The camera is an important part of the optical telescope observing system, and the performance of the camera is an important factor affecting the quality and efficiency of astronomical observations. EMCCD can achieve lower noise and higher detection sensitivity by charge multiplication techniques, and can be used to realize direct observations of very faint and weak targets, and relative to the traditional CCD/CMOS detectors, the noise level can be reduced by an order of magnitude to reach the Sub-electron level. However, facing the need for calibration of ultra-low noise at the sub-electron level, it is difficult to satisfy the currently available equipment and methods. Therefore, the study of EMCCD readout noise calibration method under high gain is of great significance for the theoretical study of EMCCD and the design of low-noise electronics. In this paper, we propose a calibration system of "cascaded integrating sphere + parallel light pipe" local illumination in dark room environment, through which we can obtain the light source under ultra-low brightness, which solves the problem of difficult to obtain the point light source, and the method of local illumination can avoid the fatigue attenuation problem under the high-fold gain, and we also refine the noise model, and propose a "gain-noise" model, which can be used to calibrate the EMCCD readout noise. The noise model is refined and the "gain-noise" fitting method is proposed, and finally the readout noise test at high gain achieves a noise calibration result of about 0.8e@600x.
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.
The successful achievement of the scientific objectives of the Visible Telescope (VT) in the Space Multi-band Variable Object Monitor (SVOM) mission relies heavily on high-precision quantum efficiency calibration. The calibration process for the VT CCD presents a challenge due to the requirement for extremely low radiation levels given the long integration time of the CCD. To address the difficulty in accurately measuring such low radiance, a two-step calibration method is employed. This method involves the use of two photodiodes, one positioned at the CCD location and the other in an integrating sphere. In the first step, the proportional relationship between the measured illuminance values of the two photodiodes is calibrated under high illumination conditions. This step establishes a reliable reference for subsequent calibrations. In the second step, the CCD is calibrated using the integrating sphere photodiode under low illumination conditions. The measured illuminance is then converted to the CCD position. Experimental results have demonstrated the effectiveness of this two-step calibration method, achieving a quantum efficiency calibration uncertainty of 1.7%. This approach provides a reliable and accurate means of calibrating the quantum efficiency of the CCD in the VT instrument.
According to requirements, a co-aperture design has been performed for the visible light remote sensing camera and the synthetic aperture radar, allowing the remote sensing satellite to acquire both visible light and radar images simultaneously. The front system is a two-mirror, no-focus system with a primary mirror diameter of 3 meters, serving to compress the beam. To avoid obstruction, the primary mirror is placed off-axis. The visible light component consists of an off-axis three mirror system, with the entrance pupil aligned with the exit pupil of the front system. All three mirrors are secondary mirrors with quadratic surfaces. The primary mirror size is 500mm, and the system's focal length is 7.22m. The overall ground resolution of the system reaches sub-meter level, with a full field of view measuring 0.8° × 0.03°. Optical design software ZEMAX was employed to evaluate the imaging quality within the visible light wavelength range. The results indicate that the spot size of the system is smaller than 13μm within each field of view. At the Nyquist frequency, the modulation transfer function (MTF) values for each field of view exceed 0.4, approaching the diffraction limit, showcasing good imaging quality. This design enhances the satellite's adaptability and observational capabilities, reduces the overall size of the instrument, and saves on manufacturing and launch costs.
It is difficult for normal CCD or CMOS camera to obtain high quality images under extremely low-light conditions for example the new moon or the quarter moon because the photons arriving at the detector are so few that signal to noise ratio (SNR) is much lower than what is necessary to resolve finer details in the nighttime scenario. To solve this problem, the intensified CCD or CMOS camera is adopted and the few photons is amplified to improve the SNR a lot. However, the intensifier is mainly composed of the cathode, MCP (Micro-channel-plate) and fluorescent screen and this complex structure and the multiple photoelectric conversion during the photon amplification process will lead to a big equivalent pitch size, which degrades the spatial resolution. Therefore in this manuscript, by improving the classical iterative back projection (IBP) algorithm a super-resolution reconstruction algorithm is proposed. By fusing multiple quite noisy lowlight images having sub-pixel displacements between each other, both the spatial resolution and the SNR could be enhanced. In the in-lab experiments, the spatial resolution can be increased to nearly 1.8 times the original one. Besides that, the increment in SNR bigger than 6dB and 9dB could be obtained for the quarter moon and the new moon light condition respectively. The out-door experiments show the similar results and besides that by fusing sub-pixel shifted low-light images corresponding to different low-light conditions together, the reconstructed high-resolution images will have even better visual performance.
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 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 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 free-floating planets. The six 30 cm diameter transit telescopes will be equipped with a CMOS camera which consists of 4(2×2)9K×9K CMOS sensors. A prototype camera with a 8900×9120 pixel GSENSE 1081 BSI type CMOS sensor and temperature control is designed and developed for high precision photometry measurements. In this paper, details of this camera design and performance test results are reported.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) project is a dedicated satellite developed at the cooperation of China and France, aim to make prompt multi-band observations of Gamma-Ray Bursts (GRBs), the afterglows and other high-energy transient astronomical events. The Visible Telescope (VT) is one of the four payloads onboard the SVOM. VT is designed to observe the afterglows of GRBs both in the visible and near infrared bands simultaneously. The telescope can reach a limiting magnitude of +22.5Mv and provide the redshift indicators for high-Z (z<4) GRBs. VT is also designed to measure the Relative Performance Errors (RPEs) for the satellite attitude and orbit control system (AOCS), aiming to improve the pointing stability of the platform during observation. VT adopts a Ritchey-Chrétien (RC) catadioptric optical configuration with a 440mm aperture and uses the dichroic prism before the focal plane to split the incident light into blue (visible) and red (near infrared) band. Two Fine Guidance Sensor (FGS) CCDs are mounted beside the main CCD on the blue band focal plane of VT and provide sub-arcsecond pixel resolution. Fiber reinforced plastic (CFRP) composites is selected as the material of VT’s main structure to ensure enough stiffness and strength during launch. The electrical video processing circuit is carefully designed to make the readout noise below 6e-/pix (rms) in 100s exposure time. Active and passive thermal control are used together to ensure the optical performance and thermoelectric cooler (TEC) is adopted to control the main CCDs working temperature below -65°C to reduce the noise. This paper provides a comprehensive overview of the scientific requirements and the key instrument design aspects of optics, main structure, electrics, thermal control, performance test and validation results of VT.
