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In JoVE (1)
Other Publications (4)
Articles by Christoph Baranec in JoVE
Bringing the Visible Universe into Focus with Robo-AO
Christoph Baranec1,2, Reed Riddle1, Nicholas M. Law3, A.N. Ramaprakash4, Shriharsh P. Tendulkar2, Khanh Bui1, Mahesh P. Burse4, Pravin Chordia4, Hillol K. Das4, Jack T.C. Davis1, Richard G. Dekany1, Mansi M. Kasliwal5, Shrinivas R. Kulkarni1,2, Timothy D. Morton2, Eran O. Ofek6, Sujit Punnadi4
1Caltech Optical Observatories, California Institute of Technology, 2Department of Astronomy, California Institute of Technology, 3Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 4Inter-University Centre for Astronomy & Astrophysics, 5Observatories of the Carnegie Institution for Science, 6Benoziyo Center for Astrophysics, Weizmann Institute of Science
Light from astronomical objects must travel through the earth's turbulent atmosphere before it can be imaged by ground-based telescopes. To enable direct imaging at maximum theoretical angular resolution, advanced techniques such as those employed by the Robo-AO adaptive-optics system must be used.
Other articles by Christoph Baranec on PubMed
Concept for a Laser Guide Beacon Shack-Hartmann Wave-front Sensor with Dynamically Steered Subapertures
Optics Letters. Apr, 2005 | Pubmed ID: 15832908
We describe an innovative implementation of the Shack-Hartmann wave-front sensor that is designed to correct the perspective elongation of a laser guide beacon in adaptive optics. Subapertures are defined by the segments of a deformable mirror rather than by a conventional lenslet array. A bias tilt on each segment separates the beacon images on the sensor's detector. One removes the perspective elongation by dynamically driving each segment with a predetermined open-loop signal that would, in the absence of atmospheric wave-front aberration, keep the corresponding beacon image centered on the subaperture's optical axis.
Experimental Results of Ground-layer and Tomographic Wavefront Reconstruction from Multiple Laser Guide Stars
Optics Express. Aug, 2006 | Pubmed ID: 19529120
We describe results from the first multi-laser wavefront sensing system designed to support tomographic modes of adaptive optics (AO). The system, now operating at the 6.5 m MMT telescope in Arizona, creates five beacons 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 Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate. A separate high-order Shack-Hartmann sensor records simultaneous measurements of wavefronts from a natural star. From open-loop measurements, we find the average beacon wavefront gives a good estimate of ground layer aberration. We present results of full tomographic wavefront analysis, enabled by supplementing the laser data with simultaneous fast image motion measurements from three stars in the field. We describe plans for an early demonstration at the MMT of closed-loop ground layer AO, and later tomographic AO.
Study of a MEMS-based Shack-Hartmann Wavefront Sensor with Adjustable Pupil Sampling for Astronomical Adaptive Optics
Applied Optics. Oct, 2008 | Pubmed ID: 18830305
We introduce a Shack-Hartmann wavefront sensor for adaptive optics that enables dynamic control of the spatial sampling of an incoming wavefront using a segmented mirror microelectrical mechanical systems (MEMS) device. Unlike a conventional lenslet array, subapertures are defined by either segments or groups of segments of a mirror array, with the ability to change spatial pupil sampling arbitrarily by redefining the segment grouping. Control over the spatial sampling of the wavefront allows for the minimization of wavefront reconstruction error for different intensities of guide source and different atmospheric conditions, which in turn maximizes an adaptive optics system's delivered Strehl ratio. Requirements for the MEMS devices needed in this Shack-Hartmann wavefront sensor are also presented.
Applied Optics. Nov, 2009 | Pubmed ID: 19904325
Low-Fresnel number optical systems exhibit significant diffraction effects that cause a shift in the peaks of on-axis irradiance away from the geometric focal point. This is currently interpreted as a change of the focal length of an optical system, leading optical system designers to compensate for the effect by assuming the image plane is coincident with the peak of on-axis irradiance. While this may be an appropriate interpretation for certain applications, I show that despite the shift in peak irradiance away from the geometrical focal point, a change in a system's optical power will not increase the on-axis irradiance at that distance. This is important for low-light level applications where it is necessary to mitigate diffraction induced transmission losses. I also show that low-Fresnel number systems have increased tolerance on system power at the geometrical focal point and as a result are inherently achromatic.