Characterization of a photon counting EMCCD for space-based high contrast imaging spectroscopy of extrasolar planets
ABSTRACT
We present the progress of characterization of a low-noise, photon counting Electron Multiplying Charged Coupled Device (EMCCD) operating in optical wavelengths and demonstrate possible solutions to the problems of Clock-Induced Charge (CIC) and other trapped charge through sub-bandgap illumination. Such a detector will be vital to the feasibility of future space-based direct imaging and spectroscopy missions for exoplanet characterization, and is scheduled to fly on-board the AFTA-WFIRST mission. The 512×512 EMCCD is an e2v detector housed and clocked by a N¨uv¨u Cameras controller. Through a multiplication gain register, this detector produces as many as 5000 electrons for a single, incident-photon-induced photoelectron produced in the detector, enabling single photon counting operation with read noise and dark current orders of magnitude below that of standard CCDs. With the extremely high contrasts (Earth-to-Sun flux ratio is ∼ 10−10) and extremely faint targets (an Earth analog would measure 28th - 30th magnitude or fainter), a photon-counting EMCCD is absolutely necessary to measure the signatures of habitability on an Earth-like exoplanet within the timescale of a mission’s lifetime, and we discuss the concept of operations for an EMCCD making such measurements.
1. INTRODUCTION
Photon counting, electron-multiplying CCDs (EMCCDs) are under strong consideration for missions as imminent as the AFTA-Coronagraph (technology deadline 2017),1 and larger missions further in the future, like the Advanced Technology Large Aperture Space Telescope (ATLAST).2 This technology will be matured as part of the Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies3 IFS to be installed at the High Contrast Imaging Testbed4 in 2015. EMCCDs are a highly desirable technology for exoplanet imaging and spectroscopy missions, as identified by the Astro2010 white paper.5 When performing starlight-suppressed, direct imaging/spectroscopy observations of Earth analogs, photon rates are extremely low (on the order of one photon per thousand seconds). Photon counting detectors significantly increase the efficiency of observing such systems, to the point that integration times fit significantly better into the lifespan of a typical space-based mission.
EMCCDs operate just as the name suggests: they measure the signal of a single photon on the detector after passing the corresponding photoelectron through a multiplication gain register that increases the output by two to three orders of magnitude.6 EMCCD operation improves as the source photon flux gets lower (in direct contrast to standard CCD operation), approaching the photon noise limit. Exposure (single frame) times are very short in photon counting mode due to the overlapping probability densities of recording more than one photon on a given pixel. Photon counting operation via thresholding requires each pixel to report signal in binary: pixel values are simply zero or one. As the rate of incoming photons increases – or the frame rate of reading the detector decreases – the probability of receiving more than one photon in a given pixel increases. The information is lost, and thus the accuracy is decreased, in this situations known as ‘pulse pile-up’ or ’coincidence losses’, the detector does not distinguish between single-photon events and multi-photon events.
The gain register renders read noise irrelevant, but it amplifies image-layer noise not usually of concern in standard CCD operation, known as clock-induced-charge (CIC). The major limitations of EMCCDs include both CIC and dark current,7 which, similarly, becomes more relevant at low photon rates. Further, what is generally measured and called CIC, is likely a combination of actual image-layer CIC and “CIC” generated within the EM gain register.8 Over the last decade, as the role for EMCCDs in astronomy has become more apparent and imminent, efforts to reduce the CIC (at least from the image area) have increased.7, 9–11 The lowest combined dark current and CIC background noise floor is claimed by the Canadian company, N¨uv¨u Cameras, with their HN¨u camera system housing e2v EMCCDs.12–14 The specifications for the e2v detector housed and clocked by a N¨uv¨u camera are in Table 1.
The lower that background noise floor, the more efficient an observing strategy can be adopted, as less time can be spent to achieve the same signal-to-noise, which we describe in detail and quantify in §2. This will be especially important in upcoming planet-finding direct imaging missions. The work described in what follows supports this premise by first verifying the claims of significant improvement in both CIC and dark current in the N¨uv¨u system (§3), and then seeking to improve the operations even further by applying sub-bandgap illumination to the detector. This method is an effort to mitigate the image-layer CIC by significantly shorten the lifetime of populated traps without producing unwanted signal within the dynamic range of the detector (§4).
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