Polarimetric imaging for air accident investigation

We report a trial wherein a simple 4 CCD visible-band Polarimetric Imaging (PI) camera was fielded against aircraft debris distributed across an arid terrain, a littoral region and a small number of maritime debris targets A debris field realistically simulating an aircrash and a debris grid of aircraft remains were observed from an air platform flying in dry and sunny conditions. We report PI utility in support of air accident investigation by an enhanced ability to successfully locate small targets within the scene via the use of colour enhanced and decorrelated intensity PI products. Our results indicate that handheld PI capability may represent an effective low cost, upgrade and augmentation option for existing and future imaging systems that would support air accident investigators and assist in the cueing of more sophisticated assets and/or analyst attention

On the conditional frazil ice instability in seawater

It has been suggested that the presence of frazil ice can lead to a conditional instability in seawater. Any frazil forming in the water column reduces the bulk density of a parcel of frazil-seawater mixture, causing it to rise. Due to the pressure-decrease in the freezing point, this causes more frazil to form, causing the parcel to accelerate, and so on. We use linear stability analysis and a non-hydrostatic ocean model to study this instability. We find that frazil ice growth caused by the rising of supercooled water is indeed able to generate a buoyancy-driven instability. Even in a gravitationally stable water column, the frazil ice mechanism can still generate convection. The instability does not operate in the presence of strong density stratification, high thermal driving (warm water), a small initial perturbation, high background mixing or the prevalence of large frazil ice crystals. In an unstable water column the instability is not necessarily expressed in frazil ice at all times; an initial frazil perturbation may melt and refreeze. Given a large enough initial perturbation this instability can allow significant ice growth. A model shows frazil ice growth in an Ice Shelf Water plume several kilometres from an ice shelf, under similar conditions to observations of frazil ice growth under sea ice. The presence of this instability could be a factor affecting the growth of sea ice near ice shelves, with implications for Antarctic bottom water formation

Optimal wind patterns for biological production in shelf ecosystems driven by coastal upwelling

Major upwelling systems around the world provide marine productivity and fishery yield out of proportion to their area. Upwelling winds have the counteracting effects that stronger winds upwell more nutrients to the surface for higher production, but they also transport that production off continental shelves where it may not be consumed by shelf-dwelling species. Because the patterns of wind fluctuations vary in these systems, we determined the conditions for maximal biological production using a simple conveyor belt model. The conditions are that: (1) the average cross-shelf velocity produced by the winds be the value that provides the maximum production with constant winds and (2) that the wind pattern be periodic with period equal to the cross-shelf transport time that results from maximizing production with constant winds. Examination of an example using winds in central California indicated wind patterns optimal for phytoplankton occurred more frequently than those for zooplankton

Development of the X-ray camera for the OGRE sub-orbital rocket

Current theories regarding the matter composition of the universe suggest that half of the expected baryonic matter is missing. One region this could be residing in is intergalactic filaments which absorb strongly in the X-ray regime. Present space based technology is limited when it comes to imaging at these wavelengths and so new techniques are required. The Off-Plane Grating Rocket Experiment (OGRE) aims to produce the highest resolution spectrum of the binary star system Capella, a well-known X-ray source, in the soft X-ray range (0.2keV to 2keV). This will be achieved using a specialised payload combining three low technology readiness level components placed on-board a sub-orbital rocket. These three components consist of an array of large format off-plane X-ray diffraction gratings, a Wolter Type 1 mirror made using single crystal silicon, and the use of EM-CCDs to capture soft X-rays. Each of these components have been previously reviewed with OGRE being the first project to utilise them in a space observation mission. This paper focuses on the EM-CCDs (CCD207-40 by e2v) that will be used and their optimisation with a camera purposely designed for OGRE. Electron Multiplying gain curves were produced for the back-illuminated devices at -80 degrees Celsius. Further tests which will need to be carried out are discussed and the impact of the OGRE mission on future projects mentioned

High-resolution soft x-ray spectrometry using the electron-multiplying charge-coupled device (EM-CCD)

