The Cherenkov Telescope Array Observatory (CTAO) will include telescopes of three different sizes, the smallest of which are the Small-Sized Telescopes (SSTs). In particular, the SSTs will be installed at the southern site of CTAO, on the Chilean Andes, and will cover the highest energy range of CTAO (up to ~300 TeV). The SSTs are developed by an international consortium of institutes that will provide them as an in-kind contribution to CTAO. The optical design of the SSTs is based on a Schwarzschild-Couder-like dual-mirror polynomial configuration, with a primary aperture of 4.3m diameter. They are equipped with a focal plane camera based on SiPM detectors covering a field of view of ~9°. The preliminary design of the SST telescopes was evaluated and approved during the Product Review (PR) organised with CTAO in February 2023. The SST project is now going through a consolidation phase leading to the finalisation and submission of the final design to the Critical Design Review (CDR), expected to occur late 2024, after which the production and construction of the telescopes will begin leading to a delivery of the telescopes to CTAO southern site starting at the end of 2025-early 2026 onward. In this contribution we will present the progress of the SST programme, including the results of the PDR, the consolidation phase of the project and the plan up to the on-site integration of the telescopes.
Vacuum photodetectors have a long history in ultraviolet (UV) sensing for both astronomy and remote sensing. One of the main advantages of this technology is the ability to use solar blind photocathodes to enable high sensitivity measurements of astronomical and atmospheric sources of Far UV (FUV) and Deep UV (DUV) emission in environments with high visible light (VIS) backgrounds. The use of microchannel plates (MCP) in vacuum photodetectors also allows single photon sensitivity for extremely weak signals. However, these detectors have typically suffered from lower Quantum Efficiency (QE) than their solid-state alternatives. Recent advances in photocathode technology have resulted in significant increases in QE for several UV sensitive photocathodes. We present test results of next generation high QE photocathodes appropriate for use in a wide range of FUV and DUV astronomy and remote sensing. A newly developed opaque Cesium Iodide (CsI) photocathode deposited on microchannel plates and sealed into vacuum photodetectors with a Magnesium Fluoride (MgF2) input window demonstrates QE of < 16% @ 130 nm. An optimized transmission mode solar blind (SB) alkali-telluride photocathode demonstrates 29% peak QE and 103 to 108 suppression of NUV and visible light, a significant improvement over previous alkali-telluride photocathodes. Finally, we present data from a new high QE S20 alkali-antimonide photocathode with < 40% QE at 254 nm, suitable for instruments requiring wideband DUV through VIS coverage. Improvements in collection efficiency of vacuum photodetector MCPs from 60% to 90% will also be presented, providing a further 50% boost to detective QE.
The Cherenkov Telescope Array Observatory (CTAO) consists of three types of telescopes: large-sized (LST), mediumsized (MST), and small-sized (SST), distributed in two observing sites (North and South). For the CTA South “Alpha Configuration” the construction and installation of 37 (+5) SST telescopes (a number that could increase up to 70 in future upgrades) are planned. The SSTs are developed by an international consortium of institutes that will provide them as an in-kind contribution to CTAO. The SSTs rely on a Schwarzschild-Couder-like dual-mirror polynomial optical design, with a primary mirror of 4 m diameter, and are equipped with a focal plane camera based on SiPM detectors covering a field of view of ~9°. The current SST concept was validated by developing the prototype dual-mirror ASTRI-Horn Cherenkov telescope and the CHEC-S SiPM focal plane camera. In this contribution, we will present an overview of the SST key technologies, the current status of the SST project, and the planned schedule.
KEYWORDS: Sensors, Data modeling, Single photon, Calibration, Neural networks, Electronics, Monte Carlo methods, Picosecond phenomena, Machine learning, Performance modeling
Currently new applications for single photon imaging detectors, are challenging algorithmic signal processing approaches due to increasing photon event rates. This research explores a potential solution of machine learning (ML) algorithms for data analysis and imaging with single photon timing detectors with 16 ×16 pixels and 60 ps timing resolution. This novel ML approach will accelerate the data processing pipeline, which must process huge volumes of data, up to 10 Gbps per detector, with hundreds of detectors in certain applications. The ML model processes the photon detector output, applying spatial/temporal clustering to improve the photon detector spatial resolution with a time constraint of 10 µs.
