KEYWORDS: Calibration, Data modeling, Signal to noise ratio, Education and training, Nulling interferometry, Equipment, Point spread functions, Wavefronts, Sensors, Optical path differences
Photonic technologies have enabled a generation of nulling interferometers, such as the guided light interferometric nulling technology instrument, potentially capable of imaging exoplanets and circumstellar structure at extreme contrast ratios by suppressing contaminating starlight, and paving the way to the characterization of habitable planet atmospheres. But even with cutting-edge photonic nulling instruments, the achievable starlight suppression (null-depth) is only as good as the instrument’s wavefront control and its accuracy is only as good as the instrument’s calibration. Here, we present an approach wherein outputs from non-science channels of a photonic nulling chip are used as a precise null-depth calibration method and can also be used in real time for fringe tracking. This is achieved using a deep neural network to learn the true in-situ complex transfer function of the instrument and then predict the instrumental leakage contribution (at millisecond timescales) for the science (nulled) outputs, enabling accurate calibration. In this method, this pseudo-real-time approach is used instead of the statistical methods used in other techniques (such as null self calibration, or NSC) and also resolves the severe effect of read-noise seen when NSC is used with some detector types.
In the past few years, there has been a resurgence in studies of space-based optical/infrared interferometry, particularly with the vision to use the technique to discover and characterize temperate Earth-like exoplanets around solar analogs. One of the key technological leaps needed to make such a mission feasible is demonstrating that formation flying precision at the level needed for interferometry is possible. Here, we present Pyxis, a ground-based demonstrator for a future small satellite mission with the aim to demonstrate the precision metrology needed for space-based interferometry. We describe the science potential of such a ground-based instrument and detail the various subsystems: three six-axis robots, a multi-stage metrology system, an integrated optics beam combiner, and the control systems required for the necessary precision and stability. We conclude by looking toward the next stage of Pyxis: a collection of small satellites in Earth orbit.
European Southern Observatory (ESO)’s Very Large Telescope Interferometer (VLTI), Paranal, Chile, is one of the most proficient observatories in the world for high angular resolution astronomy. It has hosted several interferometric instruments operating in various bandwidths in the infrared. As a result, the VLTI has yielded countless discoveries and technological breakthroughs. We propose to ESO a new concept for a visitor instrument for the VLTI: Asgard. It is an instrumental suite comprised of four natively collaborating instruments: High-Efficiency Multiaxial Do-it ALL Recombiner (HEIMDALLR), an all-in-one instrument performing both fringe tracking and stellar interferometry with the same optics; Baldr, a Strehl optimizer; Beam-combination Instrument for studying the Formation and fundamental paRameters of Stars and planeTary systems (BIFROST), a combiner whose main science case is studying the formation processes and properties of stellar and planetary systems; and Nulling Observations of dusT and planeTs (NOTT), a nulling interferometer dedicated to imaging young nearby planetary systems in the L band. The overlap between the science cases across different spectral bands yields the idea of making the instruments complementary to deliver sensitivity and accuracy from the J to L bands. Asgard is to be set on the former AMBER optical table. Its control architecture is a hybrid between custom and ESO-compliant developments to benefit from the flexibility offered to a visitor instrument and foresee a deeper long-term integration into VLTI for an opening to the community.
KEYWORDS: Data modeling, Calibration, Wavefronts, Signal to noise ratio, Point spread functions, Nulling interferometry, Adaptive optics, Telescopes, Sensors, Machine learning
Photonic technologies have enabled a new generation of nulling interferometers such as the GLINT instrument, potentially capable of imaging exoplanets and circumstellar structure at extreme contrast ratios by suppressing contaminating starlight, and paving the way to the characterisation of habitable planet atmospheres. But even with cutting edge photonic nulling instruments, the achievable starlight suppression (null-depth) is only as good as the instrument’s wavefront control, and its accuracy is only as good as the instrument’s calibration. Here we present a new approach wherein outputs from non-science channels of a photonic nulling chip are used as a precise null-depth calibration method, and can also be used in realtime for fringe tracking. This is achieved by using a deep neural network to learn the true in-situ complex transfer function of the instrument, and then predict the instrumental leakage contribution (at millisecond timescales) for the science (nulled) outputs, enabling accurate calibration. In this method, this pseudo-realtime approach is used instead of the statistical methods used in other techniques (such as numerical self calibration, or NSC), and also resolves the severe effect of read-noise seen when NSC is used with some detector types.
