Proc. SPIE. 9154, High Energy, Optical, and Infrared Detectors for Astronomy VI
KEYWORDS: Signal to noise ratio, Photon counting, Cameras, Sensors, Imaging spectroscopy, Exoplanets, Electron multiplying charge coupled devices, Charge-coupled devices, Analog electronics, Signal detection
We present the progress of characterization of a low-noise, photon counting Electron Multiplying Charged Coupled Device (EMCCD) operating in optical wavelengths and demonstrate possible solutions to the problems of Clock-Induced Charge (CIC) and other trapped charge through sub-bandgap illumination. Such a detector will be vital to the feasibility of future space-based direct imaging and spectroscopy missions for exoplanet characterization, and is scheduled to y on-board the AFTA-WFIRST mission. The 512×512 EMCCD is an e2v detector housed and clocked by a Nüvü Cameras controller. Through a multiplication gain register, this detector produces as many as 5000 electrons for a single, incident-photon-induced photoelectron produced in the detector, enabling single photon counting operation with read noise and dark current orders of magnitude below that of standard CCDs. With the extremely high contrasts (Earth-to-Sun flux ratio is ~ 10-10) and extremely faint targets (an Earth analog would measure 28th - 30th magnitude or fainter), a photon-counting EMCCD is absolutely necessary to measure the signatures of habitability on an Earth-like exoplanet within the timescale of a mission's lifetime, and we discuss the concept of operations for an EMCCD making such measurements.
The development of high quantum efficiency photemissive detectors is recognized as a significant advancement for astronomical missions requiring photon-counting detection. For solar-blind NUV detection, current missions (GALEX, STIS) using Cs2Te detectors are limited to ~10% DQE. Emphasis in recent years has been to develop high QE (>50%) GaN and AlGaN photocathodes (among a few others) that can then be integrated into imaging detectors suitable for future UV missions. We report on progress we have made in developing GaN photocathodes and discuss our observations related to parameters that effect efficiency and stability, including intrinsic material properties, surface preparation, and vacuum environment. We have achieved a QE in one case of 65% at 185 nm and are evaluating the stability of these high QEs. We also discuss plans for incorporating photocathodes into imaging and non-imaging sealed devices in order to demonstrate long term stability.
We describe the development of high quantum efficiency UV photocathodes for use in large area two dimensional microchannel plated based, detector arrays to enable new UV space astronomy missions. Future UV missions will require improvements in detector sensitivity, which has the most leverage for cost-effective improvements in overall telescope/instrument sensitivity. We use new materials such as p-doped GaN, AlGaN, ZnMgO, SiC and diamond. We have currently obtained QE values > 40% at 185 nm with Cesiated GaN, and hope to demonstrate higher values in the future. By using controlled internal fields and nano-structuring of the surfaces, we plan to provide field emission assistance for photoelectrons while maintaining their energy distinction from dark noise electrons. We will transfer these methods from GaN to ZnMgO a new family of wide band-gap materials more compatible with microchannel plates. We also are exploring technical parameters such as doping profiles, internal and external field strengths, angle of incidence, field emission assistance, surface preparation, etc.
We describe a novel readout system for photon-counting microchannel plate detectors, based on a custom-designed event-driven CMOS Active Pixel Sensor (APS). The event-driven APS device is fiber-optically coupled to the microchannel plate intensifier in a configuration analogous to that employed in intensified CCDs (ICCDs). APS technology permits the incorporation of comparator circuitry within each pixel. When coupled with suitable CMOS logic outside the array area, the comparators can be used to trigger the readout of small sub-array windows only when and where intensified photon events have been detected. The event-driven APS readout thus completely eliminates the local dynamic range limitation of ICCDs, while achieving a high global count rate capability and delivering high (MCP-limited) spatial resolution (through sub-pixel centroiding of the event splashes, performed off-chip). We elaborate on this concept and the design of the event-driven APS below.
