Kalman-filter control schemes for fringe tracking. Development and application to VLTI/GRAVITY

The implementation of fringe tracking for optical interferometers is inevitable when optimal exploitation of the instrumental capacities is desired. Fringe tracking allows continuous fringe observation, considerably increasing the sensitivity of the interferometric system. In addition to the correction of atmospheric path-length differences, a decent control algorithm should correct for disturbances introduced by instrumental vibrations, and deal with other errors propagating in the optical trains. We attempt to construct control schemes based on Kalman filters. Kalman filtering is an optimal data processing algorithm for tracking and correcting a system on which observations are performed. As a direct application, control schemes are designed for GRAVITY, a future four-telescope near-infrared beam combiner for the Very Large Telescope Interferometer (VLTI). We base our study on recent work in adaptive-optics control. The technique is to describe perturbations of fringe phases in terms of an a priori model. The model allows us to optimize the tracking of fringes, in that it is adapted to the prevailing perturbations. Since the model is of a parametric nature, a parameter identification needs to be included. Different possibilities exist to generalize to the four-telescope fringe tracking that is useful for GRAVITY. On the basis of a two-telescope Kalman-filtering control algorithm, a set of two properly working control algorithms for four-telescope fringe tracking is constructed. The control schemes are designed to take into account flux problems and low-signal baselines. First simulations of the fringe-tracking process indicate that the defined schemes meet the requirements for GRAVITY and allow us to distinguish in performance. In a future paper, we will compare the performances of classical fringe tracking to our Kalman-filter control.Comment: 17 pages, 8 figures, accepted for publication in A&

Distinguishing an ejected blob from alternative flare models at the Galactic centre with GRAVITY

The black hole at the Galactic centre exhibits regularly flares of radiation, the origin of which is still not understood. In this article, we study the ability of the near-future GRAVITY infrared instrument to constrain the nature of these events. We develop realistic simulations of GRAVITY astrometric data sets for various flare models. We show that the instrument will be able to distinguish an ejected blob from alternative flare models, provided the blob inclination is >= 45deg, the flare brightest magnitude is 14 <= mK <= 15 and the flare duration is >= 1h30.Comment: 11 pages, 9 figures, accepted by MNRA

A Design Aid for Determining Width of Filter Strips

Watershed planners need a tool for determining width of filter strips that is accurate enough for developing cost-effective site designs and easy enough to use for making quick determinations on a large number and variety of sites. This study employed the process-based Vegetative Filter Strip Model to evaluate the relationship between filter strip width and trap¬ping efficiency for sediment and water and to produce a design aid for use where specific water quality targets must be met. Model simulations illustrate that relatively narrow filter strips can have high impact in some situations, while in others even a modest impact cannot be achieved at any practical width. A graphical design aid was developed for estimating the width needed to achieve target trapping efficiencies for different pollutants under a broad range of agricultural site conditions. Using the model simulations for sediment and water, a graph was produced containing a family of seven lines that divide the full range of possible relationships between width and trapping efficiency into fairly even increments. Simple rules guide the selection of one line that best describes a given field situation by considering field length and cover management, slope, and soil texture. Relationships for sediment-bound and dissolved pollutants are interpreted from the modeled relationships for sediment and water. Interpolation between lines can refine the results and account for additional variables, if needed. The design aid is easy to use, accounts for several major variables that determine filter strip performance, and is based on a validated, process-based, mathematical model. This design aid strikes a balance between accuracy and utility that fills a wide gap between existing design guides and mathematical models

A Design Aid for Determining Width of Filter Strips

Watershed planners need a tool for determining width of filter strips that is accurate enough for developing cost-effective site designs and easy enough to use for making quick determinations on a large number and variety of sites. This study employed the process-based Vegetative Filter Strip Model to evaluate the relationship between filter strip width and trap¬ping efficiency for sediment and water and to produce a design aid for use where specific water quality targets must be met. Model simulations illustrate that relatively narrow filter strips can have high impact in some situations, while in others even a modest impact cannot be achieved at any practical width. A graphical design aid was developed for estimating the width needed to achieve target trapping efficiencies for different pollutants under a broad range of agricultural site conditions. Using the model simulations for sediment and water, a graph was produced containing a family of seven lines that divide the full range of possible relationships between width and trapping efficiency into fairly even increments. Simple rules guide the selection of one line that best describes a given field situation by considering field length and cover management, slope, and soil texture. Relationships for sediment-bound and dissolved pollutants are interpreted from the modeled relationships for sediment and water. Interpolation between lines can refine the results and account for additional variables, if needed. The design aid is easy to use, accounts for several major variables that determine filter strip performance, and is based on a validated, process-based, mathematical model. This design aid strikes a balance between accuracy and utility that fills a wide gap between existing design guides and mathematical models