The prototype Schwarzschild-Couder Telescope (pSCT) is a candidate for a medium-sized telescope in the Cherenkov Telescope Array. The pSCT is based on a dual-mirror optics design that reduces the plate scale and allows for the use of silicon photomultipliers as photodetectors. The prototype pSCT camera currently has only the central sector instrumented with 25 camera modules (1600 pixels), providing a 2.68-deg field of view (FoV). The camera electronics are based on custom TARGET (TeV array readout with GSa/s sampling and event trigger) application-specific integrated circuits. Field programmable gate arrays sample incoming signals at a gigasample per second. A single backplane provides camera-wide triggers. An upgrade of the pSCT camera that will fully populate the focal plane is in progress. This will increase the number of pixels to 11,328, the number of backplanes to 9, and the FoV to 8.04 deg. Here, we give a detailed description of the pSCT camera, including the basic concept, mechanical design, detectors, electronics, current status, and first light.
The Gamma-ray Cherenkov Telescope (GCT) is one of the telescopes proposed for the Small Sized Telescope (SST) section of CTA. Based on a dual-mirror Schwarzschild-Couder design, which allows for more compact telescopes and cameras than the usual single-mirror designs, it will be equipped with a Compact High-Energy Camera (CHEC) based on silicon photomultipliers (SiPM). In 2015, the GCT prototype was the first dual-mirror telescope constructed in the prospect of CTA to record Cherenkov light on the night sky. Further tests and observations have been performed since then. This report describes the current status of the GCT, the results of tests performed to demonstrate its compliance with CTA requirements, and the optimisation of the design for mass production. The GCT collaboration, including teams from Australia, France, Germany, Japan, the Netherlands and the United Kingdom, plans to install the first telescopes on site in Chile for 2019-2020 as part of the CTA pre-production phase.
J. L. Dournaux, A. Abchiche, D. Allan, J. P. Amans, T. P. Armstrong, A. Balzer, D. Berge, C. Boisson, J.-J. Bousquet, A. Brown, M. Bryan, G. Buchholtz, P. Chadwick, H. Costantini, G. Cotter, L. Dangeon, M. Daniel, A. De Franco, F. De Frondat, D. Dumas, J. P. Ernenwein, G. Fasola, S. Funk, J. Gironnet, J. Graham, T. Greenshaw, B. Hameau, O. Hervet, N. Hidaka, J.A. Hinton, J.M. Huet, I. Jégouzo, T. Jogler, T. Kawashima, M. Kraush, J. Lapington, P. Laporte, J. Lefaucheur, S. Markoff, T. Melse, L. Mohrmann, P. Molyneux, S. Nolan, A. Okumura, J. Osborne, R. Parsons, S. Rosen, D. Ross, G. Rowell, C. Rulten, Y. Sato, F. Sayède, J. Schmoll, H. Schoorlemmer, M. Servillat, H. Sol, V. Stamatescu, M. Stephan, R. Stuik, J. Sykes, H. Tajima, J. Thornhill, L. Tibaldo, C. Trichard, J. Vink, J. Watson, R. White, N. Yamane, A. Zech, A. Zink
The GCT (Gamma-ray Cherenkov Telescope) is a dual-mirror prototype of Small-Sized-Telescopes proposed for the Cherenkov Telescope Array (CTA) and made by an Australian-Dutch-French-German-Indian-Japanese-UK-US consortium. The integration of this end-to-end telescope was achieved in 2015. On-site tests and measurements of the first Cherenkov images on the night sky began on November 2015. This contribution describes the telescope and plans for the pre-production and a large scale production within CTA.
A. Brown, A. Abchiche, D. Allan, J.-P. Amans, T. Armstrong, A. Balzer, D. Berge, C. Boisson, J.-J. Bousquet, M. Bryan, G. Buchholtz, P. Chadwick, H. Costantini, G. Cotter, M. Daniel, A. De Franco, F. de Frondat, J.-L. Dournaux, D. Dumas, G Fasola, S. Funk, J. Gironnet, J. Graham, T. Greenshaw, O. Hervet, N. Hidaka, J. Hinton, J.-M. Huet, I. Jégouzo, T. Jogler, M. Kraus, J. Lapington, P. Laporte, J. Lefaucheur, S. Markoff, T. Melse, L. Mohrmann, P. Molyneux, S. Nolan, A. Okumura, J. Osborne, R. Parsons, S. Rosen, D. Ross, G. Rowell, Y. Sato, F. Sayede, J. Schmoll, H. Schoorlemmer, M. Servillat, H. Sol, V. Stamatescu, M. Stephan, R. Stuik, J. Sykes, H. Tajima, J. Thornhill, L. Tibaldo, C. Trichard, J. Vink, J. Watson, R. White, N. Yamane, A. Zech, A. Zink, J. Zorn
The Gamma-ray Cherenkov Telescope (GCT) is proposed for the Small-Sized Telescope component of the Cherenkov Telescope Array (CTA). GCT's dual-mirror Schwarzschild-Couder (SC) optical system allows the use of a compact camera with small form-factor photosensors. The GCT camera is ~ 0:4 m in diameter and has 2048 pixels; each pixel has a ~ 0:2° angular size, resulting in a wide field-of-view. The design of the GCT camera is high performance at low cost, with the camera housing 32 front-end electronics modules providing full waveform information for all of the camera's 2048 pixels. The first GCT camera prototype, CHEC-M, was commissioned during 2015, culminating in the first Cherenkov images recorded by a SC telescope and the first light of a CTA prototype. In this contribution we give a detailed description of the GCT camera and present preliminary results from CHEC-M's commissioning.
