Extreme Faint Flux Imaging with an EMCCD
ABSTRACT
An EMCCD camera, designed from the ground up for extreme faint flux imaging, is presented. CCCP, the CCD Controller for Counting Photons, has been integrated with a CCD97 EMCCD from e2v technologies into a scientific camera at the Laboratoire d’Astrophysique Expérimentale (LAE), Université de Montréal. This new camera achieves subelectron readout noise and very low clock-induced charge (CIC) levels, which are mandatory for extreme faint flux imaging. It has been characterized in laboratory and used on the Observatoire du Mont Mégantic 1.6 m telescope. The performance of the camera is discussed and experimental data with the first scientific data are presented.
1. INTRODUCTION
The advent of electron-multiplying charge-coupled devices (EMCCD) allows subelectron readout noise to be achieved. However, the multiplication process involved in rendering this low noise level is stochastic. The statistical behavior of the gain that is generated by the electron multiplying register adds an excess noise factor (ENF) that reaches a value of 21=2 at high gains (Stanford & Hadwen 2002). The effect on the signal-tonoise ratio (SNR) of the system is the same as if the quantum efficiency (QE) of the EMCCD were halved. In this regime, the EMCCD is said to be in analog mode (AM) operation.
Some authors proposed offline data processing to lower the impact of the ENF (Lantz et al. 2008; Basden et al. 2003) in AM operation. However, one can overcome the ENF completely, without making any assumption on the signal’s stability across multiple images, only by considering the pixel binary and by applying a single threshold to the pixel value. The pixel will be considered as having detected a single photon if its value is higher than the threshold and none if it is lower. In this way, the SNR will not be affected by the ENF and the full QE of the EMCCD can be recovered. In this regime, where the EMCCD is said to be in photon-counting (PC) operation, the highest observable flux rate will be dictated by the rate at which the images are read out; a frame rate that is too low will induce losses by coincidence.
However, at a high frame rate, the clock-induced charges (CIC) become dominant over the other sources of noise affecting the EMCCD (mainly dark noise). CIC levels in the range of 0.01–0.1 were typically measured (Tulloch 2008; Wen et al. 2006; e2v Technologies 2004) on a 512 × 512 CCD97 frame transfer EMCCD from e2v Technologies. Even at a low readout speed of 1 frame s!1, these CIC levels are at least an order of magnitude higher than the dark noise. Thus, one wanting to do faint flux imaging with an EMCCD is stuck with two conflicting problems: a low frame rate is needed to lower the impact of the CIC while a high frame rate is needed if a reasonable dynamic range is to be achieved.
In order to make faint flux imaging efficient with an EMCCD, the CIC must be reduced to a minimum. Some techniques have been proposed to reduce the CIC (Tulloch 2006; Daigle et al. 2004; Mackay et al. 2004; Gach et al. 2004; e2v Technologies 2004; Janesick 2001) but until now, no commercially available CCD controller nor commercial cameras were able to implement all of them and get satisfying results. CCCP, the CCD Controller for Counting Photons, has been designed with the aim of reducing the CIC generated when an EMCCD is read out. It is optimized for driving EMCCDs at high speed (≥10 MHz ), but may be used also for driving conventional CCDs (or the conventional output of an EMCCD) at high, moderate, or low speed. This new controller provides an arbitrary clock generator, yielding a timing resolution of ∼20 ps and a voltage resolution of ∼2 mV of the overlap of the clocks used to drive the EMCCD. The frequency components of the clocks can be precisely controlled, and the interclock capacitance effect of the CCD can be nulled to avoid overshoot and undershoots. Using this controller, CIC levels as low as 0.001–0.002 event pixel!1 per frame were measured on the 512 × 512 CCD97 operating in inverted mode. A CCD97 driven by CCCP was placed at the focus of the FaNTOmM instrument (Gach et al. 2002; Hernandez et al. 2003) to replace its GaAs photocathode-based Image Photon Counting System (IPCS). In this article, the important aspects of PC and AM operations with an EMCCD under low fluxes are outlined in § 2. In § 3, CCCP performance regarding these aspects is presented. Finally, in § 4, scientific results obtained at the telescope are presented.
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