CCCP: A CCD Controller for Counting Photons
ABSTRACT
CCCP, a CCD Controller for Counting Photons, is presented. This new controller uses a totally new clocking architecture and allows to drive the CCD in a special way. Its design is optimized for the driving of EMCCDs at up to 20MHz of pixel rate and fast vertical transfer. Using this controller, the dominant source of noise of EMCCDs at low flux level and high frame rate, the Clock Induced Charges, were reduced to 0.001 – 0.0018 electron/pixel/frame (depending of the electron multiplying gain), making efficient photon counting possible. CCCP will be deployed in 2009 on the ESO NTT through the 3D-NTT1 project and on the SOAR through the BTFI project.
1. INTRODUCTION
Electron Multiplying Charge Coupled Devices (EMCCD) allows one to apply a gain to the pixel’s charge before it reaches the noisy output amplifier where the charge-tension conversion is made.2 A gain G in the charge domain affects the effective readout noise by the relation σef f = σreal G . Sub-electron effective readout noise levels are thus achievable. However, the electron multiplying process is stochastic. This statistical behaviour adds an excess noise factor that reaches a value of 2 1/2 at high gains.3 The effect on the signal-to-noise ratio (SNR) of the system is the same as if the quantum efficiency (QE) of the CCD would be halved.
Some authors proposed offline data processing to lower the excess noise factor induced by the multiplication register.4, 5 However, the only way to overcome the excess noise factor without any a priori knowledge or stability assumption on the signal is to consider the pixel binary by applying a single threshold to the output signal. This way, only one photon per pixel per frame can be counted and the full QE of the silicon can be recovered, making the EMCCD a theoretically perfect photon counting device. The highest flux rate that can be observed in this mode will thus depend of the frame rate at which the EMCCD is operated. However, charges are generated as the EMCCD is read out. Clock Induced Charges (CIC), a well know source of noise affecting all kinds of CCDs, were typically measured in the range of 0.1 to 0.01 electron per pixel per frame6–8 (for a 512 x 512 CCD97 frame transfer EMCCD from E2V Technologies) and quickly dominate the dark current or even the photon flux as the frame rate in increased. Thus, in order to make photon counting efficient at low flux with an EMCCD, the CIC must be reduced to a minimum. Some techniques were proposed to reduce the CIC8–13 but so far, no commercially available CCD controller was able to implement all of them and get efficient results.
CCCP, a 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 the driving of EMCCDs at high speed, both vertically and horizontally, but may be used for driving classical CCDs as well. Using this controller, CIC levels as low as 0.001 – 0.0018 event per pixel per frame (as opposed to per transfer) were measured on the 512 x 512 CCD97 EMCCD from E2V Technologies operating in inverted mode (as opposed to non-inverted mode). The impact of this level of CIC on the photon counting efficiency of an EMCCD will be discussed in this article. Data gathered using the controller will also be presented.
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