Photon counting pioneers
As pioneers, astronomers always aim at the foremost innovation in imaging technologies. Over the past centuries, they ensured the field’s constant progress in their never-ending discovery of the cosmos. Within only 5 years, Nüvü Camēras’ EMCCD and photon counting technologies have successfully paired up with leading international research groups. The colossal potential of this technology is starting to emerge, and yet needs to be fully grasped. The following EMCCD applications are only the tip of the iceberg.
Nüvü Camēras’ EM N2 camera at Mont Mégantic Observatory (Québec).
Higher SNR for overcoming research frontiers
New instruments designed and optimized for Nüvü Camēras’ cutting edge technology are generating certain interest for the conception of versatile space astronomy instruments.
To learn more about spaceborne applications considered for Nüvü Camēras’ EMCCD technology, read the article linked below, courtesy of Canadian Space Agency’s Dr. Oleg Djazovski. It was presented during the 2013 Photonics North Conference held in Ottawa. Nüvü Camēras and the University of Montreal participated as coauthors.
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EMCCD ADVANTAGES FOR FUTURE SPACE APPLICATIONS
Direct exoplanet observation
As of September 2014, nearly 2000 extrasolar planets have been identified, most being Jupiter- and Neptune-class. Yet astronomers believe it is just the tip of the iceberg: recent studies predict an average of one planet per star in the Milky Way. Nevertheless, most exoplanet detections are indirect and based on star monitoring for minute luminosity changes induced by its planetary companion (see the Transient Photometry section). In the quest to uncover more Earth twins, astronomers now turn themselves to direct observation, studying their host’s light reflected onto their surface by masking the star’s brightness with a coronagraph.
10 billion star to planet light contrast
Such emission is extremely weak. A Jupiter-like planet’s is about 100,000 times dimmer than its host. For Earth analogs, their brightness drops even more, being 10 billion times lesser than the nearby star. Direct exoplanet imaging or spectroscopy thus calls for a device capable of detecting the few photons emitted from the planet’ surface that reach us, and they are scarce.
Higher sensitivities to discover new Earths
Direct extrasolar planet observation requires the most sensitive sensor there is, and Nüvü Camēras’ technology is perfectly suited for such demanding application. Nüvü’s revolutionary CCCP technology ensures the lowest clock-induced charge levels while its innovative packaging guarantees minimal dark current. The result is a camera that can detect every photon of an Earth-like planet that may support life.
Christian Marois of the National Research Council of Canada is currently developing a project known as the Mission to find A Planet Like Earth (MAPLE) to acquire an extrasolar planet image using Nüvü’s HNü 512 and 1024 cameras. He presented MAPLE’s design at the SPIE Astronomical Telescopes + Instrumentation 2014 meeting in Montreal.
Asteroids Tracking and Space Debris Monitoring
Due to the rapid increase of space missions, a high density of space debris has been accumulating in the upper atmosphere. The need for monitoring these objects in order to protect space-based infrastructures, such as communication satellites, is growing.
Farther out, sometimes at the edge of the solar system, asteroids and comets may cross path with Earth’s orbit. Such space bodies, referred to as Near Earth-Objects (NEOs), may threat the very existence of life if they were to collide with our planet. It is thus of utmost interest to scan the night sky and identify every NEOs of 100 meter + size that may impact the Earth.
Faint magnitudes and cosmic ray impacts
Space debris may cause important damage when their dimensions reach 1 cm, but are extremely difficult to detect; they require an excellent sensitivity threshold to even be seen. In addition, imaging device must be capable to discriminate objects from noise sources like cosmic rays, which can significantly blur an image. Similarly, tracking NEOs and plotting their trajectories calls for an excellent sensitivity, as they are very dim objects.
Better sensitivity and mitigation of cosmic ray impacts
With sub-electron readout noise due to its reduced clock-induced charge (CIC), Nüvü Camēras’ technology was made for high precision faint flux imaging. Its sensitivity is unrivalled to this day. Furthermore, Nüvü cameras have very high quantum efficiency and fast frame acquisition that lead to shorter integration time which, in turn, limit the effects of cosmic rays impacts on overall image quality.
Researchers have investigated the abilities of a ground-based satellite tracking system for On-Orbit Service (OOS) — which may lead to space debris reduction — using a Nüvü EM N2 1024 EMCCD camera mounted to the 1.6 m Mont Mégantic telescope. They were able to estimate the relative trajectories of close-orbiting satellites and demonstrated the potential of ground-based tracking for future satellite servicing missions.
The proof of concept by R.L. Scott (Defence Research and Development Canada) and A. Ellery (University of Carleton) is detailed in the paper below.
