Lockheed Martin has built a Space Object Tracking (SPOT) facility at our Santa Cruz test site in Northern California. SPOT consists of three 1 meter optical telescopes controlled by a common site management system to individually or cooperatively task each system to observe orbital debris and earth orbiting satellites. The telescopes are mounted in Az/El fork mounts capable of rapid repointing and arc-sec class open loop tracking. Each telescope is installed in a separate clam shell dome and has aft mounted benches to facilitate installing various instrument suites. The telescope domes are mounted on movable rail carts that can be positioned arbitrarily along tracks to provide variable baselines for sparse aperture imaging. The individual telescopes achieved first light in June 2012 and have been used since to observe satellites and orbital debris. Typical observations consist of direct photometric imaging at visible and near infrared wavelengths, and also include spectroscopic and hypertemporal measurements.
Rayleigh beacon adaptive optical systems for atmospheric aberration correction and high rate J-Band trackers for each telescope will be added in 2015. Coherent combinations of the three telescopes as an interferometric imaging array using actively stabilized free space variable delay optical paths and fringe tracking sensors is also planned. The first narrow band (I band) interferometric fringes will be formed in the summer of 2014, with wide band (R, I, H) interferometric imaging occurring by early 2015.
We present the first high spatial resolution, passively-illuminated polarimetric images of boosting rocket exhaust
plumes. The images shown here show significant linear and circular polarization, and the ability to resolve
the polarization signals into images allows us to make some preliminary arguments as to their origins. Our
observations are consistent with polarization caused by Rayleigh and Mie scattering (linear) and interaction
with plume plasma-generated magnetic fields (circular). We also present nearly simultaneous, two-color, narrowband
(633 ± 5 and 750 ± 5 nm) exhaust plume images, where significant structural differences are observed in
the plumes despite a relative small difference in the two wavelengths.
We have developed a computer simulation to demonstrate the use of a periodic grating structure in the near field of a
telescope to super-resolve objects in the far field. The separation between the telescope pupil and the grating provides a
periodic anisoplanatism for the telescope, modulating the measured brightness of a point source as it moves across the
field normal to the grating. The resulting periodic modulation of an extended source can thus produce a spatial frequency
heterodyning effect, where frequency components outside the diffraction passband are aliased inside the passband and
measured. Using the simulation, we have quantitatively validated the analytically-predicted periodic blur function for the
case of single near-field grating. Further, we have shown the heterodyning effect is observed when the distance between
the grating and pupil corresponds to the Talbot distance, where the grating forms a "self-image" in the plane of the pupil.
By techniques described in this paper, high spatial frequencies in a scene that are beyond the diffraction limit of an
optical system can modulate user-generated low spatial frequency patterns prior to image formation and detection. The
resulting low spatial frequency modulations or "moiré patterns" lie within the optical pass-band and will therefore be
detectable. In favorable and controlled situations the scene's high spatial frequencies can be reconstructed from multiple
images containing these low-frequency modulations and a single super-resolved image is synthesized. This approach to
image super-resolution is feasible and does not violate well-established physical principles.
The paper describes two phases of this ongoing research. In phase one, we investigate active remote imaging methods in
which the low-frequency modulations are produced by controlling active illumination patterns projected onto the scene.
In phase two we investigate passive remote imaging methods in which diffracting structures are interposed between the
scene and the camera to modulate the light fields prior to image formation and detection.
Superresolution of images by data inversion is defined as extrapolating measured Fourier data into regions of Fourier space where no measurements have been taken. This type of superresolution can only occur by data inversion. There exist two camps of thought regarding the efficacy of this type of superresolution: the first is that meaningful superresolution is unachievable due to signal-to-noise limitations, and the second is that meaningful superresolution is possible. Here we present a framework for describing superresolution in a way that accommodates both points of view. In particular, we define the twin concepts of primary and secondary superresolution and show that the first camp is referring to primary superresolution while the second group is referring to secondary superresolution. We discuss the implications of both types of superresolution on the ability of data inversion to achieve meaningful superresolution.
