Towards optimization of pulsed sodium laser guide stars

Pulsed sodium laser guide stars (LGS) are useful because they allow for Rayleigh blanking and fratricide avoidance in multiple-LGS systems. Bloch-equation simulations of sodium-light interactions show that these may be able to achieve photon returns nearly equal to, and in some cases greater than, what is seen from continuous-wave (CW) excitation. In this work, we study the time-dependent characteristics of sodium fluorescence, and investigate the optimal format for the new fiber laser LGS that will be part of the upgraded adaptive optics (AO) system on the Shane telescope at Mt. Hamilton. Results of this analysis are examined in the context of their general applicability to other LGS systems and the potential benefits of uplink correction are considered. Comparisons of simulation predictions with measurements from existing LGS are also presented and discussed.Comment: 9 pages, 7 figures, accepted by JOSA

IGR J17544-2619 in depth with Suzaku: direct evidence for clumpy winds in a supergiant fast X-ray transient

We present the first direct evidence for dense clumps of matter in the companion wind in a Supergiant Fast X-ray Transient (SFXT) binary. This is seen as a brief period of enhanced absorption during one of the bright, fast flares that distinguish these systems. The object under study was IGR J17544-2619, and a total of 236 ks of data were accumulated with the Japanese satellite Suzaku. The activity in this period spans a dynamic range of almost 10000 in luminosity and gives a detailed look at SFXT behavior.Comment: 14 pages, 8 figures, accepted for The Astrophysical Journal. Version 3 includes minor changes made during the refereeing process: primarily an assortment of references to results appearing since the original submission, but also a new spectral fit (Figure 8

On Divestment

Foreword to On Divestment The writings collected here are an online supplement to a class zine title On Divestment. The zine and online archive were conceived, written, and designed by the members of the Spring 2015 Literature and Environment course (ENGL 374) at the University of Puget Sound. Considerations of timeliness and sustainability encouraged us to keep the size of the printed zine compact by including only excerpts of each author’s work; the zine is in a sense an advertisement for the more substantial body of writing that you find here on Sound Ideas, which reproduces student creative work in its entirety. Informed by our study of works of ecocriticism such as Rob Nixon’s Slow Violence and the Environmentalism of the Poor (Harvard UP, 2011), which explores the role that imaginative writing can play in illuminating ecological issues, our class seized the opportunity of a collaborative final project to see how students’ own writing—whether creative, critical, or some combination of the two—might engage with a topical environmental concern. The selection of a final project was arrived at through a democratic process. Working in groups of five individuals, students presented proposals for a final project, arguing both for the primacy of the environmental concern they selected and the efficacy of the writing task they were asking the class to undertake. All the proposals were excellent, but the class had to settle on a single undertaking; the project that students ultimately selected was the fossil fuel divestment movement, currently a topic of debate on many university campuses, including the University of Puget Sound. Largely led by students, this ongoing environmental movement encourages colleges and universities to divest their endowment holdings from companies whose primary business is fossil fuel. Since we knew from recent reportage in The Trail that UPS’s own ECO Club is advocating for fossil fuel divestment, we invited ECO Club representatives to speak to the class. From their visit, we learned about the broad contours of the divestment movement. Observing that the majority of scientists understand anthropogenic climate change to be the result of burning fossil fuels, the divestment movement argues that colleges and universities should exert pressure on these companies by withdrawing their investments from them. Several universities have already made commitments to divest some component of their endowment, among them Stanford University, Pitzer College, and the University of Glasgow, and campaigns to do so are underway at other institutions of higher learning, as well as many cities and municipalities. In formalizing the class proposal into an assignment, the terms were intentionally crafted to remain open-ended and non-prescriptive. Students were invited simply “to create a work of literature or literary analysis that engages with the issue of university fossil fuel divestment.” (Students could also opt out of the zine and write a more conventional final essay). Here are some excerpts from an email that I sent to the class elaborating on this non-traditional assignment. There\u27s a quotation by W.B. Yeats that strikes me as relevant here: Out of the quarrel with others we make rhetoric; out of the quarrel with ourselves we make poetry. What I understand Yeats to mean is that honest creative writing explores its subject, rather than argues a pre-established position. (For his word poetry in that quotation, you could equally substitute drama, or fiction, creative nonfiction, etc.) Our goal is to write literature, not propaganda . . . . While the work you create should shed light on some aspect of the divestment issue, it needn\u27t do so directly. In fact, it’s possible to shed light on the issue without even mentioning the word “divestment.” Consider, for example, the first novel we read this semester, Margaret Atwood’s Oryx and Crake. That novel engages with a host of environmental topics, among them global climate change, genetic engineering, population growth, the treatment of animals, and environmental justice. It doesn\u27t tell the reader what to do or think about any of these issues; rather, it explores them imaginatively, but it does so in a way that I believe encourages readers to be more reflective about each of those issues than they were before starting the novel. It sheds light on these issues. [K]eep in mind that this assignment does not expect that you will take a particular position. It does not assume that you will create a work of literature or literary criticism that is “for” fossil fuel divestment. (I\u27m not even sure that a work of art can ever be “for” or “against” a particular action. Is Oryx and Crake for or against the human manipulation of the environment? Who knows?—it\u27s a work of imaginative writing that tells a story. Readers can engage their own sense of values based upon the encounter with the imaginative work). I hope that this advice is helpful and that you feel authorized to write freely. I created the opportunity for the assignment because I believe that your voice—both individually and collectively—does matter and I wanted to facilitate a way for it to be heard, in however modest a fashion. What that voice says is entirely up to you. Those twenty voices from the class combine to form the collective voice you encounter here in the Sound Ideas archive On Divestment. On behalf of the writers and critics in ENGL 374, I invite you explore these remarkable poems, short stories, essays, and plays in their entirety. Best wishes, Prof. William Kupinse Department of Englis