The Capacitive Division Image Readout (C-DIR) is a simple and novel image readout for photon counting detectors
offering major performance advantages. C-DIR is a charge centroiding device comprising three elements; (i) a resistive
anode providing event charge localization, event current return path and electrical isolation from detector high voltage,
(ii) a dielectric substrate which capacitively couples the event transient signal to the third element, (iii) the readout
device; an array of capacitively coupled electrodes which divides the signal among the readout charge measurement
nodes.
The resistive anode and dielectric substrate constitute the rear interface of the detector and capacitively couple the signal
to the external C-DIR readout device. The C-DIR device is a passive, multilayer printed circuit board type device
comprising a matrix of isolated electrodes whose geometries define the capacitive network. C-DIR is manufactured using
conventional PCB geometries and is straightforward and economical to construct.
C-DIR’s robustness and simplicity belie its performance advantages. Its capacitive nature avoids partition noise, the
Poisson noise associated with collection of discrete charges. The dominant noise limiting position resolution is electronic
noise. However C-DIR also presents a low input capacitance to the readout electronics, minimising this noise component
thus maximising spatial resolution. Optimisation of the C-DIR pattern-edge geometry can provide ~90% linear dynamic
range.
We present data showing image resolution and linearity of the C-DIR device in a microchannel plate detector and
describe various electronic charge measurement scheme designed to exploit the full performance potential of the C-DIR
device.
Adaptive high speed low noise detector electronics are being developed for a UV imaging instrument for application to
astronomy and planetary science space missions and for various other terrestrial high speed imaging applications.
Forthcoming space missions such as ESA JUICE1 and the World Space Observatory2 have requirements for UV photon
counting imaging detectors with high dynamic range, high spatial resolution and high radiation tolerance. Imaging
techniques that can adapt to different luminosity conditions and optimise the image spatial resolution against the
incoming photon event rate can provide significant performance advantages.
We introduce an imaging photon counting Microchannel Plate (MCP) detector utilising a low noise Capacitive Division
Image Readout (C-DIR)3 with adaptive pulse shaping capability. Our experimental setup provides controllable photon
count rates for end-to-end detector performance measurement and system calibration. It uses a four channel fast digitiser
which enables us to easily investigate various digital pulse shaping techniques and vary shaping time constants to assess
their impact on detector performance.
In this paper we describe our laboratory experimental setup, illustrate the method of imaging from photon counting and
describe techniques for quantifying the image spatial resolution. Finally we present our current set of results comparing
the measured spatial resolution with the theoretical determined from the measured intrinsic electronic noise of the
system.
The World Space Observatory - Ultraviolet (WSO-UV) is a space astronomy project led by Russia, with contributions
from China, Germany, Italy, Spain, United Kingdom and a number of other countries in the world. WSO-UV consists of
a 1.7-meter diameter telescope and three focal plane science instruments. The Long Slit Spectrograph instrument on-board
WSO-UV will produce moderate spectral resolution (R=1000-2500) spectra in the 102nm ~ 320nm wavelength
range along a slit of 75 arcsec in length and 1 arcsec in width. The spatial resolution of the instrument will be ~1 arcsec.
A two-channel scheme is proposed to optimize performance, with each of these using a Rowland Circle optical design
with Microchannel Plate detectors in the focal plane. We will discuss the detailed design of the spectrograph and its
expected performance in this paper.
We have designed and built a sealed tube microchannel plate (MCP) intensifier for optical/NUV photon counting applications suitable for 18, 25 and 40 mm diameter formats. The intensifier uses an electronic image readout to provide direct conversion of event position into electronic signals, without the drawbacks associated with phosphor screens and subsequent optical detection. The Image Charge technique is used to remove the readout from the intensifier vacuum enclosure, obviating the requirement for additional electrical vacuum feedthroughs and for the readout pattern to be UHV compatible. The charge signal from an MCP intensifier is capacitively coupled via a thin dielectric vacuum window to the electronic image readout, which is external to the sealed intensifier tube. The readout pattern is a separate item held in proximity to the dielectric window and can be easily detached, making the system easily reconfigurable. Since the readout pattern detects induced charge and is external to the tube, it can be constructed as a multilayer, eliminating the requirement for narrow insulator gaps and allowing it to be constructed using standard PCB manufacturing tolerances. We describe two readout patterns, the tetra wedge anode (TWA), an optimized 4 electrode device similar to the wedge and strip anode (WSA) but with a factor 2 improvement in resolution, and an 8 channel high speed 50 ohm device, both manufactured as multilayer PCBs. We present results of the detector imaging performance, image resolution, linearity and stability, and discuss the development of an integrated readout and electronics device based on these designs.
