Spectrograph is one of the most important tools in astronomical observation and can be used in research areas ranging from cosmology to exoplanet research. Conventional astronomical spectrograph using a diffraction grating is huge, posing great challenges to their thermal and mechanical stability, and they are also very expensive. This inevitably determines the need for new original innovations in future optical and near-infrared spectrograph technologies. The application of photonics in astronomical spectrograph in recent years has shown a great potential for miniaturizing the spectrograph which is mounted on the large telescopes. The new dispersion element named waveguide spectral lens (WSL)is proposed by Westlake University that different from the independent optical element in the conventional spectrograph, and it can realize the dual functions of both wavelength separation and focus. This kind of chip technology makes the structure more compact, and improves the design to expand the devices working in the communication band to the visible and near-infrared band, enabling the spectrograph based on this new technology to achieve astronomical observation in the visible band in the future. In order to fully understand the performance of this new dispersion element and its application potential in astronomy, we established two chip test platforms in the optical laboratory of Shanghai Astronomical Observatory, and analyzed the dispersion capability of the device by using the wavelength calibration method. In order to expand the range of the spectra, the two-dimensional cross-dispersion spectrum was realized by adding a cylindrical lens and a blazed grating in the laboratory. The solar spectrum is also observed using these two chips. The experimental results show that this new optical waveguide chip can be applied to the visible light band, and can be used as the dispersion element of astronomical spectrograph for astronomical applications. At present, the optical and mechanical design of the prototype of the spectrograph has been completed. In the future, the laboratory installation of the prototype will be completed to realize the on-sky observation as soon as possible.
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.
Aperture masking observations for binary stars have been done with a 1.56-m telescope at Shanghai Astronomical Observatory(SHAO) during 2019-2024. In order to ensure ample light available to restore the high resolution images even at short exposure times and reach the diffraction limit of the 1.56-m telescope, we select some binary stars and reference stars nearby with magnitude 5-8 and angular distance 0.2-4 arcsec in the WDS catalog as observation targets. This article achieved high-resolution restoration of binary stars using a hybrid data processing method, including data reduction, a spatial domain method named ISA or a frequency domain method named SI to suppress atmospheric turbulence, and OS-EM to reduce image degradation caused by multi-aperture interference. The results show that this method can effectively obtain high-resolution images of binary stars, and the measured angular distance is basically consistent with the given value in WDS catalog.
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.
Co-phasing technique is used to detect and compensate the fluctuation of optical path difference (OPD) between subtelescopes of long baseline optical interferometers caused by atmospheric turbulence. At present, the sensitivity of cophasing technology is limited, which cannot meet the observation needs for more and darker objects such as Broad Line Region of Active Galactic Nucleus and Quasars. A kind of chromatic phase diversity method (CPD) used to obtain OPD by extracting phase and unwrapping phase difference from the optical transfer function of multi-aperture interferometry was proposed in 2014. Compared to other traditional co-phasing or fringe tracking methods, this method is of some significant advantages such as less power loss, larger capture range of piston error and synchronous sensing for all apertures. In the past, the method was verified by bulk optics or integrated photonics chip only combining beams. Wavelength Separation was still accomplished by many dichroic beamsplitters and fold mirrors which would increase light loss. We present a new compact solution that Fizeau multi-axial beam combination and multiple spectral channels can be merged on single photonic chip. Our works including Verifying of CPD algorithm, model simulation and design of the chip on H-band will be introduced respectively.
By placing a mask over the pupil of the optical telescope, the aperture masking technique transforms the telescope into a Fizeau interferometry telescope. Thanks to reasonable aperture configuration and baseline rotation techniques, it is possible to achieve almost the same imaging quality as a full aperture telescope. This technique has shown great potential in astrometry and astrophysics research, such as: exoplanet detection, protoplanetary disk, brown dwarf, etc. In order to verify the image restoration algorithm, we carried out binary stars observations on 1.56-m telescope. We presented the numerical simulation of aperture configuration and baseline rotation, and designed the mask and the experimental system. We select some binary stars with magnitude from 5 to 7 and angular distance from 0.2 to 2arcsec as observation targets. Combined with the short exposure observation, a two-step image restoration method is proposed, the results of high-resolution image reconstruction and angular distance measurement are verified. The above results will be applied to the first-generation Fizeau interferometry prototype at the Shanghai Astronomical Observatory (SHAO).
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.
