Optical synthesis aperture telescope technology can be used to get more rich astronomical information. Interference fringe scanning method is commonly used to eliminate optical path difference between different optical delayed lines, but due to the polarization difference between the interference arms will lead to interference fringe contrast degradation especially in interference type instrument. Especially when observing faint, more distant targets, it is more necessary to consider the polarization effects caused by the instrument itself. In this paper, the Fizeau-type Y-4 prototype developing by Shanghai Astronomical Observatory, Chinese Academy of Sciences is introduced first of all. Based on the principle of the vector-wave superposition, this paper focuses on fringe contrast degradation caused by polarization effects and the changes of polarization states caused by coating of different material. The simulation results show that the interference fringe contrast is sensitive to the polarization effects. Similarly, the changes of polarization state of the beam caused by the coating needs to be considered when designing the optical interferometer. Finally, a polarization compensator is proposed to compensate the polarization difference.
The Fizeau type interferometric telescope forms an array of several sub telescopes for direct imaging on the image plane based on the principle of optical interferometry. Compared to the optical long baseline interferometer, this kind of telescope can be used for real time imaging of celestial body due to some excellent characteristics such as sufficient spatial frequencies coverage, single mounting avoiding outer optical delay lines and so on. We have built an interferometric imaging telescope with four apertures. Although each aperture size is 100mm, but this telescope can reach the higher angular resolution which is equivalent to a monolithic telescope of 280mm aperture size through optimal array configuration. Some novel opto-mechanical structure design and error control methods have been applied to this telescope successfully. For example, in order to enhance the rigidity of mechanical system, a unique C-shape structure to replace the traditional azimuth axis is adapted. Piston, tip/tilt errors between all apertures can be detected at the same time by extracting signals from Modulation Transfer Function (MTF), so some classical beam splitters can be removed which will reduce light loss significantly. At present, we have finished the final assembly, co-phasing calibration and verifying of dynamic co-phasing close-loop methods at laboratory. The FWHM of far field image spot is 0.43 arcsecond which is consistent with theoretical values. The out-door astronomical observation will be carried out soon.
A double-focus optical telescope (DOT) has been built for public observation and scientific research. The unique optical property of the DOT is that, both the Ritchey-Chretien (R-C) and Prime Focus systems are achieved on one telescope, using a common primary mirror. Switching between the R-C and Prime Focus systems is accomplished by moving the secondary mirror away from the optical path. The DOT also provides public observations through the eyepiece system.
Accurate piston error detection and closed-loop control are one of the key technologies to ensure the imaging quality of the interferometric imaging telescope. In this paper, we proposed a piston error detection and control scheme based on three computers and multithreading,which has been successfully applied to a four 0.1-m apertures interferometric telescope. This scheme adopts a kind of fringe contrast measurement and climbing method to achieve closed-loop control. The results implied that the fringe contrast can be raised through piston closed-loop correction. Compared with a single telescope with 0.1-m aperture, we can get a 2.63x improvement in resolution for the new interferometric telescope with four 0.1-m apertures. It is proved that the feasibility and effectiveness of this scheme. We will further carry out astronomical observation experiments and improve the piston error detection and control scheme, in order to provide technical guarantees for the implementation of interferometric imaging telescopes.
The Earth 2.0 (ET) space mission has entered its phase B study in China. It seeks to understand how frequently habitable Earth-like planets orbit solar-type stars (Earth 2.0s), the formation and evolution of terrestrial-like planets, and the origin of free-floating planets. The final design of ET includes six 28 cm diameter transit telescope systems, each with a field of view of 550 square degrees, and one 35 cm diameter microlensing telescope with a field of view of 4 square degrees. In transit mode, ET will continuously monitor over 2 million FGKM dwarfs in the original Kepler field and its neighboring fields for four years. Simultaneously, in microlensing mode, it will observe over 30 million I < 20.5 stars in the Galactic bulge direction. Simulations indicate that ET mission could identify approximately 40,000 new planets, including about 4,000 terrestrial-like planets across a wide range of orbital periods and in the interstellar space, ~1000 microlensing planets, ~10 Earth 2.0s and around 25 free-floating Earth mass planets. Coordinated observations with ground-based KMTNet telescopes will enable the measurement of masses for ~300 microlensing planets, helping determine the mass distribution functions of free-floating planets and cold planets. ET will operate from the Earth-Sun L2 halo orbit with a designed lifetime exceeding 4 years. The phase B study involves detailed design and engineering development of the transit and microlensing telescopes. Updates on this mission study are reported.