At NASA GSFC we are developing a high resolution solar-blind photon counting detector system for UV space based astronomy. The detector comprises a high gain MCP intensifier fiber- optically coupled to a charge injection device (CID). The detector system utilizes an FPGA based centroiding system to locate the center of photon events from the intensifier to high accuracy. The photon event addresses are passed via a PCI interface with a GPS derived time stamp inserted per frame to an integrating memory. Here we present imaging performance data which show resolution of MCP tube pore structure at an MCP pore diameter of 8 micrometer. This data validates the ICID concept for intensified photon counting readout. We also discuss correction techniques used in the removal of fixed pattern noise effects inherent in the centroiding algorithms used and present data which shows the local dynamic range of the device. Progress towards development of a true random access CID (RACID 810) is also discussed and astronomical data taken with the ICID detector system demonstrating the photon event time-tagging mode of the system is also presented.
We are developing a novel solar blind, high resolution, photon counting detector for applications in space UV astronomy. Our concept is to utilize a charge injection device (CID) as the readout stage behind a microchannel plate (MCP) intensifier. This detector will take advantage of the flexible readout options afforded by the addressable CID architecture to provide high local frame rates around bright features in an image. In this concept, the detector bandwidth can be used most efficiently, reading pixels around a bright star more frequently than those in a nearby dim cloud of gas, for example. The demonstration apparatus described in this paper incorporates a 25 mm diameter intensifier tube fiber optically coupled to a commercially available 30 frame-s-1, 512 X 512 pixel, progressive-scan CID2250 camera. The 10 MHz analog video from this camera is digitized and processed by a centroider module that calculates the position of each event in real time with subpixel precision, thus providing high spatial resolution limited by the MCPs. The pore-resolved images presented in this paper validate the intensified CID concept. We plan to incorporates custom driving electronics and an experimental CID with on-chip address decoders for high speed random access of detector subarrays of arbitrary size and location. Our goal is to demonstrate a solar-blind UV photon counter with 200-300 counts-s-1 point source and 3 X 105 counts-s-1 global rate capability with up to 4000 by 4000 elements.
A photon-counting intensified charge injection device (CID) detector is currently in development at the Laboratory for Astronomy and Solar Physics at the Goddard Space Flight Center. Analogous microchannel-plate-intensified CCD detectors have achieved impressive spatial resolution performance in photon-counting operation. Such detectors suffer, however, from a severe limitation on local dynamic range; local event rates must be kept low in order to minimize event overlap at the frame rates achievable for reading out a full CCD. By utilizing a random access CID for the readout stage, we plan to avoid this severe local count rate limitation by virtue of the addressable (rather than serial) readout capability of such a device. Different portions of the detector field can be framed at different rates, as appropriate to the brightness distribution of the scene, maximizing the local count rate limit for a given pixel read rate and event processing capability. A high spatial resolution, high count rate photon counting detector of this type is of interest in a number of applications in space and ground- based astronomy. In this report, we present: (1) the advantages and applications of this kind of detector, (2) remarks on the suitability of different CID architectures, (3) our system design concept, and (4) the status and plans for our fabrication and testing efforts.
A 75 mm diameter microchannel plate (MCP) intensifier has been developed for astronomical applications. The intensifier incorporates a semi-transparent photocathode, three MCPs in a Z- stack configuration, and a P20 phosphor screen in a dual proximity focused arrangement. The input MCP is a thin 40:1 channel plate which is conditioned to run at low gain and hence act as an ion barrier for the succeeding 80:1 chevron pair. The intensifier has been incorporated into a CCD readout system and has undergone extensive laboratory testing. The preconditioning of these 75 mm diameter channel plates required a large area, highly uniform electron scrub beam, this has led to the development of a novel electron gun. The design of the 75 mm intensifier and the novel electron gun are described. Results from the laboratory evaluation of the intensifier are presented. Flat field illumination showed the existence of self- exciting channels in a hexagonal pattern. Finally, a future UV or x-ray detector based on this design and incorporating large area MCPs is discussed.
