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\section{Introduction}  Photon counting imaging is a well-established low light level imaging technique where an image is assembled from individually detected photons. Originally developed for astronomy due to its sensitivity, it now has applications in many diverse fields of science and technology, including bioluminescence,\cite{Roncali2008, Baubet2000} optical tomography,\cite{Schmidt2000} DNA sequencing,\cite{Previte2015} lidar,\cite{McCarthy2009} quantum information science and encryption,\cite{Hadfield2009} and optical communications both on earth and in space.\cite{Boroson2013, Hemmati2014, Hemmati2007} More information about single-photon detection technology can be found in recent reviews.\cite{Hadfield2009, Buller2010, Eisaman2011, Seitz2011} Importantly, photon counting imaging allows the timing of photon arrival, and in life sciences photon counting imaging is often used for Fluorescence Lifetime Imaging Microscopy (FLIM), which has emerged as a key technique to image the environment and interaction of specific probes in living cells [23–27] \cite{Becker2012, Berezin2010, Borst2010}  with single-molecule sensitivity, molecular specificity, sub-cellular sub-micrometer resolution, and real-time data collection with negligible cytotoxicity.[22] cytotoxicity.\cite{Fischer2011}  In conventional photon counting imaging, photon events on the phosphor screen of a microchannel plate (MCP)-based image intensifier are imaged with a charge-coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) camera at high frame rates, and many frames are accumulated to build up an image [22–31]. However, single photon detection is also possible with electron-bombarded (EB) sensors, where the single photoelectron liberated from the photocathode is accelerated directly into the CCD or CMOS sensor, without going through a multiplication process and without being converted into light on a phosphor.[64] The resulting photon events are smaller and dimmer than MCP-intensified photon events, but have a narrow, voltage-dependent pulse height distribution and avoid distortion of the image due to the coupling of the MCP to the camera, spectral matching of the camera sensitivity and the phosphor and image lag due to the phosphor decay time. 
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The discovery of photoswitchable and photoactivatable fluorophores in the past decade has allowed the same centroiding principle to be employed in circumventing the diffraction limit in fluorescence microscopy; importance of this field was recognized with the 2014 Nobel Prize in Chemistry awarded “for the development of superresolved fluorescence microscopy.” Single-molecule localization microscopy techniques are based on the activation of a small subpopulation of the fluorophores which can then be imaged and subsequently deactivated before the process is repeated with a different subset of fluorophores. Rather than detecting a single photon, the detected event consists of many photons that are emitted from a single fluorophore with a size much smaller than the resolution of the microscope, but similar to photon counting imaging, the centroid positions of the molecules are calculated, and the final image is formed by summing many frames.  Single-molecule localization microscopy is now a well-established technique, and a variety of free software is available for image reconstruction. As the usefulness of this method crucially depends on both the localization accuracy and precision, much effort has been put into the development and optimization of many different types of centroiding algorithms, including iterative fitting algorithms. As recently reported, super-resolution software for single-molecule localisation gives good results when applied to centroiding single photon events imaged with an MCP-intensifed CMOS camera.\cite{Hirvonen_2015} In this work, we have applied super-resolution software for centroiding single photon events events detected with an EBCCD camera. Multi-emitter fitting analysis was also tested for separating overlapping photon events, an important aspect which allows an increased count rate and shorter acquisition times.