Jiangpei Dou

Speckle Noise Subtraction and Suppression with Adaptive Optics Coronagraphic Imaging

Future ground-based direct imaging of exoplanets depends critically on high-contrast coronagraph and wave-front manipulation. A coronagraph is designed to remove most of the unaberrated starlight. Because of the wave-front error, which is inherit from the atmospheric turbulence from ground observations, a coronagraph cannot deliver its theoretical performance, and speckle noise will limit the high-contrast imaging performance. Recently, extreme adaptive optics, which can deliver an extremely high Strehl ratio, is being developed for such a challenging mission. In this publication, we show that barely taking a long-exposure image does not provide much gain for coronagraphic imaging with adaptive optics. We further discuss a speckle subtraction and suppression technique that fully takes advantage of the high contrast provided by the coronagraph, as well as the wave front corrected by the adaptive optics. This technique works well for coronagraphic imaging with conventional adaptive optics with a moderate Strehl ratio, as well as for extreme adaptive optics with a high Strehl ratio. We show how to substrate and suppress speckle noise efficiently up to the third order, which is critical for future ground-based high-contrast imaging. Numerical simulations are conducted to fully demonstrate this technique.

A Transmission-Filter Coronagraph: Design and Test

We propose a transmission-filter coronagraph for direct imaging of Jupiter-like exoplanets with ground-based telescopes. The coronagraph is based on a transmission filter that consists of finite number of transmission steps. A discrete optimization algorithm is proposed for the design of the transmission filter that is optimized for ground-based telescopes with central obstructions and spider structures. We discussed the algorithm that is applied for our coronagraph design. To demonstrate the performance of the coronagraph, a filter was manufactured and laboratory tests were conducted. The test results show that the coronagraph can achieve a high contrast of 10-6.5 at an inner working angle of 5λ/D, which indicates that our coronagraph can be immediately used for the direct imaging of Jupiter-like exoplanets with ground-based telescopes.

A HIGH-CONTRAST IMAGING ALGORITHM: OPTIMIZED IMAGE ROTATION AND SUBTRACTION

Image Rotation and Subtraction (IRS) is a high-contrast imaging technique which can be used to suppress the speckles noise and facilitate the direct detection of exoplanets. IRS is different from Angular Differential Imaging (ADI), in which it will subtract a copy of the image with 180 degrees rotated around its PSF center, rather than the subtraction of the median of all of the PSF images. Since the planet itself will be rotated to the other side of the PSF, IRS does not suffer from planet self-subtraction. In this paper, we have introduced an optimization algorithm to IRS (OIRS), which can provide an extra contrast gain at small angular separations. The performance of OIRS has been demonstrated with ADI data. We then made a comparison of the signal to noise ratio (S/N) achieved by algorithms of locally optimized combination of images (LOCI) and OIRS. Finally we found that OIRS algorithm can deliver a better S/N for small angular separations.

High-contrast coronagraph for ground-based imaging of Jupiter-like planets

We propose a high-contrast coronagraph for direct imaging of young Jupiter-like planets orbiting nearby bright stars. The coronagraph employs a steptransmission filter in which the intensity is apodized with a finite number of steps with identical transmission in each step. It should be installed on a large ground-based telescope equipped with a state-of-the-art adaptive optics system. In this case, contrast ratios around 10 - 6 should be accessible within 0.1 arcsec of the central star. In recent progress, a coronagraph with a circular apodizing filter has been developed, which can be used for a ground-based telescope with a central obstruction and spider structure. It is shown that ground-based direct imaging of Jupiter-like planets is promising with current technology.

Focal plane wave-front sensing algorithm for high-contrast imaging

High-contrast imaging provided by a coronagraph is critical for the direction imaging of the Earth-like planet orbiting its bright parent star. A major limitation for such direct imaging is the speckle noise that is induced from the wave-front error of an optical system. We derive an algorithm for the wave-front measurement directly from 3 focal plane images. The 3 images are achieved through a deformable mirror to provide specific phases for the optics system. We introduce an extra amplitude modulation on one deformable mirror configuration to create an uncorrelated wave-front, which is a critical procedure for wave-front sensing. The simulation shows that the reconstructed wave-front is consistent with the original wave-front theoretically, which indicates that such an algorithm is a promising technique for the wave-front measurement for the high-contrast imaging.