University College London and The Imperial College of Science, Technology & Medicine are together developing a new large area imaging photon counting system, BIGMIC, for use primarily on very large telescopes. This detector is designed for applications requiring the highest sensitivity and resolution such as, high dispersion Echelle spectroscopy. The system incorporates a specially designed 75mm active diameter image intensifier fibre- optically coupled to a fast scanning 770 x 576 pixel frame transfer CCD. Photon events at the intensifier output screen are centroided to 1/8 of a CCD pixel in both X and Y in order to sample adequately the point spread function of the intensifier, giving a data acquisition format of 6160 x 4608 sub-pixles. The area imaged onto the CCD is 61mm x 46mm with a data acquisition pixel size of 10 micrometers . A hardware windowing facility, built into the detector system, enables the astronomer to select a subset of the imaging area for data acquisition. This permits the user to match the system to specific applications; for example in a number of spectroscopic applications an essentially one dimensional image is required and a detector format of perhaps 6160 x 100 pixels could be utilized.
The MIC photon counting detector, a very high resolution, large format system that has been developed for astronomical applications and has been proven on the major UK associated telescopes, is described. Additionally, though, this detector does have a number of applications in other fields such as bio-medical and x-ray imaging. The detector itself consists of a specially designed 40 mm diameter micro-channel plate intensifier fiber optically coupled to a CCD read-out system. Data is then centroided to 1/8th of a CCD pixel in both X and Y to provide high resolution. Accumulated data is stored in a micro-processor system with on-line display and reduction facilities. The maximum format available with the detector is 3072 X 2304 pixels, where each pixel is 10.6 micrometers square. The resolution is 27 micrometers FWHM when averaged over the field. Dependent upon the application, a dynamic range as high as 5 X 106 is achievable with this detector. The time resolution of the detector is in the range 1 ms to 12 ms. A very large format version of this detector is being designed that utilizes a 75 mm intensifier and has a maximum format of 6144 X 4608 pixels. It is expected that this detector will have the same performance figures as the 40 mm system.
The MIC, a 40-mm intensified microchannel-plate photo-counting detector being developed for the Anglo-Australian, Isaac Newton, and William Herschel telescopes, is described and illustrated with diagrams and sample spectra. The MIC is linked by optical fibers to a fast-scanning CCD detector, and an accurate centroiding technique is applied to yield an effective maximum of 3104 x 2304 10.6-micron pixels, for field-averaged resolution 27 microns FWHM. Applications include high-resolution spectroscopy, especially in the blue, and Fabry-Perot and speckle interferometry.
A prototype 40-mm-diameter proximity-focused microchannel-plate intensifier intended for photon-counting applications in both ground-based and space astronomy is described. The intensifier described is a sealed-window device and is also well suited to open-window ultraviolet applications in space astronomy. The tube makes use of a combination of an unfilmed curved-channel plate (C-plate), to prevent ion feedback, and a single straight-channel plate, aligned to prevent optical feedback. The tube has excellent cosmetic quality and shows a counting efficiency superior to that of filmed plate devices reported previously, giving an overall detective quantum efficiency at least equal to that of the best four-stage magnetically focussed intensifiers.
The Microchannel-plate Intensified CCD (MIC) photon-counting detector system has been developed as a future replacement for a common user photon counting detector, the IPCS (Image Photon Counting System) on both the Isaac Newton and William Herschel telescopes at the La Palma Observatory and at the Anglo-Australian Observatory. These detectors previously incorporated EMI 4-stage cascade image intensifiers. This paper addresses the technological aspects of the design and optimization of very high gain MCP image intensifiers for such photon counting systems, and particularly the optimization of device detective quantum efficiency.
The microchannel plate intensified CCD (MIC) photon counting detector system was developed to replace a common user photon counting detector, the image photon counting system (IPCS), at the Anglo Australian Telescope and at the William Herschel Telescope and the Isaac Newton Telescope at La Palma Observatory. The IPCS incorporated magnetically-focused four-stage cascade image intensifiers. This paper discusses technological aspects of the design and optimization of very high gain microchannel plate image intensifiers for such photon counting systems and particularly the optimization of device detective quantum efficiency.