We are currently investigating techniques to improve the count rate and time resolution of microchannel plates (MCPs) and high speed electronic image readout schemes, as the basis for the development of high performance imaging MCP detectors for space science and other disciplines. We discuss the factors limiting the ultimate count rate and time resolution of MCP detectors and review the potential of techniques such as bulk conductive glass in MCP manufacture, and the advantages conferred by small MCP pore size. We present test results indicating the improved time resolution achievable using small pore MCPs. We review developments in readout design to increase performance for high throughput detectors, and which are capable of providing suitable combinations of attributes for specific applications, including high spatial resolution, high time resolution, high count rate, and parallel event processing for detection of simultaneous events. High throughput techniques require an increase in processing channel density and we discuss how this may be achieved by integration of the readout with the electronics package. The design and manufacture of readout systems using integrated ASIC based electronics is discussed and projected performance is presented. Such devices also have uses over a wide range of other scientific disciplines, and we discuss applications ranging from biomedicine to synchrotron physics.
EUVE and the ROSAT WFC have left a tremendous legacy in astrophysics at EUV wavelengths. More recently, Chandra and XMM-Newton have demonstrated at X-ray wavelengths the power of high-resolution astronomical spectroscopy, which allows the identification of weak emission lines, the measurement of Doppler shifts and line profiles, and the detection of narrow absorption features. This leads to a thorough understanding of the density, temperature, abundance, magnetic, and dynamic structure of astrophysical plasmas. However, the termination of the EUVE mission has left a gap in spectral coverage at crucial EUV wavelengths (~100-300 Å), where hot (105 - 108 K) plasmas radiate most strongly and produce critical spectral diagnostics. CHIPS will fill this hole only partially as it is optimized for diffuse emission and has only moderate resolution (R~150). For discrete sources, we have successfully flown a follow-on instrument to the EUVE spectrometer (Aeff ~ 1 cm2, R ~ 400), the high-resolution spectrometer J-PEX (Aeff ~ 3 cm2, R ~ 3000). Here we build on the J-PEX prototype and present a strawman design for an orbiting spectroscopic observatory, APEX, a SMEX-class instrument containing a suite of 8 spectrometers that together achieve both high effective area (Aeff > 10 cm2) and high spectral resolution (R ~ 10,000) over the range 100-300 Å. We also discuss alternate configurations for shorter and longer wavelengths.
Microchannel plate (MCP) photon counting detectors using image readout devices such as the Vernier and Cross-strip anodes can now achieve spatial resolutions limited by the pore geometry of commercially available MCPs. We describe progress in the development of a new readout system, part of our program to achieve MCP limited spatial resolution and larger format sizes using the small pore MCPs.
We discuss the limitations of charge division devices that require high precision charge measurement and present a readout technique using charge comparison, with the potential to achieve large format readouts at high count rates. This scheme use a technique whereby the position coordinate of an event is represented by the order of amplitudes of a set of electrodes. Each coordinate is identified by a unique permutation of electrodes and is determined by comparing the charge collected on the electrodes. One of the major advantages offered by this scheme is a much lower signal-to-noise requirement. This will allow the detector to operate at substantially lower gain, raising the MCP limited count rate threshold.
We present a simple and practical readout design to implement the charge comparison scheme, which uses the image charge technique to enhance performance in the areas of spatial resolution, linearity and image stability. The high count rate capability of the new design is augmented by an ability to capture events in parallel without the requirement for excessive numbers of electronic channels.
We describe an electronics scheme specifically for the charge comparison readout and discuss how it can provide enhanced spatial resolution by using a charge centroiding technique based on pulse timing information. We support this with timing measurements obtained from a breadboarded electronic channel.
The SPIDR mission is designed to test predictions of cosmological models of the structure of the universe. In addition, SPIDR will provide new information about hot gas in a variety of Galactic environments. These diagnostics will bo obtained through spectral imaging of selected astrophysical fields in the 100 - 160 nm band. In this paper we will provide an overview of the SPIDR mission and its observational approach.
We report on the successful sounding rocket flight of the high resolution (R=3000-4000) J-PEX EUV spectrometer. J-PEX is a novel normal incidence instrument, which combines the focusing and dispersive elements of the spectrometer into a single optical element, a multilayer-coated grating. The high spectral resolution achieved has had to be matched by unprecedented high spatial resolution in the imaging microchannel plate detector used to record the data. We illustrate the performance of the complete instrument through an analysis of the 220-245Å spectrum of the white dwarf G191-B2B obtained with a 300 second exposure. The high resolution allows us to detect a low-density ionized helium component along the line of sight to the star and individual absorption lines from heavier elements in the photosphere.
