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GRAVITY is an adaptive optics assisted Beam Combiner for the second generation VLTI instrumentation. The instrument will provide high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the astronomical K-band for faint objects. We describe the wide range of science that will be tackled with this instrument, highlighti...
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... working principle of GRAVITY is explained for the case of Galactic Center observations ( figure 9). The field offers a bright AO reference star (IRS 7, 5.5" separation from Sgr A*, m K = 6.5) outside the 2" field of view of the VLTI, and a fringe-tracking star (IRS 16C, 1.2" separation from Sgr A*, m K = 9.7) inside the field of view. ...
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The second generation Very Large Telescope Interferometer (VLTI) instrument GRAVITY is a two-field infrared interferometer operating in the K band between 1.97 and 2.43 µm with either the four 8 m or the four 1.8 m telescopes of the Very Large Telescope (VLT). Beams collected by the telescopes are corrected with adaptive optics systems and the frin...
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... The field of astrophotonics promises to improve the observation of celestial objects by interfacing photonic integrated circuits with telescopes [74]. As a matter of fact, the confinement and the filtering capabilities of single-mode waveguides enable an improvement in the quality of the collected light [75], while the use of more complex integrated elements such as beam combiners and gratings provides for instance an enhancement of the angular resolution in the imaging of astronomical objects [76], or the possibility to perform the spectral analysis of the light emitted by a star [77]. In this regard, photonic integrated circuits inscribed in glass by FLM have some peculiar advantages if compared to other platforms. ...
Thanks to its unique properties, glass plays a fundamental role in science and technology, especially in optics and photonics. For instance, its transparency has been exploited in the last decades for efficiently guiding light in optical fibers for long distances, while its versatility makes it the perfect material in different research fields, ranging from fundamental science to biology and chemistry. On the occasion of the International Year of Glass, we would like to discuss a powerful microfabrication technique for devices in this material: femtosecond laser micromachining (FLM). This technique can process different types of glass, and thanks to the nonlinear nature of the induced modification, it enables the fabrication of complex three-dimensional micro-structures capable of guiding light or transporting fluids. The purpose of this review article is to celebrate the multidisciplinary nature of FLM by discussing, without claim for completeness and after a brief introduction about the process, a selection of its applications in the diverse fields of biology, strong-field physics, and astronomy.
... In addition to the sky rotation of the object, one can disperse the light as shown in Fig. 1.4d or vary the observed λ 0 from the stellar object using a set of optical filters. Instruments such as GRAVITY [28], and PIONIER [29] at VLTI rely not only on the sky rotation but also on the dispersion of the collected light to increase the u − v coverage. ...
... To give an example of how the different technologies and techniques that were described above are used in a cutting-edge instrument, some key features of the GRAVITY instrument [28,12] at VLTI are briefly described. The GRAVITY instrument works in the astronomical K-band (2.1 µm). ...
... It was also used to study several YSOs, where it was found that a few of them had skewed disks, potentially due to the presence of stellar halos [96]. The success of IONIC and IONIC3 led to the commissioning of the GRAVITY [97,28] and PIONIER [29] instruments that consisted of four-telescope IO-based beam combiners for the VLTI at the astronomical K-and H-band, respectively. The GRAVITY beam combiner -an astrophotonics component -has been used to study many exciting research areas of astrophysics, one being the observations of our Galactic center [12], which was discussed in detail in Section 1.4. ...