We consider the use of pupil masks with AO-compensated telescopes, calculating fringe energy SNR for a variety of object brightness values and for different degrees of AO compensation. We propose a way to specify the optimum subaperture diameter for an N-subaperture mask and argue for the use of masks, in combination with full-aperture data, to increase SNR at the highest spatial frequencies the telescope can sample. Use of full-aperture data in combination with high-SNR masked-aperture data can yield imagery with higher spatial resolution than full-aperture data alone.
We analyze the quality of reconstructions obtained when using the multi-frame blind deconvolution (MFBD) algorithm and the bispectrum algorithm to reconstruct images from atmospherically-degraded data that are corrupted by detector noise. In particular, the quality of reconstructions is analyzed in terms of the fidelity of the estimated Fourier phase spectra. Both the biases and the mean square phase errors of the Fourier spectra estimates are calculated and analyzed. The reason that the comparison is made by looking at the Fourier phase spectra is because both the MFBD and bispectrum algorithms can estimate Fourier phase information from the image data itself without requiring knowledge of the system transfer function, and because Fourier phase plays a dominant role in image quality. Computer-simulated data is used for the comparison in order to be able to calculate true biases and mean square errors in the estimated Fourier phase spectra. For the parameters in this study, the bispectrum algorithm produced less-biased phase estimates in all cases than the MFBD algorithm. The MFBD algorithm produced mean square phase errors comparable to or lower than the bispectrum algorithm for good seeing and few data frames, while the converse is true for many data frames and poor seeing.
I describe a variable-geometry pupil (VGP) to increase image resolution for ground-based near-IR and optical imaging. In this scheme, a curvature-type wavefront sensor provides an estimate of the wavefront curvature to the controller of a high-resolution spatial light modulator (SLM) or micro- electromechanical (MEM) mirror, positioned at an image of the telescope pupil. This optical element, the VGP, passes or reflects the incident beam only where the wavefront phase is sufficiently smooth, viz., where the curvature is sufficiently low. Using a computer simulation, I show the VGP can sharpen and smooth the long-exposure PSF and increase the OTF SNR for tilt-only and low-order AO systems, allowing higher resolution and more stable deconvolution with dimmer AO guidestars.
We present preliminary results from a comparison of image estimation and recovery algorithms developed for use with advanced telescope instrumentation and adaptive optics systems. Our study will quantitatively compare the potential of these techniques to boost the resolution of imagery obtained with undersampled or low-bandwidth adaptive optics; example applications are optical observations with IR- optimized AO, AO observations in server turbulence, and AO observations with dim guidestars. We will compare the algorithms in terms of morphological and relative radiometric accuracy as well as computational efficiency. Here, we present qualitative comments on image results for two levels each of seeing, object brightness, and AO compensation/wavefront sensing.
We present an extension of adaptive optics sky coverage calculations to include spectrometer slit power coupling. As an example, we show several slit coupling sky coverage calculations for the Gemini-North telescope. Our calculations quantify the effect on sky coverage of previous work, where we showed Strehl is not in general a good predictor of slit coupling, and provide an example of the utility of instrument-specific calculations.
Images of astronomical objects acquired by ground-based telescopes are blurred by atmospheric turbulence. These blurring effects can be partially overcome by post-detection processing such as speckle imaging (SI). We have developed a parallel implementation of SI to dramatically reduce the time required to reduce imaging data, allowing us to implement a near realtime (NRT) SI image feedback capability. With NRT SI feedback, telescope operators can select observing parameters to optimize data quality while the data is being taken. NRT processing also allows easy selection of the best data from a long observation for later post-detection processing using more sophisticated algorithms. Similar NRT schemes could also be implemented for non-imaging measurements, such as spectroscopy. NRT feedback will yield higher quality data products and better utilization of observatory resources.
