Imaging Mirror Research Articles

Random vibration testing of microelectromechanical deformable mirrors for space-based high-contrast imaging

Space-based stellar coronagraph instruments aim to directly image exoplanets that are a fraction of an arcsecond separated from and 10 billion times fainter than their host star. To achieve this, one or more deformable mirrors (DMs) are used in concert with coronagraph masks to control the wavefront and minimize diffracted starlight in a region of the image known as the “dark zone” or “dark hole (DH).” The DMs must have a high number of actuators (50 to 96 across) to allow for DHs that are large enough to image a range of desired exoplanet separations. In addition, the surfaces of the DMs must be controlled at the picometer level to enable the required contrast. Any defect in the mechanical structure of the DMs or electronic system could significantly impact the scientific potential of the mission. Thus NASA’s Exoplanet Exploration Program procured two 50 × 50 microelectromechanical DMs manufactured by Boston Micromachines Corporation to test their robustness to the vibrational environment that the DMs will be exposed to during launch. The DMs were subjected to a battery of functional and high-contrast imaging tests before and after exposure to flight-like random vibrations. The DMs did not show any significant functional nor performance degradation at 10 − 8 contrast levels.

Far ultraviolet mirrors for aurora imaging: design and fabrication.

The emission lines of 140-180nm are auroral bands of N 2 Lyman-Birge-Hopfield, and they have been imaging targets of many satellites that need reflective mirrors. To obtain good imaging quality, the mirrors also should have excellent out-of-band reflection suppression as well as high reflectance at working wavelengths. We designed and fabricated non-periodic multilayer L a F 3/M g F 2 mirrors with working wave bands of 140-160nm and 160-180nm, respectively. We used a match design method and deep search method to design the multilayer. Our work has been utilized in the new wide-field auroral imager of China, and the application of these notch mirrors with excellent out-of-band suppression reduces the utilization of corresponding transmissive filters in the optical system of space payload. Furthermore, our work provides new routes for the design of other reflective mirrors in the far ultraviolet region.

Development of multi-band and high-speed visible endoscope diagnostic on EAST with catadioptric optics

A new multi-band and high-speed endoscope diagnostic for the observation of visible light has been successfully developed on the Experimental Advanced Superconducting Tokamak. The mirror with an aperture is designed at the head of the optical system. Based on two dichroic mirrors, the system is divided into three imaging mirror groups with different bands, i.e. B (380–500 nm), G (500–580 nm) and R (580–750 nm) bands, and its focal length is 16 mm with a relatively large aperture of D/f = 1:4. The spatial resolution is less than 5 mm near the object distance of with the camera NAC ACS-1 M60. This optical system will be used to contrastively study both the spatial distribution and time evolution of different impurities in the same field of view. The experimental results confirm that it can be applied to the recognition of plasma boundary and related physical research.

Design of MWIR hyperspectral imagers based on acousto-optic tunable filters

Thermal infrared hyperspectral imagers have significant application values in the field of space optics. However, it is very difficult to obtain the spectral data cube and temperature field information of fast moving targets in a short time. An acousto-optic tunable filter (AOTF) has many advantages, such as fast tuning speed, light weight and no moving parts. It can obtain thermal infrared hyperspectral images of a fast-moving target in a very short integration time. In this paper, a collimating optical system is obtained by splicing optical path a telescope and a secondary imaging mirror. A middle wavelength infrared (MWIR) AOTF is introduced into the collimating optical system to modulate the spectrum and obtain thermal infrared hyperspectral images. The system with a focal length of 18 mm, a relative aperture of 1/2, a field of view (FOV) of 8°, a spectral resolution of 30.08 nm at 4 µm and a 100% cold diaphragm efficiency achieving is taken as a design example. The design results show that the modulation transfer function (MTF) of the system is greater than 0.6 at the Nyquist space frequency of 17 lp/mm, and the imaging quality is close to the diffraction limit. Finally, we develop a prototype of MWIR hyperspectral imager based on AOTFs and do experiments to prove its feasibility.