Nulling interferometry is one of the most promising techniques for imaging exoplanets at solar system scales as it simultaneously meets the stringent requirements for contrast and angular resolution. The GLINT instrument, operating at Subaru telescope behind the SCExAO extreme adaptive optics system, has delivered significant advances in performance, paving the way for a science-ready instrument. Results from previous commissioning runs have confirmed that integrated optics and self-calibration methods yield a robust pathway to detecting exoplanets below the diffraction limit of the telescope. Furthermore, limitations to the current performance have been identified. The most critical are the compensation of remaining phase fluctuations and delivering deep achromatic nulls. Specifically, one technology that offers enormous promise to resolve these issues is the use of tricouplers to perform simultaneous nulling and fringe tracking, or wavefront characterization and correction free of non-common path aberrations. By adding a broadband π radian phase-shifter, fringe tracking and path length stabilization is performed around a deep achromatic null, providing excellent high contrast performance at small angular scales. In this paper, we present the commissioning of GLINT, its current challenges and describe the modelling of devices to overcome them. We discuss the expected performance of a nuller based on these principles of phase control and null depth as implemented within the GLINT instrument. We further present laboratory characterization of 3D-written tricouplers made using ultrafast laser inscription.
Optical interferometry from space is arguably the most exciting prospect for high angular resolution astrophysics; including the analysis of exoplanet atmospheres. This was highlighted in the recent ESA Voyage 2050 plan, which pointed out the exciting potential of this technology, but also indicated the critical need for technological demonstrators. Here we present the Pyxis interferometer; a ground-based pathfinder for a CubeSat space interferometer, currently being built at Mt Stromlo Observatory. We outline its technological and scientific potential as the only visible wavelength interferometer in the Southern Hemisphere, and the optical systems designed to provide CubeSat compatible metrology for formation flying.
The Very Large Telescope Interferometer is one of the most proficient observatories in the world for high angular resolution. Since its first observations, it has hosted several interferometric instruments operating in various bandwidths in the infrared. As a result, the VLTI yields countless discoveries and technological breakthroughs. We introduce to the VLTI the new concept of Asgard: an instrumental suite including four natively collaborating instruments: BIFROST, a stellar interferometer dedicated to the study of the formation of multiple systems; Hi- 5, a nulling interferometer dedicated to imaging young nearby planetary systems in the M band; HEIMDALLR, an all-in-one instrument performing both fringe tracking and stellar interferometry with the same optics; Baldr, a fibre-injection optimiser. These instruments share common goals and technologies. Thus, the idea of this suite is to make the instruments interoperable and complementary to deliver unprecedented sensitivity and accuracy from J to M bands. The interoperability of the Asgard instruments and their integration in the VLTI are the main challenges of this project. In this paper, we introduce the overall optical design of the Asgard suite, the different modules, and the main challenges ahead.
Beam combiners are important components of an optical/infrared astrophysical interferometer, with many variants as to how to optimally combine two or more beams of light to fringe-track and obtain the complex fringe visibility. One such method is the use of an integrated optics chip that can instantaneously provide the measurement of visibility without temporal or spatial modulation of the optical path. Current asymmetric planar designs are complex, resulting in a throughput penalty, and so here, we present developments into a three-dimensional triangular tricoupler that can provide the required interferometric information with a simple design and only three outputs. Such a beam combiner is planned to be integrated into the upcoming Pyxis interferometer, where it can serve as a high-throughput beam combiner with a low size footprint. Results into the characterization of such a coupler are presented, highlighting a throughput of 85 ± 7 % and a flux splitting ratio between 33:33:33 and 52:31:17 over a 20% bandpass. We also show the response of the chip to changes in the optical path, obtaining an instantaneous complex visibility and group delay estimate at each input delay.
