Astronomy

Photon counting pioneers

As pioneers, astronomers always aim at the foremost innovation in imaging technologies. Over the past centuries, they ensured the constant progress of this field 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 as the following examples of its applications are only the tip of the iceberg.

Use of Nüvü Camēras’ EM N2 at Mont Mégantic Observatory (Québec).

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EMCCD ADVANTAGES FOR FUTURE SPACE APPLICATIONS

Asteroids Tracking and Space Debris Monitoring

Due to the rapid increase in the number of space missions, a high density of space debris has been accumulating in the 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 (NEO), pose a threat to life in case they enter in collision with our planet. It is thus of utmost interest to scan the night sky and identify every NEO of 100 meter + size that may impact the Earth.

Faint magnitudes and cosmic ray impacts

Space debris are considered of critical importance when their dimensions reach 1 cm, requiring an excellent sensitivity threshold. In addition, imaging device must be capable to discriminate objects from noise sources like cosmic ray impacts which can significantly blur an image. Similarly, NEO are very dim objects that require excellent sensitivity in order to track them over time and plot their trajectories.

Better sensitivity and mitigation of cosmic ray impacts

With sub-electron readout noise due to patented CCCP reduced clock-induced charge (CIC), Nüvü Camēras’ technology was made for high precision faint flux imaging. Its sensitivity is unrivaled to this day. Furthermore, Nüvü Camēras products 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.

UV Spectroscopy

Almost impossible to detect on the ground due to atmospheric absorption, ultraviolet radiation (UV) provides a wealth of information: for example, these wavelengths reveal the chemical compositions of the hottest stars and help in the understanding of the interstellar medium of our galaxy and beyond. However, such applications require an effective detection of 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 poor noise reduction performances. 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

In conventional CCD mode, Nüvü Camēras’ HNü 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 Camēras’ EMCCDs 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

Asteroseismology is a promising and rapidly developing technique that uses a star oscillations to probe its interior structure. Such technique provides valuable information about its 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 light pattern emitted by a star: oscillations modulate a light curve or a specific emission line within minutes. However, high resolution spectroscopy and photometry, especially for faint objects, requires 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 affect fish stock assessment, shipping routes and weather predictions. The collection of such data, acquired through 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 imaging systems to operate in the visible and near-infrared range for optimal image quality, spatial and spectral resolution, demanding 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; resulting in a very faint signal.

Meeting requirements for spaceborne coastal monitoring

Recent studies show tangible results for remote coastal sensors by making use of EMCCD cameras like Nüvü Camēras’ HNü, specifically. With remarkable sensitivity in the visible and near-infrared combined with the single photon detection capability, Nüvü Camēras’ technology could improve accuracy and diversity of data collected from coastlines and subsurface oceans phenomena.

Environmental and Climatic Behaviour Observation

Earth observation 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. This problem is solved by getting rid of the excess noise factor. However, extreme faint flux imaging faces another limiting factor, clock induced charges; as frame rates increase, it becomes the dominating source of noise.

The darkest EMCCD ever

The impact of clock induced charges and the amount of noise it generates decreases by implementing appropriate clocking of the sensor’s controller. With its innovative CCD Controller for Counting Photons, 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 links to referenced articles 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.

Characterization Results of EMCCDs for Extreme Low Light Imaging
The Darkest EMCCD Ever

PHOTON STARVING APPLICATIONS

High resolution spectroscopy

High resolution spectroscopy is used in a broad range of applications, from redshifted galaxies to nano-flares studies. It provides very precise information on an incident photon’s energy when the focus is on spectral resolution, and unfolds events happening on very short time scales if attention is put on temporal resolution.

Resolution limited by noise sources

Both spectral and temporal resolution depend on the photon count per pixel being as low as possible. For the former, having a high resolution means that every pixel is ideally measuring receiving a unique wavelength. For the latter, high resolution implies richer sampling with a lesser chance of aliasing as each frame has a very short integration time. In both cases, the signal coming through the camera is very faint. Single photons need to be processed, requiring excellent signal-to-noise ratios (SNR) in order for them not to be buried or indistinguishable from the noise. Time dependent noises (like dark noise) limit high spectral resolution spectroscopy; fixed noises (like read-out and clock-induced charges) hinders 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 its resolutely low dark current, the thermoelectrically cooled cameras support integration times of over several seconds in a high spectral resolution scheme. In fact, with Nüvü Camēras advanced devices, there is virtually no disadvantages to gather high resolution spectra.