The Electron-Multiplying Charge-Coupled Device (EM-CCD) shares a similar structure to the CCD except for the inclusion of a gain register that multiplies signal before the addition of read-noise, offering sub-electron effective readnoise at high frame-rates. EM-CCDs were proposed for the dispersive spectrometer on the International X-ray Observatory (IXO) to bring sub-300 eV X-rays above the noise, increasing the science yield. The high-speed, low-noise performance of the EMCCD brought added advantages of reduced dark current and stray-light per frame, reducing cooling and filtering requirements. To increase grating efficiency, several diffracted spectral orders were co-located so the inherent energy resolution of the detector was required for order separation. Although the spectral resolution of the EM-CCD is degraded by the gain process, it was shown that the EM-CCD could achieve the required separation. The RIXS spectrometer at the Advanced Resonant Spectroscopy beamline (ADRESS) of the Swiss Light Source (SLS) at the Paul Scherrer Institute currently uses a CCD, with charge spreading between pixels limiting the spatial resolution to 24 μm (FWHM). Through improving the spatial resolution below 5 μm alongside upgrading the grating, a factor of two energy resolution improvement could theoretically be made. With the high-speed, low-noise performance of the EM-CCD, photon-counting modes could allow the use of centroiding techniques to improve the resolution. Using various centroiding techniques, a spatial resolution of 2 μm (FWHM) has been achieved experimentally, demonstrating the benefits of this detector technology for soft X-ray spectrometry. This paper summarises the use of EM-CCDs from our first investigations for IXO through to our latest developments in ground-based testing for synchrotron-research and looks beyond to future possibilities

Comparison of Back-Thinned Detector Ultraviolet Quantum Efficiency for Two Commercially Available Passivation Treatments

Back-thinned silicon detectors offer a high response over a very broad spectrum for direct detection by providing an efficient optical path into the sensing silicon avoiding front face structures manufactured from metal, polysilicon, nitrides, and oxides that may absorb the incident light before reaching the sensing silicon. We have tested two CCDs with different back-surface shallow p+ implant thicknesses (basic and enhanced) at the M4 line (wavelength between 40 and 400 nm) at Physikalisch-Technische Bundesanstalt (PTB)’s Metrology Light Source in Berlin. This characterization in the ultraviolet spectral range extends the soft X-ray quantum efficiency (QE) data set previously acquired with the exact same devices. Due to the short absorption depth and the scope for many types of interactions of the device materials with ultraviolet photons, QE measurement and stability of the device against extended exposure in the UV are of ongoing interest. Therefore, QE measurements have been carried out before and after exposures to quantify any change in behavior. To allow characterization of the passivation processes only, the devices have no antireflection coating. The measured QE of the standard back-thinned CCD is below 10% between 70 and 370 nm. An average additional 5% efficiency is achieved in the enhanced device within the same range. At the limits of the measured spectrum, toward soft X-rays or toward the visible range, the QE increases and the difference between the standard and the enhanced process is reduced as the photon absorption length increases beyond the immediate back-surface. The measured QE after long high-flux exposures at 200 nm shows remarkable improvement

Optical design of the Off-plane Grating Rocket Experiment

The Off-plane Grating Rocket Experiment (OGRE) is a soft X-ray spectroscopy suborbital rocket payload scheduled for launch in Q3 2020 from Wallops Flight Facility. The payload will serve as a testbed for several key technologies which can help achieve the desired performance increases for the next generation of X-ray spectrographs and other space-based missions: monocrystalline silicon X-ray mirrors developed at NASA Goddard Space Flight Center, reflection gratings manufactured at The Pennsylvania State University, and electron-multiplying CCDs developed by the Open University and XCAM Ltd. With these three technologies, OGRE hopes to obtain the highest-resolution on-sky soft X-ray spectrum to date. We discuss the optical design of the OGRE payload

The Off-plane Grating Rocket Experiment (OGRE) system overview

The Off-plane Grating Rocket Experiment (OGRE) is a sub-orbital rocket payload that will make the highest spectral resolution astronomical observation of the soft X-ray Universe to date. Capella, OGRE’s science target, has a well-defined line emission spectrum and is frequently used as a calibration source for X-ray observatories such as Chandra. This makes Capella an excellent target to test the technologies on OGRE, many of which have not previously flown. Through the use of state-of-the-art X-ray optics, co-aligned arrays of off-plane reflection gratings, and an X-ray camera based around four Electron Multiplying CCDs, OGRE will act as a proving ground for next generation X-ray spectrometers

Arctic system on trajectory to new state

The Arctic system is moving toward a new state that falls outside the envelope of glacial-interglacial fluctuations that prevailed during recent Earth history. This future Arctic is likely to have dramatically less permanent ice than exists at present. At the present rate of change, a summer ice-free Arctic Ocean within a century is a real possibility, a state not witnessed for at least a million years. The change appears to be driven largely by feedback-enhanced global climate warming, and there seem to be few, if any processes or feedbacks within the Arctic system that are capable of altering the trajectory toward this “super interglacial” state