Yongan Liu

Correction of ultraviolet single photon counting image distortion

Single photon counting imaging technology has been widely used in space environment detection, astronomy observation, nuclear physics, and ultraweak bioluminescence. However, the distortion of the single photon counting image will badly affect the measurement results. Therefore, the correction of distortion for single photon counting image is very significant. Ultraviolet single photon imaging system with wedge and strip anode is introduced and the influence factor leading to image distortion is analyzed. To correct original distorted image, three different image correction methods, namely, the physical correction, the global correction, and the local correction, are applied. In addition, two parameters, i.e., the position index and the linearity index, are defined to evaluate the performance of the three methods. The results suggest that the correction methods can improve the quality of the initial image without losing gray information of each counting light spot. And the local correction can provide the best visual inspections and performance evaluation among the three methods.

Decoding algorithms for improving the imaging performance of Vernier anode readout

The Long Slit Spectrograph is one of instruments onboard The World Space Observatory-Ultraviolet. Both the FUV (102-1700nm) and NUV (160-320nm) channels of LSS choose micro-channel plates (MCP) detector with anode readout in the focal plane. The MCP detectors with anode readout are typically used to provide photon counting imaging. According to the desired performance of LSS, the Vernier anode may be the optimum readout scheme. The Vernier anode is famous for its high spatial resolution, however, the original decode algorithms of the Vernier anode is susceptible to wrongly decode, when the charge acquisition is not precise enough and the footprint size of charge cloud collected by Vernier anode is not small enough and eventually results in the deterioration of photon counting image. In this paper, the causes leading to image deterioration is analyzed. The least-squares method was used to calculate the phase value to correct imaging distortions caused by charge measurement accuracy. The area ratio of each electrode covered by charge cloud is accurately calculated to improving the imaging results. The corrected algorithms are verified by experimental results and the results show that the correction methods can obviously improve the quality of the original photon counting image.

Progress of NUV and FUV MCP-based photon-counting imaging detectors

In the World Space Observatory-Ultraviolet (WSO-UV) mission, the Long Slit Spectrograph (LSS) instrument will provide low resolution spectra in the range 102-320nm. Both the NUV (160-320nm) and the FUV (102-170nm) channels of LSS use microchannel plates (MCP) working in photon-counting modes as detectors. In this paper, the progress and parameters of NUV and FUV photon-counting imaging detectors were described. For the NUV detector, we developed the detector based on a sealed MCP-image intensifier which comprises input window, photocathode, MCP stack, Ge-layer and its ceramic substrate. To maximize the quantum efficiency, we adopted a Caesium Telluride (Cs2Te) photocathode, which was deposited on input window and mounted close to the MCP. For the FUV detector, because of the lower cut-off wavelength, there are no suitable window materials in this band and the open-faced design should be used to meet the requirements of the detection. Therefore, a Caesium Iodide (CsI) photocathode deposited on the input surface of the MCP was used to optimize detector efficiency. By using an existing wedge and strip anode (WSA), the imaging performance of the NUV and FUV detectors was tested respectively. Experimental results show that the quantum efficiency of Cs2Te is 12.1% (at 230nm), the spatial resolution of NUV and FUV detectors is better than 110μm, the dark count rate of NUV and FUV detectors is about 10.5- and 2.3-counts/s*cm2 respectively.

A new photon counting imaging detector for the Chinese ChangE-2 EUV explorer mission

This paper describes the preliminary design and performances of a new developed photon counting imaging detector for Chinese ChangE-2 EUV explorer mission. The detector consists of microchannel plate (MCP) stacks and wedge and strip anode and corresponding data read out electronics. The experimental results shows that the new developed detector has a spatial resolution of about 75μm, image distortions are small and dark noise count rate less than 0.4 counts cm-2s-1. The pulse height distribution vs MCP operating voltages and the flat field performances are also discussed.