Post Extreme Adaptive-Optics (ExAO) spectro-interferometers design allows high contrast imaging with an inner working angle down to half the theoretical angular resolution of the telescope. This regime, out of reach for conventional ExAO imaging systems, is obtained thanks to the interferometric recombination of multiple sub-apertures of a single telescope, using single mode waveguides to remove speckle noise. The SCExAO platform at the Subaru telescope hosts two instruments with such design, coupled with a spectrograph. The FIRST instrument operates in the Visible (600-800nm, R~400) and is based on pupil remapping using single-mode fibers. The GLINT instrument works in the NIR (1450-1650nm, R~160) and is based on nulling interferometry. We present here how these photonic instruments have the unique capability to simultaneously do high contrast imaging and be included in the wavefront sensing architecture of SCExAO.
Space interferometry is the inevitable endpoint of high angular resolution astrophysics, and a key technology that can be leveraged to analyse exoplanet formation and atmospheres with exceptional detail. Here, we present a feasibility study into a small scale formation flying interferometric array, flying in Low Earth Orbit, that will aim to prove the technical concepts involved with space interferometry while still making unique astrophysical measurements. We will detail the design of the mission, as well as present orbital simulations that show that the array should be stable enough to perform interferometry with <50 m/s/year Δv and one thruster per spacecraft. We also conduct observability simulations to identify what parts of the sky are visible for a given orbital configuration. We conclude with optimism that this design is achievable, but a more detailed control simulation factoring in a metrology system is the next step to demonstrate full mission feasibility.
A space interferometer could reach a sensitivity and angular resolution which is unattainable on Earth due to the distortion and absorption of the atmosphere. It would enable many unique science cases, including the direct imaging and characterisation of temperate terrestrial exoplanets. This ambitious vision relies on the formation flying of individual spacecrafts, and the demonstration of precision metrology measuring positions in to better than 1mm in at least 2 dimensions, and velocities in the range of nm/s. These significant technical challenges are one of the main reasons progress in space interferometry has been seriously hampered in the two last decades. To overcome this obstacle, we propose a novel metrology concept operating in two steps. The coarse positioning of the array elements is achieved through commercially demonstrated components, such as GPS, wide angle cameras and time-of-flight sensors. For the critical fine metrology, multiple longitudinal mode Fabry-Perot lasers in a central spacecraft are split and retro-reflected off each telescope bearing spacecraft. The reflected beams are then coherently combined in the central spacecraft and the resulting fringes are spectrally dispersed. In this manner, the phase difference is measured at the different Fabry-Perot wavelengths, allowing the unambiguous differential position measurements over a couple of mm capture range. We present the concept together with a prototype system in the laboratory.
With thousands of confirmed exoplanets, the era of discovery is giving way to that of characterization. Direct imaging is crucial, but extremely difficult due to high star-to-planet contrasts and high angular resolutions. Nulling interferometry, which suppresses contaminating starlight via destructive interference, aims to meet this challenge. A pathfinder of this technique is the GLINT nuller: a 6-baseline, spectrally dispersed pupil-remapping interferometer deployed at the Subaru telescope, in which a single photonic chip performs all the critical optical processes. We present the instrument, novel data processing based on self-calibrating methods, laboratory characterization and the latest on-sky results.