The primary goal of the Spectroscopy and Photometry of the IGM's Diffuse Radiation (SPIDR) Mission is to detect and map the huge filamentary structures, the "cosmic web", predicted to be present in the IGM. The SPIDR instrument comprises six imaging spectrographs providing 8° x 8° and 2.5° x 2.5° high-resolution spatial maps of IGM features in the OVI and CIV wavelength bands. For simplicity and economy all six spectrographs utilize virtually identical detector systems. Each detector records a two-dimensional image whose axes represent spectral and one-dimensional spatial information, the second spatial axis being obtained by tomographic reconstruction.
We describe the design of the prototype detector built for the SPIDR mission. The detector uses a conventional microchannel plate (MCP) arrangement with a charge division readout anode used in the image charge configuration. The image charge technique provides enhanced resolution, linearity and stability in a more compact mechanical design. The predictable distribution of the induced image charge footprint has allowed us to accurately simulate the readout performance in software. The conservative requirements of the SPIDR spectrograph allow the use of a conventional wedge and strip anode which benefits from the design improvements generated using our software simulation. Redesign of the boundary electrodes has enabled us to improve overall linearity and increase useful imaging area.
We describe the integrated electronics system for the SPIDR prototype, designed for low mass and power consumption. A single printed circuit board is used to house analog signal processing, digital processing, and power systems.
We have fabricated the four flight gratings for a sounding rocket high-resolution spectrometer using a holographic ion-etching technique. The gratings are spherical (4000-mm radius of curvature), large (160 mm x 90 mm), and have a laminar groove profile of high density (3600 grooves/mm). They have been coated with a high-reflectance multilayer of Mo5C/Si/Mo2Si. Using an atomic force microscope, we examined the surface characteristics before and after multilayer coating. The average roughness is approximately 2-3A rms after coating, somewhat smoothened by the multilayer. Using synchrotron radiation, we completed an efficiency calibration map of each grating over the wavelength range 225-245A. At an angle of incidence of 5 degree(s) and a wavelength of 234A, the average efficiency peaks in the first inside order at 10.3+/- 0.6% for Grating 1, 7.3+/- 0.9% for Grating 6, 7.2+/- 1.2% for Grating 3, and 9.0+/- 1.5% for Grating 4. These values exceed all previously published results for a high density grating. The first order groove efficiency for Grating 1 is 34.4+/- 1.9%, in good agreement with the best value obtained from similar test gratings and close to the theoretical limit of 40.5%.
We have previously described the Vernier Anode, a conductive charge division readout device for use in microchannel plate detectors. The readout pattern comprises nine electrodes each of which varies cyclically, having a sinusoidal form. One of the major benefits offered by the Vernier design is that the spatial resolution can greatly exceed the charge measurement accuracy, unlike devices such as the wedge and strip anode, where the electrode variation is linear. Thus the Vernier anode can exploit the potential position resolution of even the smallest pore microchannel plates at readily achievable microchannel plate gains and electronic signal to noise ratios. We describe a detector utilizing the Vernier anode using the image charge technique. The microchannel plate event charge is collected on a resistive anode composed of Germanium deposited on an insulting substrate. This serves to localize the charge while it is being measured. The Vernier anode is capacitively coupled to the reverse side of the Germanium and the event charge induces signals on the Vernier anode, which are then used to calculate the event centroid position. We present spatial resolution and linearity results from a detector using the Vernier in image charge mode, and discuss the practical and performance advantages offered by this method of operation. The intrinsic spatial resolution of the Vernier anode is shown to be less than 10 microns FWHM and detector resolution is limited by the microchannel plate pore spacing.
We describe the development of an imaging microchannel plate detector for a new class of high resolution EUV spectrometer. The detector incorporates a front MCP coated with a CsI photocathode to enhance quantum efficiency, while the rear MCP, supplied by Photonis SAS for a European Space Agency Technology Research Program, represents one of the first uses of a 6 micron pore device in astronomy. The detector uses a unique design of charge division anode, the Vernier readout, enabling it to deliver a spatial resolution better than 15 microns FWHM. The detector forms an integral component of J- PEX, a sounding rocket EUV spectrometer operating at near- normal incidence, using multilayer coated gratings to deliver a resolution and effective area 10 times that of EUVE in the 225 - 245 angstrom band.