MODELING SEDIMENT TRAPPING IN A VEGETATIVE FILTER ACCOUNTING FOR CONVERGING OVERLAND FLOW

Vegetative filters (VF) are used to remove sediment and other pollutants from overland flow. When modeling the hydrology of VF, it is often assumed that overland flow is planar, but our research indicates that it can be two-dimensional with converging and diverging pathways. Our hypothesis is that flow convergence will negatively influence the sediment trapping capability of VF. The objectives were to develop a two-dimensional modeling approach for estimating sediment trapping in VF and to investigate the impact of converging overland flow on sediment trapping by VF. In this study, the performance of a VF that has field-scale flow path lengths with uncontrolled flow direction was quantified using field experiments and hydrologic modeling. Simulations of water flow processes were performed using the physically based, distributed model MIKE SHE. A modeling approach that predicts sediment trapping and accounts for converging and diverging flow was developed based on the University of Kentucky sediment filtration model. The results revealed that as flow convergence increases, filter performance decreases, and the impacts are greater at higher flow rates and shorter filter lengths. Convergence that occurs in the contributing field (in-field) upstream of the buffer had a slightly greater impact than convergence that occurred in the filter (in-filter). An area-based convergence ratio was defined that relates the actual flow area in a VF to the theoretical flow area without flow convergence. When the convergence ratio was 0.70, in-filter convergence caused the sediment trapping efficiency to be reduced from 80% for the planar flow condition to 64% for the converging flow condition. When an equivalent convergence occurred in-field, the sediment trapping efficiency was reduced to 57%. Thus, not only is convergence important but the location where convergence occurs can also be important

Characterization of hexabundles: Initial results

New multi-core imaging fibre bundles -- hexabundles -- being developed at the University of Sydney will provide simultaneous integral field spectroscopy for hundreds of celestial sources across a wide angular field. These are a natural progression from the use of single fibres in existing galaxy surveys. Hexabundles will allow us to address fundamental questions in astronomy without the biases introduced by a fixed entrance aperture. We have begun to consider instrument concepts that exploit hundreds of hexabundles over the widest possible field of view. To this end, we have compared the performance of a 61-core fully-fused hexabundle and 5 lightly-fused bundles with 7 cores each. All fibres in the bundles have 100 micron cores. In the fully-fused bundle, the cores are distorted from a circular shape in order to achieve a higher fill fraction. The lightly-fused bundles have circular cores and five different cladding thicknesses which affect the fill fraction. We compare the optical performance of all 6 bundles and find that the advantage of smaller interstitial holes (higher fill fraction) is outweighed by the increase in modal coupling, cross-talk and the poor optical performance caused by the deformation of the fibre cores. Uniformly high throughput and low cross-talk are essential for imaging faint astronomical targets with sufficient resolution to disentangle the dynamical structure. Devices already under development will have between 100 and 200 lightly-fused cores, although larger formats are feasible. The light-weight packaging of hexabundles is sufficiently flexible to allow existing robotic positioners to make use of them.Comment: Accepted for publication in MNRAS. See also a complimentary paper on the development of hexabundles - Bland-Hawthorn et al. 2011, Optics Express, vol. 19, p. 2649 (http://www.opticsinfobase.org/abstract.cfm?URI=oe-19-3-2649

Holocene reef growth and recent carbonate production in the Red Sea

Holocene reef growth, present date bioerosion .and recorded carbonate production were studied in the fringing reef at Aqaba, Red Sea. Water depth, wave impact as well as nutrient availability were considered. The carbonate production was measured for several coral samples. Samples of Porites-colonies were collected from several depths and sites near the Marine Science Station at Aqaba. Growth rate depends on water depth, size and age of colonies. Within the coral optimum of water depth growth rates vary between 5 and 16 mm/yr. Coral carbonate production was calculated on the base of annual growth increments and skeletal density using transects from shallow subtidal down to 40 m water depth. High resolution stable isotope data were measured to prove the origin of growth increments. Long-term trends of sea surface temperature and carbon isotope shift (1800-today) fit to the known global deviations. Bioerosion rates were determined using standard dead coral substrates exposed in different water depths and environmental settings. Rates vary between 0.6 and 1.4 kg/m2yr. Sediment export evaluated by means of simple sediment traps ranges between 0.3 and 0.7 kg/m2yr. Gross carbonate production, mainly built up by scleractinian corals, amounts to ca. 1.57 kg/m2yr. Bioerosion alters approx. 1.3 kg/m2yr of hard substrates into sediment. Sediment export is estimated to be ca. 0.4-0.6 kg/m2yr. Thus a net production of ca. 0.7 to 0.9 kg/m2yr should remain in the present reef, which is proved by the recorded carbonate production (reef drillings). Net production preserved in the reef can be given with ca. 800 kg/m2kyr (=0.8 kg/m2yr)