David M. Walton

Spatial charge cloud distribution of microchannel plates

We describe an experiment to measure the spatial charge distribution produced by a microchannel plate intensifier with a new type of charge division readout, the ‘‘split strip’’ anode. This anode is a modified strip and strip anode which determines both the amount of charge deposited on each half of the anode and the centroid position for each event. We present experimental measurements of microchannel plate charge cloud distributions for a variety of detector operating conditions. We find that, as a first‐order approximation, one can assume the charge cloud to be azimuthally symmetric. Additionally, the charge cloud remains virtually unchanged from event to event and pore to pore. The general form of the radial distribution is best described by the sum of two exponential components whose scale and relative weights vary with detector operating conditions. The central component of the distribution is three to six times smaller than the outer, or ‘‘wing’’ component. Typically most of the charge is in the ce...

Spatial resolution limitations of microchannel plate/conductive charge division readout devices

Abstract We describe the conductive analogue charge division read-out as typified by the Wedge and Strip Anode (WSA), used in conjunction with a microchannel pl A mathematical model has been used to evaluate the positional modulation produced by various spatial charge distributions on the WSA readout. The spati Finally, we describe an application for a sealed intensifier comprising an optical/UV photocathode, a chevron pair of microchannel plates and a Tetra Wedge Anode which is presently under construction.

Microchannel Plate Pore Size Limited Imaging with Ultra-Thin Wedge and Strip Anodes

We describe a detector employing a curved channel microchannel plate (MCP) in front of an optimised, thin substrate wedge and strip anode (WSA) imaging readout. We obtain a position resolution better than 20¿m FWHM allowing the images of individual MCP pores to be distinguished. The collimation provided by a single pore is used to probe the fundamental limitations in resolution of the WSA. Finally, having experimentally verified the limitations imposed by electronic and partition noise, we describe a detector which will produce 20¿m FWHM resolution over a 50mm diameter imaging area.

Modulation effects in wedge and strip anodes

We present results on the image distortion or ‘‘modulation’’ produced when a wedge and strip anode (WSA) position readout is used with a charge cloud of size comparable with that of the pattern repeat pitch. We use a standard detector consisting of a microchannel plate (MCP) intensifier and wedge and strip anode to investigate the effect of detector operating parameters on image modulation. We find that modulation depends on the MCP gain of each event and on the MCP voltage, and is minimized when the plates are run at high gains and with high rear MCP operating voltages. We compare the experimental results obtained using this detector with the response calculated numerically in a computer simulation. From the simulation it becomes apparent that for a detailed understanding of wedge and strip performance the microchannel plate charge cloud distribution must be considered. However, some of the general features of the wedge and strip anode performance are independent of the charge cloud form. The most import...

EMCCDs for space applications

This paper describes a qualification programme for Electron-Multiplication Charge Coupled Devices (EMCCDs) for use in space applications. While the presented results are generally applicable, the programme was carried out in the context of CCD development for the Radial Velocity Spectrometer (RVS) instrument on the European Space Agencys cornerstone Gaia mission. We discuss the issues of device radiation tolerance, charge transfer efficiency at low signal levels and life time effects on the electron-multiplication gain. The development of EMCCD technology to allow operation at longer wavelengths using high resistivity silicon, and the cryogenic characterisation of EMCCDs are also described.

Determination of phytoplankton abundances (Chlorophyll-a) in the optically complex inland water - The Baltic Sea

A novel approach, termed Summed Positive Peaks (SPP), is proposed for determining phytoplankton abundances (Chlorophyll-a or Chl-a) and surface phytoplankton bloom extent in the optically complex Baltic Sea. The SPP approach is established on the basis of a baseline subtraction method using Rayleigh corrected top-of-atmosphere data from the Medium Resolution Imaging Spectrometer (MERIS) measurements. It calculates the reflectance differences between phytoplankton related signals observed in the MERIS red and near infrared (NIR) bands, such as sun-induced chlorophyll fluorescence (SICF) and the backscattering at 709nm, and considers the summation of the positive line heights for estimating Chl-a concentrations. The SPP algorithm is calibrated against near coincident in situ data collected from three types of phytoplankton dominant waters encountered in the Baltic Sea during 2010 (N=379). The validation results show that the algorithm is capable of retrieving Chl-a concentrations ranging from 0.5 to 3mgm-3, with an RMSE of 0.24mgm-3 (R2=0.69, N=264). Additionally, the comparison results with several Chl-a algorithms demonstrates the robustness of the SPP approach and its sensitivity to low to medium biomass waters. Based on the red and NIR reflectance features, a flagging method is also proposed to distinguish intensive surface phytoplankton blooms from the background water.