Previously we have described several types of charge division electronic image readouts for microchannel plate based imaging detectors developed at MSSL, primarily for space astronomy applications. These have included the wedge and strip anode1 (WSA), the Vernier anode2 - a high resolution readout, capable of exploiting the limiting spatial resolution offered by the microchannel plate, and FIRE3 - an imaging device operating at event rates in excess of 10 MHz. MSSL and Photek have now joined in collaboration to develop an intensifier based imaging system designed to employ this range of readout systems for general laboratory use. The image intensifier uses the image charge technique4,5 whereby the event charge is used to induce electrical signals on the capacitively coupled readout pattern, obviating the requirement for the readout to be inside the vacuum enclosure. The image readout is manufactured as a separate component, and can be interchanged to suit the specific application requirements. The intensifier tube design can be generic enabling it to be used with a variety if image readouts designs. We describe the image intensifier and electronic design, including the common charge amplifier, event timing and computer interface. We discuss the anticipated performance of the various readout systems - Wedge and Strip, Vernier and FIRE in terms of spatial resolution, maximum count rate, and timing resolution.
We investigate the role of the mechanical and chemical composition of the anode on the performance of charge division readout systems in microchannel plate (MCP) based detectors. Typically, in these detectors, electrons from the MCP gain stage have sufficient energy to excite secondary electrons from the anode surface. Normally, these are recollected by the anode and can thus modify the effective charge footprint. These secondary electrons can also mediate charge redistribution between the anode electrodes in the presence of differential voltages. We describe an experiment to investigate the error in electrode charge ratio of a charge division anode pattern. The detector used, comprised a microchannel plate intensifier stack with an intermediate grid between MCP and anode for secondary electron control and measurement. The intermediate grid is used to either suppress or collect secondary electrons produced by the anode depending on the configuration of detector voltages. Anode patterns were manufactured on a variety of substrates. One anode pattern design was used for all experiments and consisted of several sets of fixed electrode ratios. The effect of the anode surface finish and electrode composition on the charge ratios was measured for different substrates.
Traditional charge division readouts used in microchannel plate detectors, such as the Wedge and Strip anode, while simple in operation, can suffer from positional nonlinearities and instability in absolute positioning. The cause of both effects is due to the ratio of charges collected on the individual electrodes not accurately representing the electrode geometry. This is primarily a result of redistribution of secondary electron is produced from the anode surface among the anode electrodes. The Vernier position readout is an analogue charge division electronic readout capable of exceptional position resolution and linearity. In order to exploit this performance to the full and produce a device with absolute position stability, the problem of second are electron redistribution has had to be overcome. We describe the result of a series of experiments to determine the physical processes producing charge redistribution in the Vernier anode. Understanding of the mechanisms underlying this phenomenon has allowed the modification of the detector, anode pattern design and data acquisition software to alleviate the limitations imposed. These modifications are also applicable to other anodes relying on analogue charge division and provide improvements in absolute positional stability and linearity. We present measurements of the imaging performance of a microchannel plate detector using the Vernier anode. These results show the high spatial resolution, improved positional stability and linearity that can be achievable by controlling secondary electron redistribution.
Image intensification systems have been used for photon counting applications in ground based and space based astronomy for many years. Various charge amplification and image readout techniques have been used, but some of the most successful to date utilize the microchannel plate (MCP) intensifier in a 'Gen II' configuration with an electronic image sensor. The two major competitive solutions for image sensor are the CCD, which reads the optical image from a phosphor, and the electronic image readout, which collects the charge from the MCP directly, replacing the phosphor. We describe the advantages of the electronic charge division image readout and compare its performance with other imaging techniques. For example, intensifiers using charge division readouts have distinct advantages over intensified CCDs for applications where localized count rate capability (point source count rate) and temporal resolution (event time tagging accuracy) are important. We discuss existing and potential astronomical applications for these detector systems. We discuss recent improvements in imaging performance obtained with charge division readouts, comparing the performance of traditional resistive anode readouts with the latest generation of pattern designs including the Vernier anode. We present results showing the imaging performance of the Vernier anode. We describe a photon counting imaging intensifier system for ground and space based applications in astronomy. The intensifier, which is manufactured by Photek Ltd., uses a proximity focused photocathode, microchannel plate intensifier and conductive charge division electronic readout. A variety of pattern designs offer performance ranging up to 4000 X 4000 pixel format, with position resolution down to 10 microns FWHM and sub-microsecond timing accuracy.
