Wavefront sensing with a brightest pixel selection algorithm

Astronomical adaptive optics systems with open-loop deformable mirror control have recently come on-line. In these systems, the deformable mirror surface is not included in the wavefront sensor paths, and so changes made to the deformable mirror are not fed back to the wavefront sensors. This gives rise to all sorts of linearity and control issues mainly centred on one question: Has the mirror taken the shape requested? Non-linearities in wavefront measurement and in the deformable mirror shape can lead to significant deviations in mirror shape from the requested shape. Here, wavefront sensor measurements made using a brightest pixel selection method are discussed along with the implications that this has for open-loop AO systems. Discussion includes elongated laser guide star spots and also computational efficiency.Comment: 10 pages, 12 figures, accepted by MNRA

An explorative philosophical study of envisaging the electrical energy infrastructure of the future

The electrical energy infrastructure is one of the key life-sustaining technologies of contemporary Western society. This infrastructure is extremely complex due to its size, its multifarious technologies, and its interweaving with societal structures. Smart grids are important in future infrastructure, yet extant literature does not adequately address this complexity. This paper argues that different elements of the philosophy of Dooyeweerd offer a key to understanding this intricate complexity more fundamentally. Key concepts are the ideas of normative practices, enkapsis (intertwinement) of practices, individuality structures, and ideals and basic beliefs. By developing these ideas in the context of smart grid engineering, our research contributes to philosophy of technology, philosophy of design, and philosophy of sustainability. It offers an ontological analysis of these infrastructures, pointing a direction to the development of workable infrastructures and supporting the transition to a sustainable society

Suitability of GPUs for real-time control of large astronomical adaptive optics instruments

Adaptive optics (AO) is a technique for correcting aberrations introduced when light propagates through a medium, for example, the light from stars propagating through the turbulent atmosphere. The components of an AO instrument are: (1) a camera to record the aberrations, (2) a corrective mechanism to correct them, (3) a real-time controller (RTC) that processes the camera images and steers the corrective mechanism on milliseconds timescales. We have accelerated the image processing for the AO RTC with the use of graphics processing units (GPUs). It is crucial that the image is processed before the atmospheric turbulence has changed, i.e., in one or two milliseconds. The main task is to transfer the images to the GPU memory with a minimum delay. The key result of this paper is a demonstration that this can be done fast enough using commercial frame grabbers and standard CUDA tools. Our benchmarking image consists of 1.6×1061.6×106 pixels out of which 1.2×1061.2×106 are used in processing. The images are characterized and reduced into a set of 9248 numbers; about one-third of the total processing time is spent on this characterization. This set of numbers is then used to calculate the commands for the corrective system, which takes about two-third of the total time. The processing rate achieved on a single GPU is about 700 frames per second (fps). This increases to 1100 fps (1565 fps) if we use two (four) GPUs. The variation in processing time (jitter) has a root-mean-square value of 20–30 μμ s and about one outlier in a million cycles

Analysis of on-sky MOAO performance of CANARY using natural guide stars

The first on-sky results obtained by CANARY, the multi-object adaptive optics (MOAO) demonstrator, are analysed. The data were recorded at the William Herschel Telescope, at the end of September 2010. We describe the command and calibrations algorithms used during the run and present the observing conditions. The processed data are MOAO-loop engaged or disengaged slopes buffers, comprising the synchronised measurements of the four natural guide stars (NGS) wavefront sensors running in parallel, and near infrared (IR) images. We describe the method we use to establish the error budget of CANARY. We are able to evaluate the tomographic and the open loop errors, having median values around 216 nm and 110 nm respectively. In addition, we identify an unexpected residual quasi-static field aberration term of mean value 110 nm. We present the detailed error budget analysed for three sets of data for three different asterisms. We compare the experimental budgets with the numerically simulated ones and demonstrate a good agreement. We find also a good agreement between the computed error budget from the slope buffers and the measured Strehl ratio on the IR images, ranging between 10% and 20% at 1530 nm. These results make us confident in our ability to establish the error budget of future MOAO instruments

On the Formation of Collective Memories: The Role of a Dominant Narrator.

To test our hypothesis that conversations can contribute to the formation of collective memory, we asked participants to study stories and to recall them individually (pregroup recollection), then as a group (group recounting), and then once again individually (postgroup recollection). One way that postgroup collective memories can be formed under these circumstances is if unshared pregroup recollections in the group recounting influences others\u27 postgroup recollections. In the present research, we explored (using tests of recall and recognition) whether the presence of a dominant narrator can facilitate the emergence of unshared pregroup recollections in a group recounting and whether this emergence is associated with changes in postgroup recollections. We argue that the formation of a collective memory through conversation is not inevitable but is limited by cognitive factors, such as conditions for social contagion, and by situational factors, such as the presence of a narrator