Imaging Simulation Method for Novel Rotating Synthetic Aperture System Based on Conditional Convolutional Neural Network

The novel rotating synthetic aperture (RSA) is a new optical imaging system that uses the method of rotating the rectangular primary mirror for dynamic imaging. It has the advantage of being lightweight, with no need for splicing and real-time surface shape maintenance on orbit. The novel imaging method leads to complex image quality degradation characteristics. Therefore, it is vital to use the image quality improvement method to restore and improve the image quality to meet the application requirements. For the RSA system, a new system that has not been applied in orbit, it is difficult to construct suitable large datasets. Therefore, it is necessary to study and establish the dynamic imaging characteristic model of the RSA system, and on this basis provide data support for the corresponding image super resolution and restoration method through simulation. In this paper, we first analyze the imaging characteristics and mathematically model the rectangular rotary pupil of the RSA system. On this basis, combined with the analysis of the physical interpretation of the blur kernel, we find that the optimal blur kernel is not the point spread function (PSF) of the imaging system. Therefore, the simulation method of convolving the input image directly with the PSF is flawed. Furthermore, the weights of a convolutional neural network (CNN) are the same for each input. This means that the normal convolutional layer is not only difficult to accurately estimate the time-varying blur kernel, but also difficult to adapt to the change in the length–width ratio of the primary mirror. To that end, we propose a blur kernel estimation conditional convolutional neural network (CCNN) that is equivalent to multiple normal CNNs. We extend the CNN to a conditional model by taking an encoding as an additional input and using conditionally parameterized convolutions instead of normal convolutions. The CCNN can simulate the imaging characteristics of the rectangular pupil with different length–width ratios and different rotation angles in a controllable manner. The results of semi-physical experiments show that the proposed simulation method achieves a satisfactory performance, which can provide data and theoretical support for the image restoration and super-resolution method of the RSA system.

Imaging model of the off-axis non-central spherical mirrored stereo vision sensor

In order to expand the field-of-view of mirrored stereo vision sensor, replacing planar mirrors with spherical mirrors has become an effective solution. However, using the convex spherical mirror for reflection imaging will lead to image curl deformation. In this paper, to address the limitation of the structure, we proposed a novel imaging model of the off-axis non-central spherical mirrored vision sensor. This model can realize the points mapping from three-dimensional space to the two-dimensional image plane through the spherical mirror reflection. We derived the imaging model based on the optical path tracing principle, and established the corresponding relationship between spherical and planar mirrored virtual points to realize spherical distortion rectification. The imaging effect of the model was verified, and noises were added for error analysis. The research quantitatively describes the mapping relationship and expands the field-of-view, providing a new idea for establishing the measurement model of mirrored stereo vision sensor.

Dual-resonant scanning multiphoton microscope with ultrasound lens and resonant mirror for rapid volumetric imaging

A dual-resonant scanning multiphoton (DRSM) microscope incorporating a tunable acoustic gradient index of refraction lens with a resonant mirror is developed for high-speed volumetric imaging. In the proposed microscope, the pulse train signal of a femtosecond laser is used to trigger an embedded field programmable gate array to sample the multiphoton excited fluorescence signal at the rate of one pixel per laser pulse. It is shown that a frame rate of around 8000 Hz can be obtained in the x–z plane for an image region with a size of 256 × 80 pixels. Moreover, a volumetric imaging rate of over 30 Hz can be obtained for a large image volume of 343 × 343 × 120 μm3 with an image size of 256 × 256 × 80 voxels. Moreover, a volumetric imaging rate of over 30 Hz can be obtained for a large image volume of 256 × 256 × 80 voxels, which represents 343 × 343 × 120 μm3 in field-of-view. The rapid volumetric imaging rate eliminates the aliasing effect for observed temporal frequencies lower than 15 Hz. The practical feasibility of the DRSM microscope is demonstrated by observing the mushroom bodies of a drosophila brain and performing 3D dynamic observations of moving 10-μm fluorescent beads.

Perspective clipping and fast rendering of light field images for holographic stereograms using RGBD data.