The microsphere plate (MSP) is a new type of electron multiplier device operating along similar lines to the well known microchannel plate (MCP). The MSP is manufactured by El- Mul Technologies Ltd., using glass beads 20 to 60 micrometer diameter, sintered together to form a wafer less than 1 mm thick. Conductive coatings are applied to the upper and lower surfaces, and a high voltage is applied between these two electrodes, allowing secondary electron multiplication to take place. The device uses the surfaces of the randomly arranged interstices of the sintered glass beads as dynodes, whereas in the MCP, dynodes are constituted by the inner surfaces of the longitudinal pores. The homogeneous composition of the MSP causes charge to spread laterally during multiplication, resulting in a spatial resolution of about 2 linepairs/mm when proximity focused to a phosphor. Charge division readouts benefit from this charge spreading, such as the wedge and strip anode which requires a charge footprint of order 1 - 2 mm diameter. We present results of experiments on the imaging performance of detectors using MSPs with readouts such as the wedge and strip anode. We discuss and quantify the potential advantages to be gained from MSPs, such as the higher gain achievable per stage, reduced susceptibility to paralysis owing to their isotropic conductivity, etc. Potential MSP disadvantages, such as image nonlinearities, quantum efficiency variability, and pulse height saturation are analyzed.
We have previously demonstrated the enhanced performance of progressive geometry readouts, such as the SPAN two dimensional charge division readout, in MCP based detectors. The nine electrode two-dimensional Vernier readout described in this paper is the latest development in this series of devices. The Vernier readout uses three triplets of electrodes, each triplet consisting of three electrodes whose areas vary sinusoidally and whose phases are each displaced by 120 degrees. The co-ordinate of an event at any point in the active area of the readout is encoded uniquely by the three phases, one phase being generated by each triplet. The Vernier technique uses only the phase information encoded by the electrode areas, whereas previous designs, such as SPAN, also relied on modulation of the sinusoidal amplitude to encode a co-ordinate uniquely. The sensitivity to varying charge footprint size caused by this reliance is thus avoided in the Vernier readout. In addition, fixed pattern noise in the image, resulting from signal digitization prior to phase calculation, is avoided by encoding the final x and y co-ordinates as linear combinations of all three phases. The pattern geometry can be chosen to ensure that the fixed pattern noise in each phase never coincides with that in other phases when combined in the decoding algorithm. We present images and performance data from an MCP based detector using a one Vernier anode, with an active diameter of 25 mm. A first image from the nine electrode two dimensional Vernier anode is shown and its preliminary performance is discussed.
We describe a new position readout scheme, applicable to proportional counters, which provides the attributes required for large format, high energy x-ray detectors, such as that proposed for the Eixon x-ray monitor instrument on the ESA INTEGRAL mission. Large format detectors for coded mask imaging require a position resolution of typically less than 1 mm, in order to over-sample the projected mask pixel. Background rejection at higher energies can be improved by using fluorescence gating. However, this technique requires the position readout to be capable of detecting the simultaneous double event signature. The scheme we propose combines both excellent position resolution with the ability to resolve simultaneous events. The readout scheme consists of an array of charge measurement electronic channels connected to groups of cathode strips. The particular cathode grouping arrangement allows a large reduction (approximately 1/6) in the number of channels required compared to the fully parallel scheme, with one channel per electrode. However, the new design still retains the charge centroiding and parallel processing capabilities of the fully parallel scheme, enabling it to provide high spatial resolution and resolve multiple simultaneous events. We present results of a Monte Carlo simulation of the detector and readout. The simulation models the physics involved in each x ray interaction and predicts the primary ionization distribution. Simulation of electron diffusion and gas multiplication are used to predict the charge induced on each cathode strip. Electronic noise and other signal degradation factors are included for a realistic assessment of readout performance. Thus far, the position readout is modeled in one axis only. The success of the new scheme is assessed by comparison with the fully parallel readout.
The image linearity of a charge division position readout anode, for instance, a Wedge and Strip Anode (WSA), is controlled by several factors. In addition to the drift field uniformity and pattern geometry requirements, linearity can be affected by the surface characteristics of the anode itself. We have undertaken experiments to establish the mechanisms which lead to non-linear response and suggest ways to avoid it. We present measured image non linearity of image intensifier tubes using two different types of charge division position readout anode; the Wedge and Strip anode (WSA) and the Spiral Anode (SPAN) devices. Both intensifier tubes were manufactured by DEP bv., Holland and utilize S20 or Caesium Telluride semi- transparent photocathodes on Magnesium Fluoride, Fused Silica or Calcium fluoride windows, proximity focused to a microchannel plate (MCP) stack. All detector variants built thus far use a stack of three MCPs in a Z configuration. We have undertaken experiments using a de- mountable detector of similar design but without window or photocathode. The role of secondary electrons produced on the anode in charge redistribution was investigated by using an intermediate conductive grid of high open area between the MCP and anode. The grid was used to define the electric field in a region just above the anode in order to either suppress the range of the anode secondary electrons, or prevent them from returning to the anode by collecting them on the grid. We present some preliminary results from these experiments and suggest modifications to the intensifier design to maximize the detector performance.
