unexpected, it is possible to increase Strehl over the nominally aligned system by as
much as 11% in closed loop operation when phase errors are purposefully induced.
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