As confirmed exoplanets climb into the thousands, the era of exoplanets discovery is giving way to exoplanet characterization. The most desirable scenario is one where the exoplanet can be directly imaged. Direct imaging not only delivers orbital parameters, but also yields the chemical composition of the atmosphere. The potential for habitable zone exoplanets to exhibit biosignatures in such data from a visionary future instrument drives intense interest. However, this requires to simultaneously reach extremely high star-to-planet contrast (from 104 to 108) and extremely high angular resolution (around and below the diffraction limit). Accomplishing all this through the atmosphere blurred by turbulence remains a critical challenge, yet it is one that nulling interferometry in combination with extreme adaptive optics aims to meet. This technique overcomes the contrast problem by removing the starlight with destructive interference, permitting the faint light coming from the planet to remain. In this paper, we present the latest evolution of nulling interferometry instrumentation: the integrated- photonic nuller. It allows spatial filtering, multiple simultaneous baselines, simultaneous photometric channels and simultaneous measurement of the "nulled" signal (the light emitted from the planet after cancelling the starlight) as well as the "anti-nulled" signal (the channel containing the redirected starlight). Exploiting these fundamental optical principles, the delivery of imaging and differential spectroscopy of exoplanetary systems becomes possible. This paper describes a pathfinder that has implemented these ideas into a robust and compact photonic-chip platform known as the GLINT (Guided-Light Interferometric Nulling Technology) project.
With many thousands of exoplanets discovered one of the important next steps in astronomy is to be able to characterise them. This presents a great challenge and calls for new observational capabilities with both high angular resolution and extreme high contrast in order to efficiently separate the bright light of a host star to that of a faint companion. Glint South is an instrument that uses photonic technology to perform nulling interferometry. The light of a star is cancelled out by means of destructive interference in a photonic chip. One of the challenges is the star light injection into the chip. This is done by a unique active system that optimises the injection and provide low order correction for the atmospheric turbulence. We are reporting on the latest progress following several tests on the Anglo Australian Telescope.
SCExAO is the premier high-contrast imaging platform for the Subaru Telescope. It offers high Strehl ratios at near-IR wavelengths (y-K band) with stable pointing and coronagraphs with extremely small inner working angles, optimized for imaging faint companions very close to the host. In the visible, it has several interferometric imagers which offer polarimetric and spectroscopic capabilities. A recent addition is the RHEA spectrograph enabling spatially resolved high resolution spectroscopy of the surfaces of giant stars, for example. New capabilities on the horizon include post-coronagraphic spectroscopy, spectral differential imaging, nulling interferometry as well as an integral field spectrograph and an MKID array. Here we present the new modules of SCExAO, give an overview of the current commissioning status of each of the modules and present preliminary results.
A long-held astronomical vision is to realize diffraction-limited optical aperture synthesis over kilometer baselines. This will enable imaging of stellar surfaces and their environments, show their evolution over time, and reveal interactions of stellar winds and gas flows in binary star systems. An opportunity is now opening up with the large telescope arrays primarily erected for measuring Cherenkov light in air induced by gamma rays. With suitable software, such telescopes could be electronically connected and used also for intensity interferometry. With no optical connection between the telescopes, the error budget is set by the electronic time resolution of a few nanoseconds. Corresponding light-travel distances are on the order of one meter, making the method practically insensitive to atmospheric turbulence or optical imperfections, permitting both very long baselines and observing at short optical wavelengths. Theoretical modeling has shown how stellar surface images can be retrieved from such observations and here we report on experimental simulations. In an optical laboratory, artificial stars (single and double, round and elliptic) are observed by an array of telescopes. Using high-speed photon-counting solid-state detectors and real-time electronics, intensity fluctuations are cross correlated between up to a hundred baselines between pairs of telescopes, producing maps of the second-order spatial coherence across the interferometric Fourier-transform plane. These experiments serve to verify the concepts and to optimize the instrumentation and observing procedures for future observations with (in particular) CTA, the Cherenkov Telescope Array, aiming at order-of-magnitude improvements of the angular resolution in optical astronomy.
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