We describe an image intensifier tube of flexible design, capable of being used for a number of different ground and space-based applications. The tube was originally designed to meet a requirement for an optical photon-counting detector sensitive between 150 and 650 nm with a spatial resolution of less than 20 micrometers . Other performance criteria aimed for were a peak quantum efficiency of greater than 20% and a 200 kHz maximum count rate. The tube has a 25 mm diameter image format which is nominally 2D, though 1D and non-Cartesian imaging formats could be accommodated. Photon detection is via a semitransparent photocathode coated on the detector input window. Liberated photoelectrons are proximity focused on to a microchannel plate (MCP) stack which provides electron gain with a saturated pulse height distribution for photon counting. Imaging is accomplished with conductive charge division readout. The detector geometry is easily modified to accommodate different types of this generic readout type. The imaging electronics consist of several parallel charge measurement channels, the number depending on the readout type. A decoding algorithm provides an x,y photon event coordinate. The detector was manufactured by DEP bv, Holland. The original design utilized an S20 photocathode on a magnesium fluoride or fused silica window but recently detectors have been made with calcium fluoride windows and caesium telluride photocathodes. All detector variants built thus far use a stack of three MCPs in a Z configuration producing an electron gain of between 106 and 2 X 107 depending on MCP conditioning. We present images and performance data from the two types of detector so far built; the original optical intensifier design with a SPAN six electrode image readout, and a variant with a UV photocathode and wedge and strip anode readout. We discuss the problems encountered during the tube development and the solutions arrived at for each type of device.
It has been found that the gain depression in MCP's operated at high gains is a relatively long range phenomenon. Active pores can significantly depress the gain in the surrounding quiescent pores at distances of the order of millimeters. This is of fundamental importance for detectors in which high point source count rates are encountered. We have measured this effect for a variety of plate operating conditions and point source count rates and find that in all cases there is a constant limiting radius. We have also determined that the gain depression has a long term effect on the MCP.
Microchannel plate (MCP) detectors are often used with charge division anode readouts, such as the SPAN anode, to provide high position resolution. This paper discusses the effect on image quality, of digitization (causing fixed patterning), electronic noise, pulse height distribution (PHD) and charge cloud size. The discussion is supported by experimental data obtained from a one dimensional SPAN anode, developed for the SOHO Coronal Diagnostic Spectrometer (CDS) Grazing Incidence Spectrometer (GIS). Results from a computer model of this detector, and from a charge cloud simulation model, are also included. The SPAN anode normally has three sinusoidal electrodes with phase differences of 120 degree(s)C. An alternative configuration is to use a phase difference of 90 degree(s)C. This paper compares the advantages and disadvantages of these arrangements.
The SPAN position readout device uses a charge division and measurement method to encode the coordinates of the centroid of a charge cloud and thus provides a technique for imaging with photon counting detectors of various formats; for example, microchannel plate intensifiers and gas proportional counters. Its principle of operation causes the position resolution to substantially exceed the charge measurement precision. The reduced signal to noise requirement compared with the competitive devices of comparable imaging format size enables the SPAN readout system to operate at higher input count rates. We present imaging performance results from SPAN readout systems incorporated in several detector formats. The dependence of the physical parameters of the SPAN pattern design on the detector type and geometry together with the performance trade-offs between speed and resolution for these particular detectors are discussed. The practical implementation of the SPAN readout decoding algorithm is outlined. We describe the experimental applications for which the SPAN readout system has been proposed.
The concept for the 2D position-readout device for the SPAN photon-counting detector is presented with attention to the count rates, spatial resolution, and charge-measurement precision. The electrodes which are deposited on the planar substrate result from charge division induced by a charge cloud, the centroid position of which is encoded by the ratio of charge magnitudes. The SPAN electrode design is analyzed and theorized to permit 1000 x 1000-pixel resolution at 1 MHz. The SPAN spiral-anode six-electrode design is compared to the Vernier-anode twelve-electrode structure for encoding 2D position, and digital precision is analyzed at count rates up to 1 MHz. The SPAN readout affords resolution levels of up to 1/1000 across the entire active area at 8-bit digitization.
The compact photon-counting detector SPAN is described which offers 25-micron spatial resolution and a 25-mm imaging diam. The SPAN detector incorporates position readout within a vacuum-sealed optical-intensifier tube, and a photocathode is used to sense the images with high-blue and near-UV sensitivity. A microchannel-plate intensifier generates an electron cloud that is measured with a position-sensitive readout developed for this application. The position-sensitive readout is a conductive device that, in the context of the SPAN, permits a high count rate and spatial resolution greater than the charge-measurement precision of each electrode. Preliminary photon counting is demonstrated, and the results suggest that the SPAN has a resolution of better than 1/1000 and effective linearity with 8-bit digitization.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.