The production of holographic stereogram (HS) requires a huge amount of light field data. How to efficiently clip and render these image data remains a challenge in the field. This work focuses on the perspective clipping and fast rendering algorithm for light field images using RGBD data without explicit 3D reconstruction. The RGBD data is expanded to RGBDθ data by introducing a light cone for each point, which gives a new degree of freedom for light field image rendering. Using the light cone and perspective coherence, the visibility of 3D image points can be clipped programmatically. Optical imaging effects including mirror imaging and half mirror imaging effects of 3D images can also be rendered with the help of light cones during the light field rendering process. The perspective coherence is also used to accelerate the rendering, which has been shown to be on average 168% faster than traditional DIBR algorithms. A homemade holographic printing system was developed to make the HSs using the rendered light field images. The vivid 3D effects of the HS have validated the effectiveness of the proposed method. It can also be used in holographic dynamic 3D display, augmented reality, virtual reality, and other fields.

A potential way of detecting circulating tumor cells via laser scanning

Circulating tumor cells (CTCs) are generally considered as seeds for metastases, serving as a clinical biomarker for cancer diagnosis. Due to their low content in the blood, detection of CTCs normally requires an antibody-mediated biological enrichment process, which is rather complicated and time-consuming. In this work, we demonstrate a potential, fast, and cost-effective way of detecting CTCs. That is, CTCs are physically isolated by size using filtration and imaged optically via laser scanning. The filtered and stained blood sample, which is placed on a moving and spinning stage, is illuminated by a 405 nm diode laser. Scattered optical signals are collected by using an ellipsoidal reflective mirror for dark-field imaging. Then CTCs in dark-field images are screened and counted by the clustering algorithm. Our method possesses the advantages of antibody-free and low cost, holding great promise for early diagnosis and prognosis prediction of cancer patients.

Spectral reduction model for an echelle spectrometer based on a digital micromirror device and photomultiplier.

For an echelle spectrometer based on a digital micromirror device and photomultiplier, a lens group is placed in front of the digital micromirror device and used to converge the two-dimensional spectrum and correct aberration. The matching spectral reduction model is deduced by establishing the mathematical models of the imaging mirror and lens group. According to the known wavelength and corresponding coordinates, the refractive index of the prism is fitted, and the spectrum is divided into three parts to calibrate. Experimental results of a mercury lamp show that the maximum deviations of x and y coordinates are 3.4 pixels and 2.3 pixels in the corrected spectral reduction model. The deviations are in the range of the errors permitted, which means the extraction of order and wavelength satisfy the requirement of the spectrometer.

Study of the Effect of Intake Layout on the Wavefront in a Beam Expanding System of a Telescope

The main disadvantage of windowless beam expansion systems is that they cannot achieve a good sealing effect. Turbulence and impurities in the environment can easily affect the imaging and primary mirror. Thus, in this study, a matrix of small holes was introduced for inflation to form a stable and smooth flow inside the system to avoid these disadvantages. In order to study the layout of the matrix, the flow state of the model was analysed, and the Lorentz–Lorenz formula and Barron gradient operator were used for ray tracing. Simulation results show that when the matrix of small holes is arranged in 16 rows with 360 holes in each row, inflation has a lesser effect on the wavefront aberration of the system. Moreover, the root mean square (RMS) of wavefront aberration was only 0.077 μm, which was superior to the other layouts considered. Experimental results show that the RMS was 0.08 μm in this state, which is consistent with the analysis. This indicates that this analysis method can meet actual work needs. The calculation methods and calculation results have high reliability and, thus, can be also used in similar situations.

Comparing focal plane wavefront control techniques: Numerical simulations and laboratory experiments