Positivity and support have long been used to improve image quality beyond that achievable from the measured data alone. In this paper we analyze how positivity functions to reduce noise levels in measured Fourier data and the corresponding images. We show that positivity can be viewed as a signal- dependent support constraint, and thus it functions by enforcing Fourier-domain correlations. Using computer simulated data, we show the effects that positivity has upon measured Fourier data and upon images. We compare these results to equivalent result obtained using support as constraint. We show that support is a more powerful constraint than positivity in several ways: (1) more super- resolution is possible, (2) more Fourier domain noise reduction can occur, and (3) more image-domain noise reduction can occur.
In this paper the problem of optimally using object model information in image reconstruction is addressed. A closed form solution for the estimated object spectrum is derived using the Lagrange multiplier technique which assumes a measured image, knowledge of the optical transfer function, statistical information about the measurement noise, and a model of the object. This reconstruction algorithm is iterative in nature for two reasons: (1) because the optimal Lagrange multiplier is not generally known at the start of the problem; and (2) we can use the object estimate obtained from one step of the algorithm as the model input for the next step. In this paper we derive the estimator, describe one technique for determining the optimal Lagrange multiplier, demonstrate a stopping criterion based on the mean squared error between a noise free image and the photon-limited version of the image, and show representative results for a sparse aperture imaging application.
We review recent arguments for using increased spectral bandwidth and exposure times to optimize the signal-to-noise ratio of speckle imaging estimators and discuss the tradeoff between camera exposure time and the number of data frames collected when observing time is fixed. We compare experimental results with a previously-derived expression for optimal exposure time and find reasonable agreement after accounting for frequency-dependent camera noise.
The Florida Institue of Technology and the Phillips Laboratory have developed several advanced visible (0.4-0.8 micrometers ) imaging fourier transform spectrometer (IFTS) brassboards, which simultaneously acquire one spatial and one spectral dimension of the hyperspectral image cube. The initial versions of these instruments may be scanned across a scene or configured with a scan mirror to pick up the second spatial dimension of the image cube. The current visible hyperspectral imagers possess a combination of features, including (1) low to moderate spectral resolution for hundreds/thousands of spectral channels, (2) robust design, with no moving parts, (3) detector limited free spectral range, (4) detector-limited spatial and spectral resolution, and (5) field widened operation. The utility of this type of instrument reaches its logical conclusion however, with an instrument designed to acquire all three dimensions of the hyperspectral image cube (both spatial and one spectral) simultaneously. In this paper we present the (1) detailed optical system designs for the brassboard instruments, (2) the current data acquisition system, (3) data reduction and analysis techniques unique to hyperspectral sensor systems which operate with photometric accuracy, and (4) several methods to modify the basic instrument design to allow simultaneous acquistion of the entire hyperspectral image cube. The hyperspectral sensor systems which are being developed and whose utility is being pioneered by Florida Tech and the Phillips Laboratory are applicable to numerous DoD and civil applications including (1) space object identification, (2) radiometrically correct satellite image and spectral signature database observations, (3) simultaneous spactial/spectral observations of booster plumes for strategic and surrogate tactical missile signature identification, and (4) spatial/spectral visible and IR astronomical observations with photometric accuracy.
We derive an expression for the wavelength (lambda) o giving maximum resolution for an adaptive-optics compensated telescope. An approximate expression for average on-axis intensity is written to account for the competing effects of diffraction and residual (post-compression) phase error; this expression is then differentiated with respect to the imaging wavelength (lambda) to yield (lambda) o. The analytically predicted (lambda) o is compared to simulation results and correspondence is shown to be good at widely separated seeing conditions and adaptive optics geometries.
Over the last several years the practical implementation of adaptive optics to compensate for the atmospheric distortions in large telescopes has become a reality. Of the elements that must be considered in the design of an adaptive optics system, the expected atmospheric turbulence is one of the most important. The usual method for estimating these criteria is to use standard atmospheric models or site specific adaptations of these models. An implicit assumption in these models is that the atmosphere can be treated as an isotropic mass and that the index of refraction variations follow Kolmogorov theory. An analysis of two of these features, smoothed data vice a set of individual turbulence profiles and the influence of a partially non-Kolmogorov atmosphere, was performed for a large adaptive optics system. The results show that performance expectations can vary significantly. Smoothed data tends to over estimate atmospheric effects up to 50%. Non-Kolmogorov effects are less significant introducing differences on the order of 10% for zenith observations. The conclusion is that the designer must pay careful attention to the atmospheric model and the method in which it is employed. The use of multiple phase screens created directly from sonde data are recommended.