Stellar interferometry is the only method in observational astronomy for obtaining the highest resolution images of astronomical targets. This method is based on combining light from two or more separate telescopes to obtain the complex visibility that contains information about the brightness distribution of an astronomical source. The applications of stellar interferometry have made significant contributions in the exciting research areas of astronomy and astrophysics, including the precise measurement of stellar diameters, imaging of stellar surfaces, observations of circumstellar disks around young stellar objects, predictions of Einstein's General relativity at the galactic center, and the direct search for exoplanets to name a few. One important related technique is aperture masking interferometry, pioneered in the 1960s, which uses a mask with holes at the re-imaged pupil of the telescope, where the light from the holes is combined using the principle of stellar interferometry. While this can increase the resolution, it comes with a disadvantage. Due to the finite size of the holes, the majority of the starlight (typically > 80 %) is lost at the mask, thus limiting the signal-to-noise ratio (SNR) of the output images. This restriction of aperture masking only to the bright targets can be avoided using pupil remapping interferometry - a technique combining aperture masking interferometry and advances in photonic technologies using single-mode fibers. Due to the inherent spatial filtering properties, the single-mode fibers can be placed at the focal plane of the re-imaged pupil, allowing the utilization of the whole pupil of the telescope to produce a high-dynamic range along with high-resolution images. Thus, pupil remapping interferometry is one of the most promising application areas in the emerging field of astrophotonics. At the heart of an interferometric facility, a beam combiner exists whose primary function is to combine light to obtain high-contrast fringes. A beam combiner can be as simple as a beam splitter or an anamorphic lens to combine light from 2 apertures (or telescopes) or as complex as a cascade of beam splitters and lenses to combine light for > 2 apertures. However, with the field of astrophotonics, interferometric facilities across the globe are increasingly employing some form of photonics technologies by using single-mode fibers or integrated optics (IO) chips as an efficient way to combine light from several apertures. The state-of-the-art instrument - GRAVITY at the very large telescope interferometer (VLTI) facility uses an IO-based beam combiner device reaching visibilities accuracy of better than < 0.25 %, which is roughly 50× as precise as a few decades back. Therefore, in the context of IO-based components for applications in stellar interferometry, this Thesis describes the work towards the development of a 3-dimensional (3-D) IO device - a monolithic astrophotonics component containing both the pupil remappers and a discrete beam combiner (DBC). In this work, the pupil remappers are 3-D single-mode waveguides in a glass substrate collecting light from the re-imaged pupil of the telescope and feeding the light to a DBC, where the combination takes place. The DBC is a lattice of 3-D single-mode waveguides, which interact through evanescent coupling. By observing the output power of single-mode waveguides of the DBC, the visibilities are retrieved by using a calibrated transfer matrix ({U}) of the device. The feasibility of the DBC in retrieving the visibilities theoretically and experimentally had already been studied in the literature but was only limited to laboratory tests with monochromatic light sources. Thus, a part of this work extends these studies by investigating the response of a 4-input DBC to a broad-band light source. Hence, the objectives of this Thesis are the following: 1) Design an IO device for broad-band light operation such that accurate and precise visibilities could be retrieved experimentally at astronomical H-band (1.5-1.65 μm), and 2) Validation of the DBC as a possible beam combination scheme for future interferometric facilities through on-sky testing at the William Herschel Telescope (WHT). This work consisted of designing three different 3-D IO devices. One of the popular methods for fabricating 3-D photonic components in a glass substrate is ultra-fast laser inscription (ULI). Thus, manufacturing of the designed devices was outsourced to Politecnico di Milano as part of an iterative fabrication process using their state-of-the-art ULI facility. The devices were then characterized using a 2-beam Michelson interferometric setup obtaining both the monochromatic and polychromatic visibilities. The retrieved visibilities for all devices were in good agreement as predicted by the simulation results of a DBC, which confirms both the repeatability of the ULI process and the stability of the Michelson setup, thus fulfilling the first objective. The best-performing device was then selected for the pupil-remapping of the WHT using a different optical setup consisting of a deformable mirror and a microlens array. The device successfully collected stellar photons from Vega and Altair. The visibilities were retrieved using a previously calibrated {U} but showed significant deviations from the expected results. Based on the analysis of comparable simulations, it was found that such deviations were primarily caused by the limited SNR of the stellar observations, thus constituting a first step towards the fulfillment of the second objective.
... Sampling three pixels per fringe cycle is often seen as a good compromise between sampling the interference fringes well enough while minimising the number of pixels to be read out. In the case of FOURIER I opted to sample four pixels per cycle on the highest spatial frequency interference fringe pattern, the same number of samples is used in some wave-guided optics beam combiners such as GRAVITY (Gillessen et al. 2010). Figure 7.26 plots the maximum fringe contrast as a function of how well sampled the interference fringes are. ...
In this thesis I present and discuss the work carried out during my PhD in the astrophysics group of the Cavendish Laboratory. The majority of this work has been to develop the Free-space Optical multi-apertUre combineR for IntERferometry (FOURIER), a novel, high sensitivity, near-infrared beam combiner intended to be the first generation science beam combiner for the Magdalena Ridge Observatory Interferometer (MROI). FOURIER is a three telescope, J, H and K band image plane beam combiner. I first highlight the scientific motivation for a faint limiting magnitude beam combiner and the design requirements this placed on FOURIER. This is followed by a discussion of how the optical design was derived from these requirements. The simulated performance of the instrument is then discussed including the alignment and manufacturing error budget, limiting magnitude estimation, spectral resolution and modelling of thermal effects on cooling the instrument to its liquid nitrogen operating temperature. The cryogenic optomechanics are described as well as a series of laboratory tests of the fringe contrast, spectral resolution and throughput to verify the instruments performance. In addition to the development of FOURIER I discuss a numerical simulation of an optical interferometer subject to atmospheric seeing, used to quantify previously unaccounted for image plane crosstalk effects arising due to various combinations of atmospheric seeing, long propagation distances and finite sized optics present in long baseline optical interferometers.
