B. L. Morgan

OPTIMISATION OF CENTROIDING ALGORITHMS FOR PHOTON EVENT COUNTING IMAGING

Abstract Approaches to photon event counting imaging in which the output events of an image intensifier are located using a centroiding technique have long been plagued by fixed pattern noise in which a grid of dimensions similar to those of the CCD pixels is superimposed on the image. This is caused by a mismatch between the photon event shape and the centroiding algorithm. We have used hyperbolic cosine, Gaussian, Lorentzian, parabolic as well as 3-, 5-, and 7-point centre of gravity algorithms, and hybrids thereof, to assess means of minimising this fixed pattern noise. We show that fixed pattern noise generated by the widely used centre of gravity centroiding is due to intrinsic features of the algorithm. Our results confirm that the recently proposed use of Gaussian centroiding does indeed show a significant reduction of fixed pattern noise compared to centre of gravity centroiding (Michel et al., Mon. Not. R. Astron. Soc. 292 (1997) 611–620). However, the disadvantage of a Gaussian algorithm is a centroiding failure for small pulses, caused by a division by zero, which leads to a loss of detective quantum efficiency (DQE) and to small amounts of residual fixed pattern noise. Using both real data from an image intensifier system employing a progressive scan camera, framegrabber and PC, and also synthetic data from Monte-Carlo simulations, we find that hybrid centroiding algorithms can reduce the fixed pattern noise without loss of resolution or loss of DQE. Imaging a test pattern to assess the features of the different algorithms shows that a hybrid of Gaussian and 3-point centre of gravity centroiding algorithms results in an optimum combination of low fixed pattern noise (lower than a simple Gaussian), high DQE, and high resolution. The Lorentzian algorithm gives the worst results in terms of high fixed pattern noise and low resolution, and the Gaussian and hyperbolic cosine algorithms have the lowest DQEs.

A position-sensitive photon event counting detector applied to fluorescence imaging of dyes in sol-gel matrices

A photon event counting imaging detector, originally developed for astronomical applications, has been adapted for use on a low light level fluorescence microscope. It is based on a 40 mm diameter, three-microchannel plate image intensifier with a photon gain of 107 when operating in the photon event counting mode. The intensifiers output screen is lens coupled to a full frame progressive scan CCD camera which transfers the photon event data into a PC via a framegrabber. The image is built up from the photon events in the frames using software-based event processing. The high sensitivity of the photon counting approach means that low excitation light levels and/or a low probe concentration can be used. Moreover, the inherent linearity between incident intensity and the number of detected events provides intrinsic photometric accuracy independent of camera gain and offset. To demonstrate the viability of photon event counting imaging applied to fluorescence studies, we have imaged 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran- (DCM-) and rhodamine 6G-doped thin films produced by the sol-gel technique using tetraethylorthosilicate (TEOS) as a precursor. We show that fluorescence imaging is a feasible tool for finding cracks, streaks or other defects and for assessing the uniform thickness of the finished films. In addition, we image the fluorescence in response to an excitation beam illuminating rhodamine 6G-doped bulk monolith sol-gel samples. The results are analysed using the Lambert-Beer law.

Minimization of fixed pattern noise in photon event counting imaging

Low light-level ultraviolet and optical imaging with a photon counting image intensifier coupled to a charge coupled device camera generally results in varying levels of fixed pattern noise in the image. Here, we demonstrate that this can be minimized by the appropriate choice of photon event centroiding algorithm. We compare the fixed pattern noise generated by a center of gravity centroiding algorithm, a Gaussian centroiding algorithm, and a hybrid centroiding algorithm which uses center of gravity centroiding when one wing is zero, and Gaussian centroiding otherwise. This approach yields the best image quality with a lower fixed pattern noise parameter (9.99%) than the sole use of Gaussian centroiding (16.4%), and there is no need for a look-up table correction. In addition, the hybrid algorithm also yields maximum detective quantum efficiency by overcoming small pulse centroiding failure associated with Gaussian centroiding. The digitization error when recording the events is modeled with a Monte Carl...

Conversion of methane to higher hydrocarbons by the interaction between electrons and ions at a surface

This paper reports experiments in which methyl ions and hydrogen ions are generated and accelerated within a plasma plate array so that they interact with neutral methane molecules on the inside surfaces of a set of microchannels. The impact energies of the ions are in the range 10 eV to greater than 100 eV. The energy delivered in interactions at the surfaces causes the production of larger hydrocarbon molecules such as , and as well as molecules containing , , , , and .

Line spread function degradation in photon-counting detectors

An investigation of the degradation of the line spread function due to electron scattering in a micro-channel plate intensifier employing a semi-transparent photocathode and proximity focusing is reported. Data obtained using the Anglo-Australian Telescope (AAT) are used to measure emission line broadening and infilling of absorption features in astronomical spectra, due to scattering in the micro-channel plate intensifier. A comparison with a magnetically focused four-stage cascade intensifier is made. Values are obtained with the two intensifiers for equivalent width and residual infilling of a totally saturated absorption feature. The micro-channel plate intensifier is found to scatter into the wings of emission lines a larger fraction of the input signal than the cascade intensifier, amounting to 6.5% of the total signal. A model of the line spread function degradation observed in the micro-channel plate intensifier, based on electron scattering in the input proximity focused lens, is also presented. The modified line spread function obtained from this model is found to be a remarkably good fit to the data obtained at the AAT, and the values for infilling of absorption line features calculated from the model are close to those observed in the spectra. Finally, suggestions are made as to how the design of future micro-channel plate intensifiers can be modified to reduce scattering from the input micro-channel plate.

DQE enhancement of MCP intensifiers for astronomy results of the MIC II program

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.

MCP image intensifier with improved DQE

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.

Evaluation of Photon-event-counting Intensifiers

Publisher Summary This chapter discusses the evaluation of photon-event-counting intensifiers. It is proposed to consider some of the measurements that have been carried out to evaluate the performance of the detector for photon-event counting. Measurements carried out on the prototype 40 mm microchannel plate (MCP) image intensifiers that have been produced as part of a programme to develop an improved front-end detector for photon-event-counting systems indicate that the tubes have several desirable characteristics for use in this application. Device operating parameters such as gain, resolution, pulse height distribution, noise, and detective quantum efficiency can be measured. Light entering the device may suffer several reflections in the intensifier input faceplate or inside the vacuum envelope. Measurements have been made of a number of critical parameters that are of particular importance for photon-event-counting systems and that are often not quoted by commercial manufacturers. MCP counting efficiency can be improved by modifications to the manufacturing procedures.

Performance of a large-area microchannel plate photon-counting intensifier

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.

Speckle interferometry of candidate stars for two space telescope programs

This paper describes the preflight programs developed for two telescope missions, the U.S. Hubble Space Telescope and the European Hipparcos, in order to remove stars showing multiple nature from a set of proposed reference stars. It was found that, in a large unbiased sample of stars, 10 percent of stars at apparent magnitudes between 9 and 13 mag are resolved in the range 0.05-1.00 arcsec. The consequence is a significant increase in the difficulty of making observations with the space telescopes. In the case of Hubble telescope, this will add to the acquisition time of Guide stars by about 11 percent. In the case of Hipparcos, it will introduce small uncorrelated proper motion errors into a number of unidentified unresolved binary stars.