In recent years, Low-Light-Level (LLL) remote -sensing camera has become a novel subject for the development of aerospace optical remote-sensing payloads. LLL remote sensing camera works in ultra-low light conditions, the image signal is very weak and requires image intensified technology to achieve. In order to make better use of LLL remote sensing data, it is necessary to establish the quantitative relationship between the amount of radiation received by the camera and the digital signal output to process the image. That's radiation calibration technology. Therefore, the radiation calibration of the LLL remote sensing camera is particularly important. In this article, first the requirements for calibration of LLL remote sensing cameras are analyzed in theory. After that, a radiation calibration scheme of the LLL remote sensing camera is put forward. Finally, the radiation calibration test is carried out, and the calibration data are analyzed. The results show that the calibration scheme of LLL remote sensing camera is reasonable and feasible.
With the continuous development of navigation technology, celestial navigation system has gradually become the essential of celestial navigation system with its advantages of autonomy is strong, good concealment and high precision. The key component of the astronomical navigation system is the all-day star sensor. With the improvement of navigation accuracy, higher requirements have been put forward for the design of all-day star sensor, which is required to be able to work normally within 24 hours. The temperature range of the star sensor working on the airborne platform is very wide, and the temperature has a great impact on the imaging performance of the star sensor, thus affecting the positioning accuracy of navigation. Therefore, it is of great practical significance to ensure that its optical performance does not change within such a wide range of temperature, so as to significantly improve its navigation and positioning accuracy.
With the continuous development of astronomy theory and space exploration technology, searching for extrasolar planets has become one of the most active research topics in astronomy. In recent decades, countries around the world have invested a lot of ground and space projects in this search field and obtained abundant results. Firstly, this paper summarizes the mainstream exoplanet detection methods such as radial velocity, transit and direct imaging method with the outline of the principles and features. Then, several instruments for obtaining the spectrum of exoplanets are introduced, focusing on the optical system parameters of telescopes and spectrometers. Finally, according to the comprehensive discussion above, the future development trend of exploration missions and instrument design in this field is predicted, and it is recognized that these survey missions for detecting and characterizing exoplanets are of great significance for searching biological signals outside the solar system.
KEYWORDS: Star sensors, Target detection, Stars, Sensors, Imaging systems, Signal to noise ratio, Stray light, Optical design, Signal detection, Mirrors
The star sensor is used to detect the position of the stars in space. By recognizing and analyzing star maps, satellites or spacecraft can automatically change the direction of movements to realize the navigation function. However, the strong background radiation in the sky during the day results in a low contrast of the star image. This brings great difficulties to star sensors that work on atmospheric platforms observing stars all the time. To overcome the adverse impacts of the stray lights from the sky during the whole day through the atmosphere, a catadioptric all-day star sensor optical system is presented. In comparison to Cassegrain System, the design has a smaller size of aperture of housing. Therefore, it has the advantage of superb suppression of the stray lights caused by external sky background radiation and other factors. By adopting a plane mirror to compress the light path, the size of the system is decreased, realizing a light and miniaturized design. Based on the analysis of the characteristics of sky background radiation and star radiation, the optical system parameters are selected. The system has a focal length of 800mm, an effective aperture of 70mm, and an instantaneous field of view of 2 °. Meanwhile, with a steering mirror, it can observe an area between 40° and 70° airspace at all day. Finally, the results of the analysis show that the optical system spot shape approaches to a circle in the wide spectrum of 800 nm ~ 1700 nm, and the energy of which is close to the Gaussian distribution and highly concentrated. The modulation transfer function curve is close to the diffraction limit with small chromatic aberration of magnification.
The number of space debris has been increasing dramatically in the last few years, and is expected to increase as much in the future. As the orbital debris population grows, the risk of collision between debris and other orbital objects also grows. Therefore, space debris detection is a particularly important task for space environment security, and then supports for space debris modeling, protection and mitigation. This paper aims to review space debris detection systematically and completely. Firstly, the research status of space debris detection at home and abroad is presented. Then, three kinds of optical observation methods of space debris are summarized. Finally, we propose a space-based detection scheme for space debris by photometric and polarimetric characteristics.
KEYWORDS: Turbulence, Star sensors, Modulation transfer functions, Signal to noise ratio, Atmospheric turbulence, Point spread functions, Stars, Electro optical modeling, Quantum efficiency, Atmospheric sensing
All-day star sensor makes it possible to observe stars in all-day time in the atmosphere. But the detecting performance is influenced by atmospheric turbulence. According to the characteristic of turbulence in long-exposure model, the modulation transfer function, point spread function and encircled power of the imaging system have been analyzed. Combined with typical star sensor optical system, the signal to noise ratio and the detectable stellar magnitude limit affected by turbulence have been calculated. The result shows the ratio of aperture diameter to atmospheric coherence length is main basis for the evaluation of the impact of turbulence. In condition of medium turbulence in day time, signal to noise ratio of the star sensor with diameter 120mm will drop about 4dB at most in typical work environment, and the detectable stellar limit will drop 1 magnitude.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.