Almost impossible to detect on the ground due to atmospheric absorption, ultraviolet radiation (UV) originating from space provides a wealth of information: for example, these wavelengths reveal the chemical compositions of the hottest stars and help understanding the interstellar medium of our galaxy and beyond. However, such applications require an effective mean to detect this energetic radiation.
Shortcomings of current UV sensors
Although CCD technology has come far in terms of enhanced UV light detection, it shows limited performance with low quantum efficiency and little noise reduction. Such factors affect signal-to-noise ratios (SNR) and lead to poor results in low light conditions or during fast acquisitions.
EMCCDs for near-UV light detection
Nüvü Camēras offers top quality components and meticulous manufacturing, hence reaching the maximum quantum efficiency achievable with today’s Si detector technology. Moreover, through electron multiplication and proper clocking control via CCCP, Nüvü EMCCD cameras reduce inherent noise and increase image resolution. It has thus been actively considered for integration in powerful space telescopes for wide field imaging in both UV and visible spectrums.
Asteroseismology is a promising and rapidly developing technique that uses star oscillations to probe their internal structure. Such technique provides valuable information about their mass, radius, chemical composition, and age, information that is otherwise not available via traditional means of astronomical observations.
Dispersed light and rapid observations
To study stellar vibrations, one examines their effects over the star light pattern: oscillations modulate the light curve or a particular emission line within minutes. However, high-resolution spectroscopy and photometry with traditional sensors, especially for faint objects, calls for long uninterrupted acquisitions, incompatible with fast light variations or the collection of a sizable amount of photons, achievable only with large aperture telescopes.
Drastic read noise reduction for high resolution
EMCCDs low readout noise and improvements made by limiting clock-induced charges with Nüvü Camēras’ CCCP controller allows for faster frame rates during faint stars photometry or spectroscopy. Such technology provides great flexibility for unique asteroseismic diagnostics without requiring a large telescope to achieve high-resolution data.
Ocean and Coastal Monitoring
Government agencies need to monitor coastline phenomena to provide useful information that may lead to better fish stock assessment or shipping routes and weather predictions. The collection of such data, acquired by probing oceanic water columns, helps mitigate risks for marine navigation as well as offshore exploration and coastal settlement.
Damped signal and spectral range constraints
Such monitoring requires that imaging systems operate in the visible and near-infrared range for optimal image quality, spatial and spectral resolution, and thus demand high quantum efficiency. Moreover, atmospheric scattering and reflection over the sea surface result in only 20% of the total signal being returned from below the surface; hence resulting in very faint emissions.
Meeting requirements for spaceborne coastal monitoring
Recent studies show tangible remote coastal sensors results by making use of EMCCD cameras like Nüvü Camēras’ HNü. With remarkable sensitivity in the visible and near-infrared light combined with the single photon detection capability, Nüvü Camēras’ technology could improve both accuracy and diversity of data collected from coastlines and subsurface oceans phenomena.
Environmental and Climatic Behaviour Observation
Observing Earth from space provides fundamental insights on environmental protection and sustainability. New possibilities involving imaging of the dark side of the Earth may allow for a more comprehensive and global vision of environmental changes and climatic behaviours affecting human life.
Limited precision in darkness
EMCCD technology is precise in low light imaging. Yet signal-to-noise ratios (SNR) are insufficient to collect interpretable data in near total darkness conditions. Getting rid of the excess noise factor solves this problem. However, extreme faint flux imaging faces another limiting factor, namely clock-induced charges: as frame rates increase, they become the dominant noise source.
The darkest EMCCD ever
The impact of clock-induced charges and the amount of noise they generate decreases by implementing appropriate clocking with the sensor’s controller. With its innovative CCD Controller for Counting Photons (CCCP), Nüvü Camēras offers sub-electron readout noise, therefore enabling photon detection with top quantum efficiency and charge transfer efficiency for conclusive results even in near total darkness conditions.
Following: Direct link to a referenced article by Dr. Daigle, CTO at Nüvü Caméras, and the Canadian Space Agency presented during SPIE 2010 and SPIE 2012 meetings on the topic of High Energy, Optical and Infrared Detectors for Astronomy.
PHOTON STARVING APPLICATIONS
Transiting in front of a star, an exoplanet alters its host incoming light flux. Information about this object, such as its atmospheric composition, size, mass, and radius, are deducted by precisely measuring these minute variations over time.
Crucial information lost during the integration time
During the exoplanet’s transit in front of its host star, the first and last moments are of paramount importance: they unveil many of the planet’s features and occur on very short time scales. Such meaningful information can be lost in the integration time necessary for a less sensitive camera to capture images of moving objects at low light levels. Likewise, for planets with short revolution periods, the light curve shape may be hard to recover and is sometimes unreliable.