... SMFs also act as a spatial filter and couple very little sky background [7]. This makes them highly suitable for direct exoplanet spectroscopy [8] and interferometry [9][10][11][12][13]. When coupled to a high resolution spectrograph, SMFs also remove conventional modal noise, allowing an increase in the achievable radial velocity (RV) precision [14]. ...
We present the first on-sky results of the microlens ring tip-tilt sensor. This sensor uses a 3D printed microlens ring feeding six multimode fibers to sense misaligned light, allowing centroid reconstruction. A tip-tilt mirror allows the beam to be corrected, increasing the amount of light coupled into a centrally positioned single-mode (science) fiber. The sensor was tested with the iLocater acquisition camera at the Large Binocular Telescope in Tucson, Arizona, in November 2019. The limit on the maximum achieved rms reconstruction accuracy was found to be $0.19 \lambda /{\rm{D}}$ 0.19 λ / D in both tip and tilt, of which approximately 50% of the power originates at frequencies below 10 Hz. We show the reconstruction accuracy is highly dependent on the estimated Strehl ratio and simulations support the assumption that residual adaptive optics aberrations are the main limit to the reconstruction accuracy. We conclude that this sensor is ideally suited to remove post-adaptive optics noncommon path tip-tilt residuals. We discuss the next steps for concept development, including optimization of the lens and the fiber, tuning of the correction algorithm, and selection of optimal science cases.
... SMFs also act as a spatial filter and couple very little sky background [7]. This makes them highly suitable for direct exoplanet spectroscopy [8] and interferometry [9][10][11][12][13]. When coupled to a high resolution spectrograph, SMFs also remove conventional modal noise, allowing an increase in the achievable radial velocity (RV) precision [14]. ...
We present the first on-sky results of the micro-lens ring tip-tilt (MLR-TT) sensor. This sensor utilizes a 3D printed micro-lens ring feeding six multi-mode fibers to sense misaligned light, allowing centroid reconstruction. A tip-tilt mirror allows the beam to be corrected, increasing the amount of light coupled into a centrally positioned single-mode (science) fiber. The sensor was tested with the iLocater acquisition camera at the Large Binocular Telescope in November 2019. The limit on the maximum achieved root mean square reconstruction accuracy was found to be 0.19 $\lambda$/D in both tip and tilt, of which approximately 50% of the power originates at frequencies below 10 Hz. We show the reconstruction accuracy is highly dependent on the estimated Strehl ratio and simulations support the assumption that residual adaptive optics aberrations are the main limit to the reconstruction accuracy. We conclude that this sensor is ideally suited to remove post-adaptive optics non-common path tip tilt residuals. We discuss the next steps for the concept development, including optimizations of the lens and fiber, tuning of the correction algorithm and selection of optimal science cases.
... A photonic nulling interferometer called GLINT has recently been demonstrated which can be used for observing exoplanets, possibly able to measure their spectra in the future [53]. A current analogue which offers these functionalities (R ∼ 20, 500, 4000 and angular resolution of tens of milliarcseconds in the K-band) is the GRAVITY instrument-a VLT interferometer [54][55][56]. A photonic counterpart of GRAVITY will be shoebox-sized, with improved mechanical stability, thus allowing deeper observations of faint, background-limited sources. ...
Astrophotonics is the application of photonic technologies to channel, manipulate, and disperse light from one or more telescopes to achieve scientific objectives in astronomy in an efficient and cost-effective way. Utilizing photonic advantage for astronomical spectroscopy is a promising approach to miniaturizing the next generation of spectrometers for large telescopes. It can be primarily attained by leveraging the two-dimensional nature of photonic structures on a chip or a set of fibers, thus reducing the size of spectroscopic instrumentation to a few centimeters and the weight to a few hundred grams. A wide variety of astrophotonic spectrometers is currently being developed, including arrayed waveguide gratings (AWGs), photonic echelle gratings (PEGs), and Fourier-transform spectrometer (FTS). These astrophotonic devices are flexible, cheaper to mass produce, easier to control, and much less susceptible to vibrations and flexure than conventional astronomical spectrographs. The applications of these spectrographs range from astronomy to biomedical analysis. This paper provides a brief review of this new class of astronomical spectrographs.