Context. Fewer than 1% of all exoplanets detected to date have been characterized on the basis of spectroscopic observations of their atmosphere. Unlike indirect methods, high-contrast imaging offers access to atmospheric signatures by separating the light of a faint off-axis source from that of its parent star. Forthcoming space facilities, such as WFIRST/LUVOIR/HabEX, are expected to use coronagraphic instruments capable of imaging and spectroscopy in order to understand the physical properties of remote worlds. The primary technological challenge that drives the design of these instruments involves the precision control of wavefront phase and amplitude errors. To suppress the stellar intensity to acceptable levels, it is necessary to reduce phase aberrations to less than several picometers across the pupil of the telescope. Aims. Several focal plane wavefront sensing and control techniques have been proposed and demonstrated in laboratory to achieve the required accuracy. However, these techniques have never been tested and compared under the same laboratory conditions. This paper compares two of these techniques in a closed loop in visible light: the pair-wise (PW) associated with electric field conjugation (EFC) and self-coherent camera (SCC). Methods. We first ran numerical simulations to optimize PW wavefront sensing and to predict the performance of a coronagraphic instrument with PW associated to EFC wavefront control, assuming modeling errors for both PW and EFC. Then we implemented the techniques on a laboratory testbed. We introduced known aberrations into the system and compared the wavefront sensing using both PW and SCC. The speckle intensity in the coronagraphic image was then minimized using PW+EFC and SCC independently. Results. We demonstrate that both techniques – SCC, based on spatial modulation of the speckle intensity using an empirical model of the instrument, and PW, based on temporal modulation using a synthetic model – can estimate the wavefront errors with the same precision. We also demonstrate that both SCC and PW+EFC can generate a dark hole in space-like conditions in a few iterations. Both techniques reach the current limitation of our laboratory bench and provide coronagraphic contrast levels of ∼5 × 10−9 in a narrow spectral band (< 0.25% bandwidth). Conclusions. Our results indicate that both techniques are mature enough to be implemented in future space telescopes equipped with deformable mirrors for high-contrast imaging of exoplanets.

Design and status of the Micro-X microcalorimeter sounding rocket

The Micro-X High Resolution Microcalorimeter X-Ray Imaging Rocket is a sounding rocket mission that will observe Supernova Remnants and search for keV-scale sterile neutrino dark matter. Micro-X will combine the excellent energy resolution of Transition Edge Sensor microcalorimeters with the imaging capabilities of a conical imaging mirror to map extended and point X-ray sources with an unprecedented combination of energy and spatial resolution. The payload has been designed to operate in the challenging conditions of a sounding rocket flight and to achieve sensitive results, in a single five-minute exposure, for each of these science goals. Micro-X’s unique design considerations are presented here, along with the status of the instrument and projections for the upcoming flights. The first Micro-X flight in 2018 will observe the Puppis A supernova remnant, where it will attain nearly 13,000 counts in the 300 s exposure. The second Micro-X flight will observe the Galactic Center to search for keV-scale dark matter and explore the nature of the unexplained 3.5 keV line observed by X-ray satellites.

Full-field X-ray fluorescence microscope based on total-reflection advanced Kirkpatrick-Baez mirror optics.

A novel full-field X-ray fluorescence microscope based on total-reflection advanced Kirkpatrick-Baez mirror optics was developed. The total-reflection imaging mirror optics arrangement, with four reflections, has the advantage of being able to function both as a powerful low-pass energy filter, completely rejecting incident excitation X-rays, and as an achromatic optical imaging system. Isolated X-ray fluorescence signals can be imaged, avoiding imaging-detector saturation, with low background noise. A prototype fluorescence microscope constructed at SPring-8 demonstrated the capability to simultaneously image elemental distributions using various X-ray fluorescence signals (Ni, Cu, Zn, Ge, and Bi). A half-period spatial resolution of ~0.5-1 µm (1000-500 LP/mm) was achieved, owing to the achromaticity of the imaging mirrors and the photon-counting scheme of the CCD camera used for fluorescence detection.