In general, most of the adaptive optical systems for human eye aberration detection are based on the wavefront slope measurement provided by the Shark-Hartman wavefront sensor (SHWS), and then the wavefront slope is fed back to the deformable mirror to correct the human eye aberrations. Compared with the SHWS, the pyramid wavefront sensor (PWS) has the characteristics of fast sampling speed, wide linear capture range, and high sensitivity. Our works show that the modulation angle of the dynamic high-frequency modulator affects the dynamic measurement range, linearity and sensitivity of the pyramid sensing. The dynamic measurement range and the linear fitting residuals are both proportional to the modulation angle, and the sensitivity is inversely proportional to the modulation angle. Pixel combination affects the sensitivity of the detection signals of the pyramid sensor. The pixel combination mode of 1 × 1, 2 × 2, and 3 × 3 is tested respectively. When the pixel combination mode of 2 × 2 is used, the sensitivity of the signals will be highest significantly. In addition, the beacon light used to detect the human eye should not be too strong. The grinding “blind zone” of the spires and edges will have a scattering effect on the incident light and cause loss of light energy. Therefore, it is necessary to optimize the parameters of the pyramid sensor and further improve the processing technology of the pyramid prism.
Deformable mirror (DM) is the most main wavefront corrector in adaptive optics, which can be used to compensate optical aberrations through changing the reflective mirror’s surface frequently. However, a commercial piezoelectric DM can’t have an ideal flat initial surface under zero-voltage condition due to limitation of thin mirror fabrication and support structure of actuators behind of mirror. Optical aberrations generated by this initial distortion will seriously attenuate the performance of DM’s close-loop control, so a flat-surface calibration of mirror needs to be carried out before DM properly correct optical aberrations. In order to properly control the optical figure of the DM we have to obtain an interactive matrix which is the response of optical surface to the DM actuator’s stroke. We measured a serious of surface phase data of OKO 109-channel DM through self-collimation using a ZYGO-GPI interferometer directly, then construct the interactive matrix by zonal and modal methods. After several close-loop iterations, the initial RMS surface error of OKO 109-channel deformable mirror, 1.506λ has been remarkably reduced to 0.145λ.
High resolution observation of celestial objects has always been the goal of optical interferometry. In this paper, we concentrated on two aspects of image reconstruction for Fizeau interferometric telescope. 1. The influence of piston error on imaging quality was studied, which provides a basis for the technical specifications of telescopes. 2. We proposed to use speckle imaging technology in interferometric telescopes, this method can reduce the effect of atmospheric turbulence on the resolution. In summary, a method combining denoising algorithm and speckle imaging technology is used to suppress noise, remove turbulence and reconstruct high-resolution images of real objects. The simulation results show that speckle imaging technology is also applicable to the interferometric telescope, and got good image reconstruction effect. The research results can be further extended to other mosaic telescopes.
Fringe test is the method which can detect the relative optical path difference in optical synthetic aperture telescope array.
To get to the interference fringes, the two beams of light in the meeting point must be within the coherence length. Step
scanning method is within its coherence length, selecting a specific step, changing one-way’s optical path of both by
changing position of micro displacement actuator. At the same time, every fringe pattern can be recorded. The process of
fringe patterns is from appearing to clear to disappearing. Firstly, a particular pixel is selected. Then, we keep tract of the
intensity of every picture in the same position. From the intensity change, the best position of relative optical path
difference can be made sure. The best position of relative optical path difference is also the position of the clearest fringe.
The wavelength of the infrared source is 1290nm and the bandwidth is 63.6nm. In this experiment, the coherence length
of infrared source is detected by cube reflection experiment. The coherence length is 30μm by data collection and data
processing, and that result of 30μm is less different from the 26μm of theoretical calculated. In order to further test the
relative optical path of optical synthetic aperture using step scanning method, the infrared source is placed into optical
route of optical synthesis aperture telescope double aperture. The precision position of actuator can be obtained when the
fringe is the clearest. By the experiment, we found that the actuating step affects the degree of precision of equivalent
optical path. The smaller step size, the more accurate position. But the smaller the step length, means that more steps
within the coherence length measurement and the longer time.
Speckle interferometry has beenwidely used in the observational astronomy, especially in binary stars.This paper is the
part of a series dedicated to the speckle imaging of binary stars carried out by the research team of Shanghai
Astronomical Observatory.The observation experiments were carried out with 1.56-m telescope using a speckle
camera,and the high resolution image were reconstructed successfully using speckle interferometry and iterative shiftand-
add. In order to speed up the computation speed, we also prepared a reconstruction software based on GPU
technology and CUDA programming model, compared with C++ program based on CPU, the speed ratio can reach
about 7 times.
Fizeau interferometry is one of the most important technique to measure astronomical objects with high angle resolution.
This paper is the part of a series dedicated to research of the Fizeau interferometry carried out by the research team of
Shanghai Astronomical Observatory. This paper is mainly concerned the simulation of image restoration based on
Y-type telescope and segmented mirrors telescope. It is proved that we can get the high resolution image using RL and
OS-EM method.
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