An ultra-compact optical spectrograph (~43x16x13cm) is developed using a new optical arrayed waveguide technique based on waveguide spectral lenses (WSL). The WSL is an evolved version from the arrayed waveguide grating design can achieve simultaneous spectral dispersion and image focusing onto the detector plane at designed distance. Despite its compact size, the instrument maintains high optical throughput and provides a wide range of spectral resolution (R~200-2000 at 600-950 nm). The spectrograph's design and the results of laboratory testing will be reported.
Large field-of-view rapid sky surveys are pivotal methodologies for research in time-domain astronomy. The project entitled ”Antarctic Time Domain Astronomical Optical Observational Array Based on Drift Scanning CCD Technology”, also known as the Antarctic Tianmu Plan, is based in Antarctica and comprises an assemblage of numerous small wide-field optical telescope systems. This expansive array covers a sky area of 10,000 square degrees. Each wide-field optical telescope unit is fitted with a low noise drift scanning CCD camera, facilitating rapid observations without the necessity of moving parts within the telescope system. The advantageous clear polar night conditions of Antarctica are employed to enable repeated and continuous wide-area observations, providing an optimal environment for the monitoring of transient events. A prototype of the Antarctic Tianmu Program was constructed from 2020 to 2022 and subsequently dispatched to the Zhongshan Station in Antarctica for installation, debugging, and operational observation in concurrence with the 39th scientific research vessel at the culmination of 2022. This manuscript primarily presents the quality of raw images and limiting magnitudes acquired at varying exposure times, observational results encompassing photometry precision, and the long-term stability of the system operation during the initial year of observation.
To achieve high-resolution image using optical synthesis aperture telescope, it’s necessary to co-phase accurately of all the telescopes so as to reduce the effect of co-phase errors including piston error, tip/tilt error, and mapping error, etc. Though simulation analysis of the optical system, error sources can be identified and thus save time of alignment. This paper introduces the Fizeau-type Y-4 prototype under development, including the layout of the Y-4 prototype, the layout of the reflective mirrors in the delayed light paths and the beam combiner. With the optical transfer function as the evaluation index, the actual equivalent diameter of Y-4 prototype is calculated. Furthermore, the effect of polarization introduced by coating and polarization differences on the contrast of interference fringe is analyzed. At present, the installation and alignment of the prototype in laboratory have been completed, and the interference synthesis of 4 light paths has been realized. One aim of this paper is to share some experiences in optical design and detection for the development of optical synthetic aperture telescopes. Another aim is to expand these new techniques to the larger optical synthesis aperture telescope project in the future.
The Earth 2.0 (ET) mission is a space mission in China which will be operated at the Earth-Sun L2 orbit with a designed lifetime longer than 4 years. ET’s scientific payload consist of six 30cm diameter transit telescopes with each field of view of 500 square degrees and one 35 cm diameter microlensing telescope with a field of view of 4 square degrees. Each telescope is equipped with a camera with 2×2 9K×9K CMOS detectors, and Front-end Electronics (FEE). Each transit telescope is an f/1.57 eightlens refractive optical system while the microlensing telescope is an f/17.2 catadioptric optical system with diffraction-limited design. The diameter of 90% Encircled Energy (EE90) for transit telescopes is within 5×5 pixels while the FWHM of PSF for the microlensing telescope is less than 0.78 arcsec. Fine Guidance Sensors are mounted at the four edges of the CMOS camera. All seven telescopes are fixed on a common mounting reference plate, and a large sun shield is used to block the heat flow from the Sun and provide a stable thermal environment for the telescopes. It also blocks straylight form the Sun, Earth, and the Moon. Each telescope has an additional top hood to block straylight incident at a large angle while the top hood is also used as a radiator to cool the detectors to below - 40°C. With PID heating loops, each telescope will work at -30±0.3°C while the detectors work at - 40±0.1°C. Details of the conceptual design for the scientific payload will be presented.
