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Widefield Astronomical Image Compensation with Multiple Laser Guided Adaptive Optics

Date: August 2009

Authors: M. Hart, N. M. Milton, C. Baranec, T. Stalcup, K. Powell, E. Bendek, D. McCarthy, C. Kulesa

Abstract: We report closed-loop results obtained from the first adaptive optics system to deploy multiple laser guide beacons.  The system is mounted on the 6.5 m MMT telescope in Arizona, and is designed to explore advanced altitude-conjugated techniques for wide-field image compensation.  Five beacons are made by Rayleigh scattering of laser beams at 532 nm integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics.  The return light is analyzed by a unique Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate.  Wavefront correction is applied with the telescope's unique deformable secondary mirror.  The system has now begun operations as a tool for astronomical science, in a mode in which the boundary-layer turbulence, close to the telescope, is compensated.  Image quality of 0.2-0.3 arc sec is routinely delivered in the near infrared bands from 1.2–2.5 μm over a field of view of 2 arc min.  Although it does not reach the diffraction limit, this represents a 3 to 4-fold improvement in resolution over the natural seeing, and a field of view an order of magnitude larger than conventional adaptive optics systems deliver.  We present performance metrics including images of the core of the globular cluster M3 where correction is almost uniform across the full field.  We describe plans underway to develop the technology further on the twin 8.4 m Large Binocular Telescope and the future 25 m Giant Magellan Telescope. 

MFBD and the Local Minimum Trap

Date: September 2009

Authors: Nagy, J., Mejia-Bustamante, V.

Abstract: Obtaining high resolution images of space objects from ground based telescopes involves using a combination of sophisticated hardware and computational post processing techniques. An important, and often highly effective, computational post processing tool is multi-frame blind deconvolution (MFBD) for image restoration. One difficulty with using MFBD algorithms is that the nonlinear inverse problem they are designed to solve may have many local minima. Standard optimization methods that use the gradient to search for a minimum may get trapped in a local minimum, resulting in a less than optimal restored image. Moreover, in some cases the global minimum may not correspond to the best reconstructed image compared to a different local minimum. In this paper we consider a flexible optimization approach that can easily incorporate various filtering schemes and prior information, resulting in a scheme that is computationally more tractable. Practical implementation details are discussed.

Imaging Geo-Synchronous Satellites with the AEOS Telescope

Date: September 2008

Authors: Hope, D., Jefferies, S., Giebink, C.

Abstract: The USA has significant civilian and military assets in geostationary orbit. High-resolution, ground-based imaging of these assets enables us to monitor in detail their health and safety and to detect the presence of any foreign microsatellites that might be in proximity. Although adaptive optics compensation of ground-based imagery imparts some level of mitigation of the deleterious effects due to the Earth’s turbulent atmosphere, the correction is far from optimal and there is usually ample room for further improvement in resolution through image post processing. Here we show that significant gains in image fidelity and detection sensitivity can be achieved during the image post processing by the injection of prior information on the wave-front phases via wave-front sensing data and wave/phase-diverse data. The gains are such that it is possible that AEOS could become a practical resource for high-fidelity imaging and detection of GEO targets. 
 
 

Ground-Layer Adaptive Optics with Multiple Laser Guide Stars

Date: September 2008

Authors: M. Hart, N. M. Milton, C. Baranec, T. Stalcup, K. Powell, and E. K. Hege

Abstract: We report the first closed-loop results obtained from an adaptive optics system with multiple laser guide beacons. The system is mounted on the 6.5 m MMT in Arizona, and is designed to explore advanced altitude-conjugated techniques for wide-field image compensation. Five beacons are made by Rayleigh scattering of laser beams at 532 nm integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics. The return light is analyzed by a unique Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate. The wavefront sensor divides the 6.5 m telescope pupil into 60 subapertures, and wavefront correction is applied with the telescope's unique deformable secondary mirror. The first application of the system is to correct boundary-layer turbulence, resulting in image quality of 0.2 arcsec in the near infrared bands from 1.2 to 2.5 μm. In this mode we do not try to reach the diffraction limit of the 6.5 m aperture, but instead aim for improved seeing over a field of view much larger than the isoplanatic patch. In this paper we present images of the central 2 arcmin region of the globular cluster M13 in the halo of the Milky Way, and an open cluster, where correction is almost uniform across the full field. The system has particular scientific application to extragalactic survey work, typically done in dark fields where guide stars are very faint, and where large samples of objects are required.

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