MEMS Deformable Mirrors for Space-Based High-Contrast Imaging

Micro-Electro-Mechanical Systems (MEMS) Deformable Mirrors (DMs) enable precise wavefront control for optical systems. This technology can be used to meet the extreme wavefront control requirements for high contrast imaging of exoplanets with coronagraph instruments. MEMS DM technology is being demonstrated and developed in preparation for future exoplanet high contrast imaging space telescopes, including the Wide Field Infrared Survey Telescope (WFIRST) mission which supported the development of a 2040 actuator MEMS DM. In this paper, we discuss ground testing results and several projects which demonstrate the operation of MEMS DMs in the space environment. The missions include the Planet Imaging Concept Testbed Using a Recoverable Experiment (PICTURE) sounding rocket (launched 2011), the Planet Imaging Coronagraphic Technology Using a Reconfigurable Experimental Base (PICTURE-B) sounding rocket (launched 2015), the Planetary Imaging Concept Testbed Using a Recoverable Experiment - Coronagraph (PICTURE-C) high altitude balloon (expected launch 2019), the High Contrast Imaging Balloon System (HiCIBaS) high altitude balloon (launched 2018), and the Deformable Mirror Demonstration Mission (DeMi) CubeSat mission (expected launch late 2019). We summarize results from the previously flown missions and objectives for the missions that are next on the pad. PICTURE had technical difficulties with the sounding rocket telemetry system. PICTURE-B demonstrated functionality at >100 km altitude after the payload experienced 12-g RMS (Vehicle Level 2) test and sounding rocket launch loads. The PICTURE-C balloon aims to demonstrate contrast using a vector vortex coronagraph, image plane wavefront sensor, and a 952 actuator MEMS DM. The HiClBaS flight experienced a DM cabling issue, but the 37-segment hexagonal piston-tip-tilt DM is operational post-flight. The DeMi mission aims to demonstrate wavefront control to a precision of less than 100 nm RMS in space with a 140 actuator MEMS DM.

Thermal oxide patterning method for compensating coating stress in silicon substrates.

We introduce a novel method for correcting distortion in thin silicon substrates caused by coating stress. Thin substrates, such as lightweight mirrors for x-ray or optical imaging, and semiconductor wafers or flat panel substrates, are easily distorted by stress in thin film coatings. We report a new method for correcting stress-induced distortion in flat silicon substrates which utilizes a micro-patterned silicon oxide layer on the back side of the substrate. Due to the excellent lithographic precision of the patterning process, we demonstrate stress compensation control to a precision of ~0.2%. The proposed process is simple and inexpensive due to the relatively large pattern features on the photomask. The correction process has been tested on flat silicon wafers that were distorted by 30 nm-thick compressively-stressed coatings of chromium, achieving RMS surface height and slope error reductions of a factor of 68 and 50, respectively.

High-Resolution Full-Field X-ray Microscope for 20-keV X-rays with Multilayer Imaging Mirrors

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Demonstration of Focusing Wolter Mirrors for Neutron Phase and Magnetic Imaging

Image-forming focusing mirrors were employed to demonstrate their applicability to two different modalities of neutron imaging, phase imaging with a far-field interferometer, and magnetic-field imaging through the manipulation of the neutron beam polarization. For the magnetic imaging, the rotation of the neutron polarization in the magnetic field was measured by placing a solenoid at the focus of the mirrors. The beam was polarized upstream of the solenoid, while the spin analyzer was situated between the solenoid and the mirrors. Such a polarized neutron microscope provides a path toward considerably improved spatial resolution in neutron imaging of magnetic materials. For the phase imaging, we show that the focusing mirrors preserve the beam coherence and the path-length differences that give rise to the far-field moiré pattern. We demonstrated that the visibility of the moiré pattern is modified by small angle scattering from a highly porous foam. This experiment demonstrates the feasibility of using Wolter optics to significantly improve the spatial resolution of the far-field interferometer.

Developement of a gaze controller using triple automated mirrors for high-speed and high-resolution telescopic imaging

Developement of a gaze controller using triple automated mirrors for high-speed and high-resolution telescopic imaging

Characterization and modeling of Fabry–Perot ultrasound sensors with hard dielectric mirrors for photoacoustic imaging

A Fabry-Perot ultrasound sensor with nonhygroscopic dielectric mirrors made out of Ta2O5 and SiO2 for use in photoacoustic tomography is described. The sensor offers flat frequency response up to 36 MHz, low noise-equivalent pressure (70 Pa), and near-omnidirectional response up to 20 MHz as well as optical transparency for near-infrared illumination. A numerical model was developed to predict its frequency response, and the results were validated experimentally. An image of the human palm was acquired to demonstrate in vivo imaging capabilities.