High speed, high SNR cameras
CCCP, Nüvü Camēras’ EMCCD innovative controller, achieves very high speeds while multiplying the signal to obtain a much superior SNR. The inherently lower readout noise offers a tenfold increase of an optical system’s efficiency, leading to a tremendous improvement in precision, richness and reliability of transient events’ light curves.
The atmosphere is in constant motion due to wind and temperature variations. This phenomenon, atmospheric turbulence, randomly deflects light as it passes through air, hence limiting the precision and accuracy of optical systems. These fluctuations have a characteristic time scale of roughly 30 ms. In the event that a picture is taken under that time span, the turbulence appears static on the frame, leading to a technique known as lucky imaging. It combines multiple fast frames to create a single near-perfect frame without turbulence-generated artefacts.
Need for high signal to noise ratios and large field of views
Combining multiple images require a high signal to noise ratio for each frames; otherwise, the resulting image may be compromised. Furthermore, in order to track atmospheric fluctuations, lucky imaging uses comparison stars for superposition: the instrument must offer good sky coverage, thus calling for large detectors capable of acquiring images at high frame rates.
Fast acquisition speeds with large detectors
Nüvü Camēras’ HNü is the first EMCCD camera to offer readout speeds of up to 20 MHz with the largest detector size available for this type of camera (1024 × 1024 pixels) while maintaining highly satisfactory SNRs. With binning and region of interest (ROI) options, the HNü achieves the necessary speed for lucky imaging while offering a large field of view to keep all reference stars in sight.
Air in the atmosphere drifts in foot-long pockets that get carried by wind and convective currents. Having different temperature and density, each pocket possesses its own refractive index. While moving and merging randomly, they bend light, thus affecting the original light path and degrading the incoming signal. Such turbulent motions hinder observations at a telescope theoretical resolution.
Tracking air fluctuation in real time
While lucky imaging overcomes atmospheric turbulences by virtually freezing random air fluctuations, adaptive optics reconstruct the light’s path through the atmosphere, and adjust the telescope’s mirror shape to flatten the wavefront. The incoming light thusly reconstructed improves a telescope’s performance to its theoretical limit, but doing so calls for both camera speed and accuracy.
Meeting all requirements for adaptive optics
With up to 20 MHz readout rates, low noise and superior sensitivity, Nüvü Camēras’ technology is suited for adaptive optics systems. Its exceptional quantum efficiency across the visible and near-infrared spectra also allows good sampling of the wavefront distortion due to air pockets. Nüvü cameras indeed fulfill the stringent requirements set by adaptive optics to achieve the lowest wavefront error.
Astronomers resort to high-resolution spectroscopy for a broad range of applications, from redshifted galaxies to nano-flares studies. It provides very precise information on an incident photon’s energy, or unfolds events happening on very short time scales.
Resolution limited by noise sources
Both spectral and temporal resolutions depend on the photon count per pixel being as low as possible. For the former, reaching high resolution means that every pixel ideally measures a unique wavelength. For the latter, high resolution implies richer sampling and lesser chance of aliasing with short integration time. In both cases, the signal coming to the camera is very faint. Single photons need to be processed, requiring excellent signal-to-noise ratios (SNR) so they remain above the noise threshold. Time dependent noises (like dark noise) limit high spectral resolution spectroscopy; fixed noises (like read-out and clock-induced charges) hinder high temporal spectroscopy.
Drastic noise reduction and post-processing for desired SNRs
Nüvü Camēras’ controller produces the lowest clock-induced charges of any EMCCD on the market, thanks to its re-invented electronics. Its electron-multiplying ability and photon counting mode decreases read noise to a minimum, allowing for very high temporal resolutions. Moreover, with their resolutely low dark current, Nüvü thermoelectrically cooled cameras support integration times of several seconds in a high spectral resolution scheme. In fact, with Nüvü Camēras advanced devices, there are virtually no disadvantages to gather high-resolution spectrums.
Fabry-Pérot Imaging Spectroscopy
Usual diffraction grating spectrometers are not efficient when imaging large astronomical objects like the interstellar medium. The resolution of these optical systems is limited by the light cone size that enters the spectrometer. On the other hand, a Fabry-Pérot spectrometer uses a large field imager and accepts large light cones without compromising its spectral resolution. Imaging the desired area for a selected wavelength range consists in collecting a series of frames, which are transposed, into a data volume or cube.