The AIROPA software package - Milestones for testing general relativity in the strong gravity regime with AO

General relativity can be tested in the strong gravity regime by monitoring stars orbiting the supermassive black hole at the Galactic Center with adaptive optics. However, the limiting source of uncertainty is the spatial PSF variability due to atmospheric anisoplanatism and instrumental aberrations. The Galactic Center Group at UCLA has completed a project developing algorithms to predict PSF variability for Keck AO images. We have created a new software package (AIROPA), based on modified versions of StarFinder and Arroyo, that takes atmospheric turbulence profiles, instrumental aberration maps, and images as inputs and delivers improved photometry and astrometry on crowded fields. This software package will be made publicly available soon

The AIROPA software package - Milestones for testing general relativity in the strong gravity regime with AO

General relativity can be tested in the strong gravity regime by monitoring stars orbiting the supermassive black hole at the Galactic Center with adaptive optics. However, the limiting source of uncertainty is the spatial PSF variability due to atmospheric anisoplanatism and instrumental aberrations. The Galactic Center Group at UCLA has completed a project developing algorithms to predict PSF variability for Keck AO images. We have created a new software package (AIROPA), based on modified versions of StarFinder and Arroyo, that takes atmospheric turbulence profiles, instrumental aberration maps, and images as inputs and delivers improved photometry and astrometry on crowded fields. This software package will be made publicly available soon

Advancing Adaptive Optics Technology: Laboratory Turbulence Simulation and Optimization of Laser Guide Stars

Since Galileo's first telescope some 400 years ago, astronomers have been building ever-larger instruments. Yet only within the last two decades has it become possible to realize the potential angular resolutions of large ground-based telescopes, by using adaptive optics (AO) technology to counter the blurring effects of Earth's atmosphere. And only within the past decade have the development of laser guide stars (LGS) extended AO capabilities to observe science targets nearly anywhere in the sky. Improving turbulence simulation strategies and LGS are the two main topics of my research. In the first part of this thesis, I report on the development of a technique for manufacturing phase plates for simulating atmospheric turbulence in the laboratory. The process involves strategic application of clear acrylic paint onto a transparent substrate. Results of interferometric characterization of the plates are described and compared to Kolmogorov statistics. The range of r0 (Fried's parameter) achieved thus far is 0.2 - 1.2 mm at 650 nm measurement wavelength, with a Kolmogorov power law. These plates proved valuable at the Laboratory for Adaptive Optics at University of California, Santa Cruz, where they have been used in the Multi-Conjugate Adaptive Optics testbed, during integration and testing of the Gemini Planet Imager, and as part of the calibration system of the on-sky AO testbed named ViLLaGEs (Visible Light Laser Guidestar Experiments). I present a comparison of measurements taken by ViLLaGEs of the power spectrum of a plate and the real sky turbulence. The plate is demonstrated to follow Kolmogorov theory well, while the sky power spectrum does so in a third of the data. This method of fabricating phase plates has been established as an effective and low-cost means of creating simulated turbulence. Due to the demand for such devices, they are now being distributed to other members of the AO community. The second topic of this thesis pertains to understanding and optimizing the laser beacons used to bring AO correction to parts of the sky that lack a naturally bright light source for measuring atmospheric distortion. Long pulse length laser guide stars (LGS) that use fluorescence from the D2 transition in mesospheric sodium are valuable both due to their high altitude, and because they permit Rayleigh blanking and fratricide avoidance in multiple LGS systems. Bloch equation simulations of sodium-light interactions in Mathematica show that certain spectral formats and pulse lengths (on the order of 30 μs), with high duty cycles (20-50%), should be able to achieve photon returns within 10% of what is seen from continuous wave (CW) excitation. Utilizing this recently developed code (called LGSBloch), I investigated the time dependent characteristics of sodium fluorescence. I then identified the optimal format for the new LGS that will be part of the upgrade to the AO system on the Shane 3 meter telescope at the Lick Observatory. I discuss these results, along with their general applicability to other LGS systems, and provide a brief description of the potential benefits of uplink correction. Predictions from the LGSBloch simulation package are compared to data from currently operating LGS systems. For a CW LGS, the return flux measurements and theory show reasonable agreement, but for short pulse lasers, such as those at the Lick and Keck Observatories, the code seems to be overestimating the data by a factor of 2 - 3. Several tactics to explicate this discrepancy are explored, such as verifying parameters involved in the measurements and including greater detail in the modeling. Although these efforts were unsuccessful at removing the discrepancy, they illuminated other facets of the problem that deserve further consideration. Use of the sophisticated LGSBloch model has allowed detailed study of the evolution of the energy level populations and other physical effects (e.g. Larmor precession, atomic recoil, and collisions). This has determined formats that will have maximal coupling efficiency of the laser light to the sodium atoms in order to achieve the highest possible return signal per Watt of output power. These quantitative findings allow the astronomical AO community to make rational, physics-based choices of which high-power (and unavoidably high-cost) lasers to procure for implementation in future LGS systems