Fabry-Pérot Imaging Spectroscopy

Usual diffraction grating spectrometers are not as efficient when imaging astronomical events distributed over large portions of the sky like the interstellar medium. The resolution of these optical systems is limited by the size of the light cone which 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

Recorded objects are often faint, dispersed and do not emit considerable light fluxes. Fabry-Pérot imaging spectroscopy thus requires exposure times that may sometimes be quite long 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 the 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) collects. Unlike Fabry-Pérot imaging spectroscopy, an integral field unit splits the incoming light beam in multiple sections, then directs them into a spectrometer. The following datacube consists of series of spectrum 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 decreases by increasing the amount of image slices. 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, allowing an excellent wavelength coverage for IFS. 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.

Transient Phenomenon Imaging

Transiting in front of a star, an exoplanet alters its host incoming light flux. Information about an exoplanet, such as its atmospheric composition, size, mass, and radius, are deducted by precisely measuring these minute variations over time.

Light curve of a transient planet across its host star.

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 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.

Lucky Imaging

Our atmosphere is in constant motion due to wind and temperature gradients. 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 frame is taken under that period of time, the turbulence appears static on the frame, a technique know as lucky imaging. It combines multiple fast frames where the atmosphere is static to create a single near-perfect frame by isolation of redundant, verified features.

Need for high signal to noise ratios and large field of views

Combining frames require a high signal to noise ratio for each of the 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 a good sky coverage, requiring 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 speed required for lucky imaging is easily achieved with a large field of view to keep all reference points in sight.

Adaptive Optics

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, affecting the original light path and degrading the incoming signal. Such turbulent motions hinder observations at a telescope theoretical resolution, roughly given by the ratio between the wavelength and its primary mirror aperture.

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. Tracking a natural or artificial guide star in the field, a wavefront sensor samples its pattern every hundredth of a second, or even faster, and sends data to dozens of actuators that accordingly alter in real-time the primary mirror shape. The incoming light thusly reconstructed improves a telescope’s performance to its theoretical limit.

Meeting all requirements for adaptive optics

With high frame rates, 20 MHz readout rates, low noise and superior sensitivity, Nüvü Camēras’ technology is perfectly 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’s cameras indeed fulfill the stringent requirements set by adaptive optics to achieve the lowest wavefront error.

Real-time Photometry

Studying dynamic celestial objects like cataclysmic variables or supernovae requires efficient techniques to gather more data and better understand them. Correspondingly, real-time photometry acquires images of thousands of stars 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 read time. However past CCD controller technologies were not developed for fast 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) that 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 lays in the electronics of its re-invented controller, built from the ground up for the purpose of faint flux imaging. Through significant reduction of noise sources like CIC, Nüvü Camēras products 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.

Light curve of G226-29, a ZZ Ceti pulsating white dwarf, taken in V band on a 1.6-m telescope on a full moon night. Data were acquired at 30 frames per second and binned to 5 seconds.

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.

Electron-Multiplying CCDs for Future Space Instruments

Why every photon counts

For applications that are not limited by background or shot noise, an imaging device with the lowest possible readout noise is paramount for high quality observation. In that regard, the EMCCD’s sub-electron readout noise makes it the ideal candidate for such 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 Quantum Efficiency (QE) of the camera.

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 to count up to 5 photons/pixel/image. In this context, the applications are associated with very faint fluxes and/or considerably high speeds. The latter case produces spurious charges, also referred to as clock-induced charges (CIC), which becomes the dominant source of noise in EMCCDs. With high CIC levels and insufficient electron-multiplying gain, it is futile to operate the camera in Photon Counting mode.

Nüvü Camēras’ controller CCCP (CCD Controller for Counting Photons) stems 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 needs of modern astronomers as it produces an EM gain 5 times greater than that of other CCD controller while generating at least 10 times less CIC. With this level of noise reduction, there are virtually no disadvantages to take high resolution images and post-process them in order to choose the desired SNR and/or spectral resolution. In fact, this is changing the way astronomers use spectroscopy as one can choose afterwards to bin frames temporally or spectrally and/or pixels to any desired extent while keeping track of the higher dynamics inside the high resolution original images.

NGC7331 radial velocity field extracted from Fabry-Pérot spectroscopy data. The data were gathered at the 1.6-m Mont Megantic telescope at a spectral resolution of 15000 and is among the very first demonstration of the potential of photon-counting imaging.