... The observations can be followed-up with ESO's VLTI optical interferometer GRAVITY that provides milli-arcsecond angular resolution in near infrared wavelengths. An interferometric instrument GRAVITY is used for interferometric imaging, as well as for astrometry [9]. Two independent systems: fringe tracking (FT) and infrared wavefront sensing system (CIAO) help to correct automatically the residual optical path difference between the beams [9]. ...
... An interferometric instrument GRAVITY is used for interferometric imaging, as well as for astrometry [9]. Two independent systems: fringe tracking (FT) and infrared wavefront sensing system (CIAO) help to correct automatically the residual optical path difference between the beams [9]. This concept will be presented for optical interferometry, regarding to VLTI. ...
In recent years, Intermediate Mass Black Holes (IMBHs) (with masses between 100 – 105 ) attracted wide attention due to importance in understanding how the black holes at various masses are formed, in particular the super-massive ones seen in quasars [1]. It is presumed that they can be formed by various occurring phenomena, including direct collapse of unpolluted gas in very massive stars [2], from compact stellar clusters [3] and some can even originate from the mysterious dark matter and potentially be partially responsible for the dark matter content of the Universe. So far, there has been no robust detection of IMBH, with only a couple of candidates suggested [4].
... A photonic nulling interferometer called GLINT has recently been demonstrated which can be used for observing exoplanets, possibly able to measure their spectra in the future [53]. A current analogue which offers these functionalities (R ∼ 20, 500, 4000 and angular resolution of tens of milliarcseconds in the K-band) is the GRAVITY instrument-a VLT interferometer [54][55][56]. A photonic counterpart of GRAVITY will be shoebox-sized, with improved mechanical stability, thus allowing deeper observations of faint, background-limited sources. ...
Astrophotonics is the application of photonic technologies to channel, manipulate, and disperse light from one or more telescopes to achieve scientific objectives in astronomy in an efficient and cost-effective way. Utilizing photonic advantage for astronomical spectroscopy is a promising approach to miniaturizing the next generation of spectrometers for large telescopes. It can be primarily attained by leveraging the two-dimensional nature of photonic structures on a chip or a set of fibers, thus reducing the size of spectroscopic instrumentation to a few centimeters and the weight to a few hundred grams. A wide variety of astrophotonic spectrometers is currently being developed, including arrayed waveguide gratings (AWGs), photonic echelle gratings (PEGs), and Fourier-transform spectrometer (FTS). These astrophotonic devices are flexible, cheaper to mass produce, easier to control, and much less susceptible to vibrations and flexure than conventional astronomical spectrographs. The applications of these spectrographs range from astronomy to biomedical analysis. This paper provides a brief review of this new class of astronomical spectrographs.
... One combiner will be an image-plane design similar to the Michigan Infrared combiner (MIRC) 1 while a second combiner will be based on the 4-beam integrated optics combiner GRAVITY. 2 The new instrument will use a C-RED One 3 camera (First Light Imaging) based on the SELEX-SAPHIRA 4 chip. We expect "first light" in 2019. ...
We present the design for MYSTIC, the Michigan Young STar Imager at CHARA. MYSTIC will be a K-band, cryogenic, 6-beam combiner for the Georgia State University CHARA telescope array. The design follows the image-plane combination scheme of the MIRC instrument where single-mode fibers bring starlight into a non-redundant fringe pattern to feed a spectrograph. Beams will be injected in polarization-maintaining fibers outside the cryogenic dewar and then be transported through a vacuum feedthrough into the ~220K cold volume where combination is achieved and the light is dispersed. We will use a C-RED One camera (First Light Imaging) based on the eAPD SAPHIRA detector to allow for near-photon-counting performance. We also intend to support a 4-telescope mode using a leftover integrated optics component designed for the VLTI-GRAVITY experiment, allowing better sensitivity for the faintest targets. Our primary science driver motivation is to image disks around young stars in order to better understand planet formation and how forming planets might influence disk structures.
... One combiner will be an image-plane design similar to the Michigan Infrared combiner (MIRC) 1 while a second combiner will be based on the 4-beam integrated optics combiner GRAVITY. 2 The new instrument will use a C-RED One 3 camera (First Light Imaging) based on the SELEX-SAPHIRA 4 chip. We expect "first light" in 2019. ...