A high-contrast imaging polarimeter with a stepped-transmission filter based coronagraph

The light reflected from planets is polarized mainly due to Rayleigh scattering, but starlight is normally unpolarized. Thus it provides an approach to enhance the imaging contrast by inducing the imaging polarimetry technique. In this paper, we propose a high-contrast imaging polarimeter that is optimized for the direct imaging of exoplanets, combined with our recently developed stepped-transmission filter based coronagraph. Here we present the design and calibration method of the polarimetry system and the associated test of its high-contrast performance. In this polarimetry system, two liquid crystal variable retarders（LCVRs） act as a polarization modulator, which can extract the polarized signal. We show that our polarimeter can achieve a measurement accuracy of about 0.2% at a visible wavelength（632.8 nm）with linearly polarized light. Finally, the whole system demonstrates that a contrast of 10-9 at 5λ/D is achievable, which can be used for direct imaging of Jupiter-like planets with a space telescope.

The first portable solar and stellar adaptive optics

We have developed a portable solar and stellar adaptive optics (PSSAO) system, which is optimized for solar and stellar high-resolution imaging in the near infrared wavelength range. Our PSSAO features compact physical size, low cost and high performance. The AO software is based on LabVIEW programing and the mechanical and optical components are based on off-the-shelf commercial components, which make a high quality, duplicable and rapid developed AO system possible. In addition, our AO software is flexible, and can be used with different telescopes with or without central obstruction. We discuss our portable AO design philosophy, and present our recent on-site observation results. According to our knowledge, this is the first portable adaptive optics in the world that is able to work for solar and stellar high-resolution imaging with good performances.

Laboratory experiment of a coronagraph based on step-transmission filters

This paper presents the first results of a step-transmission-filter based coronagraph in the visible wavelengths. The primary goal of this work is to demonstrate the feasibility of the coronagraph that employs step-transmission filters, with a required contrast in the order of better than 10-5 at an angular distance larger than 4λ/D. Two 13-step-transmission filters were manufactured with 5% transmission accuracy. The precision of the transmitted wave distortion and the coating surface quality were not strictly controlled at this time. Although in perfect case the coronagraph can achieve theoretical contrast of 10-10, it only delivers 10-5 contrast because of the transmission error, poor surface quality and wave-front aberration stated above, which is in our estimation. Based on current techniques, step-transmission filters with better coating surface quality and high-precision transmission can be made. As a follow-up effort, high-quality step-transmission filters are being manufactured, which should deliver better performance. The step-transmission-filter based coronagraph has the potential applications for future high-contrast direct imaging of earth-like planets.

Design and experimental test of an optical vortex coronagraph

Using an optical vortex coronagraph (OVC) is one of the most promising techniques for directly imaging exoplanets because of its small inner working angle and high throughput. This paper presents the design and laboratory demonstration performance of an OVC based on liquid crystal polymers (LCPs) at 633 nm and 1520 nm. The OVC can deliver good performance in laboratory tests and achieve a contrast of 10 −6 at an angular distance of 3λ/D, which can be implemented for imaging young giant exoplanets in combination with extreme adaptive optics.

Design and calibration of a high-sensitivity and high-accuracy polarimeter based on liquid crystal variable retarders

Polarimetry plays an important role in the measurement of solar magnetic fields. We developed a high-sensitivity and high-accuracy polarimeter (HHP) based on nematic liquid crystal variable retarders (LCVRs), which has a compact setup and no mechanical moving parts. The system design and calibration methods are discussed in detail. The azimuth error of the transmission axis of the polarizer as well as the fast axes of the two LCVRs and the quarter-wave plate were determined using dedicated procedures. Linearly and circularly polarized light were employed to evaluate the performance of the HHP. The experimental results indicate that a polarimetric sensitivity of better than 5.7 × 10 −3 can be achieved by using a single short-exposure image, while an accuracy on the order of 10 −5 can be reached by using a large number of short-exposure images. This makes the HHP a high-performance system that can be used with a ground-based solar telescope for high-precision solar magnetic field investigations.