The Darkest EMCCD ever
ABSTRACT
EMCCDs are devices capable of sub-electron read-out noise at high pixel rate, together with a high quantum efficiency (QE). However, they are plagued by an excess noise factor (ENF) which has the same effect on photometric measurement as if the QE would be halved. In order to get rid of the ENF, the photon counting (PC) operation is mandatory, with the drawback of counting only one photon per pixel per frame. The high frame rate capability of the EMCCDs comes to the rescue, at the price of increased clock induced charges (CIC), which dominates the noise budget of the EMCCD. The CIC can be greatly reduced with an appropriate clocking, which renders the PC operation of the EMCCD very efficient for faint flux photometry or spectroscopy, adaptive optics, ultrafast imaging and Lucky Imaging. This clocking is achievable with a new EMCCD controller: CCCP, the CCD Controller for Counting Photons. This new controller, which is now commercialized by Nuv¨ u cam ¨ ¯eras inc., was integrated into an EMCCD camera and tested at the observatoire du mont-Megantic. ´ The results are presented in this paper.
1. INTRODUCTION
The operation of an EMCCD1 in Photon Counting (PC) mode enables one to get rid of its Excess Noise Factor (ENF).2, 3 The ENF usually plagues the EMCCD when it is operated at a high EM gain and when it is attempted to count more than one photon per pixel per image.2–4 This operating mode is referred to as the Analogic Mode (AM). The PC operation, on the other hand, implies counting only one photon per pixel per image. The reduced dynamic range of an image must be compensated by reading the EMCCD at a faster pace to avoid the losses by coincidence that occur at high flux. However, in order to be efficient, the PC operation requires the operation of the EMCCD at a very high EM gain. An EM gain that is too low will result in many photons being lost in the read-out noise. Figure 1 summarizes this effect. The operation of the EMCCD at a high EM gain will usually yield a higher Clock Induced Charges (CIC) level.5 In PC operation, the CIC, which might be negligible as compared to the dark noise for long integration times (> 30 sec), quickly arise and become the dominant source of noise.6–9 Thus, in order to avoid the ENF of an EMCCD, one must tame the CIC down to very low levels while having a high EM gain. Even if the CIC could be lowered by operating the EMCCD in Non-Inverted Mode (NIMO), the Inverted Mode operation (IMO) of the EMCCD is preferable to keep the dark current low. Figure 2 shows that at a moderate CIC level (0.005 ¯e/pixel/image), the PC operation at a G/ ratio of 20 (which is typical of a of 50 ¯e and an EM gain of 1000) will barely be more efficient than the AM operation, and for a very limited flux range. Realistically, it is useless to operate such a camera in PC mode, as there are more disadvantages (losses by coincidence, higher noise at low fluxes) than advantages (less than 10% decrease in integration time for a very limited flux range). A higher EM gain is required to render the PC operation useful. On the other hand, even with a G/ of 50, a very low CIC level is required for the PC operation to be more efficient than the AM operation over a wide range of fluxes.
In order to reduce the amount of CIC generated by the read-out process of an EMCCD, while being able to produce a very high EM gain, a whole new EMCCD controller was built. CCCP, the CCD Controller for Counting Photons, was designed specifically to address these issues. In this paper, the controller will be presented in section 2, and experimental results obtained with a camera built with CCCP and a CCD97 (hereafter CCCP/CCD97) will be shown in section 3.
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