The Earth 2.0 (ET) mission is a Chinese next-generation space mission aiming at detecting thousands of terrestrial-like planets, including habitable Earth-like planets orbiting solar type stars (i.e., Earth’s 2.0s), cold low-mass planets, and free-floating planets. The ET mission will use six 300 mm diameter wide field telescope arrays to continuously monitor 1.2 million FGKM dwarf stars in the original Kepler field and its adjacent regions for four consecutive years to search for new planets including Earth 2.0s using the transit technique. The six telescopes have the same configuration, point to the same sky area, and constitute the main scientific payload. Each telescope has an effective aperture of 300 mm with a very wide field of view (FOV) of 500 square degrees and a wavelength coverage of 450-900 nm. Each telescope is equipped with a focal plane mosaic camera. The mosaic camera is composed of 2×2, 9k×9k CMOS detectors with pixel size of 10μm. The optical design results in the diameter of the 90% encircled energy (EE90%) less than 40μm (or 4 pixels) over the entire FOV. About 20% vignetting at the edge of the FOV is introduced to provide good throughput for the entire FOV while keeping optics size and weight down to reduce manufacturing risk and scientific payload within the mass and volume limit. In this paper, we will present the optical design details, including influence analysis of various factors on image quality, e.g., glass material, detector flatness, manufacturing and assembly tolerances. In addition, we will describe temperature stability analysis of the telescope on image quality and photometry measurements.
A space mission called “Earth 2.0 (ET)” is being developed in China to address a few of fundamental questions in the exoplanet field: How frequently habitable Earth-like planets orbit solar type stars (Earth 2.0s)? How do terrestrial planets form and evolve? Where did floating planets come from? ET consists of six 30 cm diameter transit telescope systems with each field of view of 500 square degrees and one 35 cm diameter microlensing telescope with a field of view of 4 square degrees. The ET transit mode will monitor ~1.2M FGKM dwarfs in the original Kepler field and its neighboring fields continuously for four years while the microlensing mode monitors over 30M I< 20.6 stars in the Galactic bulge direction. ET will merge its photometry data with that from Kepler to increase the time baseline to 8 years. This enhances the transit signal-to-noise ratio, reduce false positives, and greatly increases the chance to discover Earth 2.0s. Simulations show that ET transit telescopes will be able to identify ~17 Earth 2.0s, about 4,900 Earth-sized terrestrial planets and about 29,000 new planets. In addition, ET will detect about 2,000 transit-timingvariation (TTV) planets and 700 of them will have mass and eccentricity measurements. The ET microlensing telescope will be able to identify over 1,000 microlensing planets. With simultaneous observations with the ground-based KMTNet telescopes, ET will be able to measure masses of over 300 microlensing planets and determine the mass distribution functions of free-floating planets and cold planets. ET will be operated at the Earth-Sun L2 orbit with a designed lifetime longer than 4 years.
The Earth 2.0 (ET) mission is a Chinese next-generation space mission to detect thousands of Earth-sized terrestrial planets, including habitable Earth-like planets orbiting solar type stars (Earth 2.0s), cold low-mass planets, and freefloating planets. To meet the scientific goals, the ET spacecraft will carry six 30 cm diameter transit telescopes with each field of view of 500 square degrees, and one 35 cm diameter microlensing telescope with a field of view of 4 square degrees, monitor ~1.2M FGKM dwarfs in the original Kepler field and its neighboring fields continuously while monitoring over 30M stars in the Galactic bulge direction. The high precision transit observations require high photometry precision and pointing stability, which is the key drive for the ET spacecraft design. In this paper, details of the overall mission modeling and analysis will be presented. The spacecraft orbit, pointing strategy, stability requirements are presented, as well as the space-ground communication analysis. The ET spacecraft adopts an ultra-high photometry precision & high stable platform, largely inherited from other space science missions. The preliminary design of spacecraft which meets mission requirements is introduced, including the spacecraft overall configuration, observation modes, avionics architecture and development plan, which pays great attention to the pointing stability and huge volume science telemetry download.
VERMILION is a VLTI visitor instrument project intended to extend the sensitivity and the spectral coverage of Optical Long Baseline Interferometry (OLBIn). It is based on a new concept of Fringe Tracker (VERMILIONFT) combined with a J band spectro-interferometer (VERMILION-J). The Fringe Tracker is the Adaptive Optics module specific to OLBIn that measures and corrects in real time the Optical Path Difference (OPD) perturbations introduced by the atmosphere and the interferometer, by providing a sensitivity gain of 2 to 3 magnitudes over all other state of the art fringe trackers. The J band spectro-interferometer will provide all interferometric measurements as a function of wavelength. In addition to a possible synergy with MATISSE, VERMILION-J, by observing at high spectral resolution many strong lines in J (Paβ-γ, HeII, TiO and other metallic monoxides), will cover several scientific topics, e.g. Exoplanets, YSOs, Binaries, Active Hot, Evolved stars, Asteroseismology, and also AGNs.
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