Imaging faint objects multiple times
The recorded objects are often faint, dispersed and do not emit considerable light fluxes. Fabry-Pérot imaging spectroscopy thus requires long exposure times to accumulate enough photons and provide a rich image at a given wavelength. This is a problem especially when considering that this process must be repeated for a number of wavelengths in order to have an accurate representation of an object. Eventually, this time inefficiency limits the scope of the work that can be done.
Superior detection and fast frame acquisitions
Nüvü Camēras EMCCD technology benefits from state-of-the-art quantum efficiency and a >91 % single photon detection probability, thus harnessing more photons in less time. Indeed, with its high gains and sub-electron readout noise, faster frame acquisition can be recorded for each wavelength range, making this spectroscopy technique much more time-affordable.
Integral Field Spectroscopy
Studying either the radial velocities of galaxy clusters or the properties of a newly born planetary system requires both spectral and location information, which integral field spectroscopy (IFS) provides. Unlike Fabry-Pérot imaging spectroscopy, an integral field unit splits the incoming light beam in multiple sections, and then directs them into a spectrometer. The following datacube is a series of spectrums, which, once combined, recreate a target’s image for a vast wavelength range.
Sensitivity and wavelength range
By splitting the incoming light into multiple spectra, IFS calls for sensitive sensors with excellent wavelength coverage as the signal strength decreases greatly. Additionally, high signal-to-noise ratios are required to achieve outstanding quality data. A larger detector also allows a greater amount of spectra to be analyzed in finer details in order to successfully reconstruct a frame.
Finely detailed spectra
Nüvü Camēras EMCCD technology is the most sensitive to date in the visible and near-infrared spectra, thus supporting the IFS wavelength coverage. It achieves high SNR even with low light flux with its patented and proven CCD Controller for Counting Photons (CCCP) by decreasing clock-induced charges and read noise to their minimum. With its 1024 × 1024 sensor, Nüvü Camēras’ HNü also offers great options for IFS by increasing split spectra surface coverage while still producing exquisite data.
Studying dynamic celestial objects like cataclysmic variables requires efficient techniques to gather more data and better understand them. To this end, real-time photometry acquires variable phenomena images at a frame rate of a few Hz to survey rapid brightness changes of a multitude of celestial objects.
Limiting noise sources
Monitoring events that take place on small time scales calls for fast readout time. However, past CCD controller technologies were not developed for rapid acquisitions. An increase in the pixel rate without a change in the technology only leads to more noise sources such as clock-induced charges (CIC), which ultimately pollute the signal and contaminate frames. Following this requirement, exposure times are shortened, decreasing the total photon count and increasing the need for lower noise levels.
A technology manufactured for fast acquisitions
The expertise developed at Nüvü Camēras lies in its re-invented controller electronics, built from the ground up for faint flux imaging. Through significant CIC level reduction, Nüvü cameras gather light with shorter exposure times. Even for large fields of view (1024 × 1024 pixel CCD detector size), frame rates higher than 6 fps with an electron-multiplying gain of 5,000 can be obtained and still yield exquisite image quality.
When every photon counts
Imaging devices with the lowest possible readout noise are paramount for high quality observation not limited by background or shot noise. In that regard, the EMCCD’s sub-electron readout noise makes it the ideal candidate for low light imaging applications. However, due to the stochastic nature of the electron-multiplying gain that counters readout noise, EMCCDs are flawed with an Excess Noise Factor (ENF) that has the same effect on the signal-to-noise ratio as halving the camera Quantum Efficiency (QE).
To recover the acquisition system’s full capabilities, it is possible to operate in Photon Counting mode in which a signal as little as a single photon per pixel per image can be recorded. For greater dynamic range, images can be superimposed and the threshold adjusted accordingly. In this context, EMCCDs are to be used for imaging very faint light sources and/or at considerable high speeds. The latter case however produces spurious charges, also referred to as clock-induced charges (CIC), which becomes the dominant noise source in EMCCDs. With high CIC levels and insufficient electron-multiplying gain, it is futile to operate the camera in Photon Counting mode.
Nüvü Camēras’ CCCP controller (CCD Controller for Counting Photons) arises from an academic research endeavour. It was developed from the ground up with the objective of elegantly mastering the major noise source in Photon Counting applications, the CIC. The CCCP is best suited for the modern astronomer needs as it produces an EM gain 5 times greater than that of other CCD controller while generating at least 10 times less CIC. With that much less noise, there are virtually no disadvantages to take high-resolution images and post-process them in order to select the desired SNR and/or spectral resolution. In fact, this is changing the way astronomers use spectroscopy, as one can choose to bin frames temporally or spectrally to any desired extent while keeping track of the higher dynamics inside the high-resolution original images.