The performance of the penning gas imager

Abstract We describe a gas proportional counter of 120 mm diameter employing two stages of charge amplification and utilising a Wedge and Strip Anode two dimensional position readout. The two stage gas amplification allows upper stage gain to be optimised for energy resolution which is further improved by the use of a Penning gas mixture. The second stage of amplification allows the overall gain to be high; a requirement for good spatial resolution. Thus optimal energy and position resolution can be obtained simultaneously. We present pre-flight performance data from the Penning Gas Imager [1] in conjunction with its flight electronics which are being prepared for inclusion in a sounding rocket experiment, the X-ray Objective Grating Spectrometer (XOGS) to be launched in mid-1988.

MERIS observations of phytoplankton phenology in the Baltic Sea

The historical data from the MEdium Resolution Imaging Spectrometer (MERIS) is an invaluable archive for studying global waters from inland lakes to open oceans. Although the MERIS sensor ceased to operate in April 2012, the data capacities are now re-established through the recently launched Sentinel-3 Ocean and Land Colour Instrument (OLCI). The development of a consistent time series for investigating phytoplankton phenology features is crucial if the potential of MERIS and OLCI data is to be fully exploited for inland water monitoring. This study presents a time series of phytoplankton abundance and bloom spatial extent for the highly eutrophic inland water of the Baltic Sea using the 10-year MERIS archive (2002-2011) and a chlorophyll-a based Summed Positive Peaks (SPP) algorithm. A gradient approach in conjunction with the histogram analysis was used to determine a global threshold from the entire collection of SPP images for identifying phytoplankton blooms. This allows spatio-temporal dynamics of daily bloom coverage, timing, phytoplankton abundance and spatial extent to be investigated for each Baltic basin. Furthermore, a number of meteorological and hydrological variables, including spring excess phosphate, summer sea surface temperature and photosynthetically active radiation, were explored using boosted regression trees and generalised additive models to understand the ecological response of phytoplankton assemblages to environmental perturbations and potential predictor variables of summer blooms. The results indicate that the surface layer excess phosphate available in February and March had paramount importance over all other variables considered in governing summer bloom abundance in the major Baltic basins. This finding allows new insights into the development of early warning systems for summer phytoplankton blooms in the Baltic Sea and elsewhere.

Optical telescope BIRT in ORIGIN for gamma ray burst observing

The ORIGIN concept is a space mission with a gamma ray, an X-ray and an optical telescope to observe the gamma ray bursts at large Z to determine the composition and density of the intergalactic matter in the line of sight. It was an answer to the ESA M3 call for proposal. The optical telescope is a 0.7-m F/1 with a very small instrument box containing 3 instruments: a slitless spectrograph with a resolution of 20, a multi-imager giving images of a field in 4 bands simultaneously, and a cross-dispersed Échelle spectrograph giving a resolution of 1000. The wavelength range is 0.5 μm to 1.7 μm. All instruments fit together in a box of 80 mm x 80 mm x 200 mm. The low resolution spectrograph uses a very compact design including a special triplet. It contains only spherical surfaces except for one tilted cylindrical surface to disperse the light. To reduce the need for a high precision pointing, an Advanced Image Slicer was added in front of the high resolution spectrograph. This spectrograph uses a simple design with only one mirror for the collimator and another for the camera. The Imager contains dichroics to separate the bandwidths and glass thicknesses to compensate the differences in path length. All 3 instruments use the same 2k x 2k detector simultaneously so that telescope pointing and tip-tilt control of a fold mirror permit to place the gamma ray burst on the desired instrument without any other mechanism.