We study the use of a cross-correlation algorithm as a sub-aperture slope estimator in a low light-level adaptive optics scheme for a two-telescope interferometer. Using a computer simulation of a Hartmann-Shack wavefront sensor to compare the algorithm against a conventional intensity-centroid estimator, we find that the cross-correlation algorithm marginally reduces the mean-square slope-measurement error under shot-noise limited conditions and significantly reduces the error in the presence of additive noise. We compare the performance of the two estimators in a two-telescope interferometer scheme by introducing wavefront sensor noise with appropriate variance in a full adaptive optics simulation, calculating measures-of-merit appropriate to interferometric imaging such as fringe visibility and coherent energy.
Speckle imaging is a statistical technique for achieving near-diffraction-limited imagery of astronomical objects with ground-based telescopes. The performance of this statistical postdetection processing technique is critically dependent on the signal-to-noise ratio (SNR) of the estimators used for various average spectra, which can be a strong function of detector characteristics. We discuss techniques for maximizing SNR under low-light conditions where so-called "read noise" becomes a factor in CCD detectors, and we derive an optimal exposure time for CCD detection when total viewing time limits the SNR. We also show that a properly optimized CCD can outperform a shot-noise-limited detector, in terms of the SNR, at much lower light levels than without optimization.
Adaptive-optics (AO) systems for imaging of exoatmospheric objects employ a wavefront sensor designed to sample the phase imposed by the atmosphere. Whether a Hartmann array or shearing interferometer, such a sensor is typically designed to sample at spatial frequencies corresponding to the expected value of the Fried coherence diameter, r(o), at the site. However, undersampling of the wavefront occurs in practice during periods of bad seeing, and the cost of adaptive-optics systems designed to sample at high frequency and control a large number of actuators make deliberately 'underdesigned' systems attractive. In this paper, we use a detailed computer simulation in a preliminary investigation of the effect of wavefront undersampling on the SNR of the power-spectrum estimate of resulting point-source images. We have found doubling subaperture size in an Hartmann-sensor-driven AO system can nearly approximate the performance given by a fully-sampled system for a 3.67-m telescope.
Short exposure imagery of single and binary stars was collected at the 1.6-m Air Force Maui Optical Station (AMOS) telescope, using a low-noise CCD camera. Atmospheric turbulence effects were partially mitigated using the Compensated Imaging System (CIS), a predetection wavefront sensor and deformable mirror adaptive optical system. We present images and power spectra from both partially compensated and uncompensated short exposure simulations and field data. Our results illustrate that the use of lower-cost partially compensating adaptive optical systems combined with post-detection processing provides a viable alternative to expensive, fully compensated adaptive imaging systems for achieving high-resolution imagery through the atmosphere.
A volumetric thermal grating (VTG) is a spatially periodic refractive index variation in a volume of gas or liquid, generated by imaging interference fringes into the medium. The fringes can be created and varied by steering laser write beams electronically with acousto- optic (A-O) cells. While the wavelength of the write beams is chosen to be absorbed by a dopant in the VTG medium, a read beam at an off-resonance wavelength can be manipulated by diffraction from the resulting index grating. Potential applications include resonator and amplifier optical isolation prepulse suppression in high-gain amplifiers, noninertial steering of large-diameter laser beams, transfer of phase information between beams to facilitate adaptive optics, Q-switching of chemical lasers, and line selection in broadband lasers. In this paper, we present a preliminary assessment of VTG utility for these optical systems applications by quantitative analysis of the medium density dynamics. In Section 2, we derive a relation between A-O acoustic frequency uncertainty and VTG pointing/steering uncertainty, which also scales desired steering range to required A-O frequency modulation bandwidth. In Section 3, we discuss the temporal response of a doped rare-gas VTG medium. Section 4 is an assessment of VTG beam-steering performance potential using available technology.
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