Optimizing use of multiple stars for near-infrared tip-tilt compensation at the W. M. Keck Observatory

The implementation of a near-infrared tip-tilt sensor in the Keck I adaptive optics (AO) system is the first of its kind, and represents a substantial step forward in AO technology. Enhanced-TRICK (Tilt Removal with IR Compensation at Keck) is a project built off this implementation which will further improve performance and versatility. Currently the system is capable of using a single star for measuring image motion, but in the off-axis case this may cause elongation in the science image due to tilt anisoplanatism. The near-infrared tip-tilt sensor (NIRTTS) has been designed with the capability of using up to three stars from around the field to correct for elongation. In order for the science object correction to be optimized, the measurements from each star must be weighted based on magnitude and separation from the science object. When weighted optimally the tip-tilt error at the science object will be a minimum. The process for assigning weights is described, and results of performance modeling of the actual systems having multiple tip-tilt stars are presented

Optimizing use of multiple stars for near-infrared tip-tilt compensation at the W. M. Keck Observatory

The implementation of a near-infrared tip-tilt sensor in the Keck I adaptive optics (AO) system is the first of its kind, and represents a substantial step forward in AO technology. Enhanced-TRICK (Tilt Removal with IR Compensation at Keck) is a project built off this implementation which will further improve performance and versatility. Currently the system is capable of using a single star for measuring image motion, but in the off-axis case this may cause elongation in the science image due to tilt anisoplanatism. The near-infrared tip-tilt sensor (NIRTTS) has been designed with the capability of using up to three stars from around the field to correct for elongation. In order for the science object correction to be optimized, the measurements from each star must be weighted based on magnitude and separation from the science object. When weighted optimally the tip-tilt error at the science object will be a minimum. The process for assigning weights is described, and results of performance modeling of the actual systems having multiple tip-tilt stars are presented

Near-infrared tip-tilt sensing at Keck: System architecture and on-sky performance

The sky coverage and performance of laser guide star (LGS) adaptive optics (AO) systems is limited by the natural guidestar (NGS) used for low order correction. This limitation can be reduced by measuring image motion of the NGS in thenear-infrared where it is partially corrected by the LGS AO system and where stars are generally several magnitudesbrighter than at visible wavelengths. We have integrated a near-infrared tip-tilt sensor with the Keck I telescope’s LGSAO system and recently began offering it for science use. The implementation involved modifications to the AO bench,real-time control system, higher-level controls and operations software. The tip-tilt sensor is a H2RG-based near-infraredcamera with 0.05 arcsecond pixels. Low noise at high sample rates is achieved by only reading a small region of interest,from 2x2 to 16x16 pixels, centered on an NGS anywhere in an 100 arc second diameter field. The sensor operates ateither Ks or H-band using light reflected by a choice of dichroic beam-splitters located in front of the OSIRIS integralfield spectrograph. This work presents an overview of the completed system along with on-sky performance results.Lessons learned and efforts to extend the capabilities and further optimize the system are also discussed