The Wide Field Survey Telescope (WFST) is a dedicated photometric surveying facility equipped with a 2.5-meter diameter primary mirror, an active optics system, and a mosaic CCD camera with 0.765 gigapixels on the primary focal plane for high quality image capture over a 6.5-square-degree field of view. The mosaic CCD camera is the key device for high precision photometric and high frequency observation and the ‘eye’ of the telescope for deep survey with wide field. The focal plane consists of three kinds of CCD including scientific imaging sensors, wavefront sensors and guiding sensors. In the scientific imaging area, there are 9 back-illuminated full frame scientific CCDs –CCD290-99 from E2V company with pixels of 9K by 9K and pixel size of 10um, which is mosaicked by 3 by 3 with flatness of 20μm PV. The R&D of the camera including the high precision large-scale mosaicking of detectors, detectors’ cryocooling and vacuum sealing, readout and driving with low noise and low power, data acquisition, imaging control, data storage and distribution. Furthermore a camera control system (CCS) was developed at same time.
Space targets are mainly divided into spacecraft and space debris working in orbit. Important near-Earth objects can be accurately detected and tracked, and their threats to the planet security can be predicted by monitoring their orbit and volume parameters. Optical telescope is one of the most important astronomical telescopes, science-grade image sensors were used to collects optical information about stars. In modern times, with the rapid development of semiconductor technology, CMOS (Complementary Metal Oxide Semiconductor) image sensor has high data transmission rate and high integration, CMOS image sensors have become the main optical imaging image sensors used in astronomical telescopes. In this presentation, the imaging principle and firmware design of a scientific CMOS camera named PX400 are introduced and tested. The PX400 uses a scientific image sensor called GSENSE400BSI (hereinafter referred to as GS400) produced by GPIXEL, which has a high data readout rate and a variety of operating modes.
For giant telescopes using mosaic technology, the flatness of the Mosaic Focal Plane Arrays (MFPAs) is an important determinant of their image quality. In order to measure the flatness through the flat dewar window of cryogenic chamber, a measurement system called differential triangulation measurement system (DTMS) is designed. The DTMS mainly includes four parts: a displacement platform, two laser triangulation sensors, the connecting structure and an optical flat as reference surface. To operate DTMS, an operational software system is developed. The operational software consists of the control and processing software developed by QT, the displacement platform control module realized by PLC and the sensor control module. Through the operational software, users can easily operate the DTMS to scan the MFPAs and calculate the flatness. As for data processing, it is implemented based on the Progressive Sample Consensus (PROSAC) algorithm which can reduce the effect of anomalous data points on flatness calculation. The DTMS and its operational software have been used to measure the flatness of the Wide Field Survey Telescope (WFST).
In low-light-level detection, glow and hot pixels in some imaging sensors become visible due to long-exposure time, leading to image quality degradation. To solve the problem of glow and hot pixels in a single image, an improved extraction algorithm based on the idea of robust principal component analysis is proposed to remove them. The image is divided into three terms in our algorithm: a low-rank matrix (image without glow and hot pixels), an extremely sparse matrix (hot pixels), and a sparse and spatially smooth matrix (glow). Specifically, the total variation norm and ℓ1-norm are exploited to describe the property of glow. Moreover, a top-hat filter and a boundary-searching method are introduced into the soft threshold operator to improve accuracy. The superiority of the proposed approach is demonstrated with evaluations on simulated datasets, quantitative metrics, and real data.
KEYWORDS: Charge-coupled devices, Cameras, Control systems design, Head, Control systems, Resistance, Electronics, CCD cameras, Telescopes, Imaging systems
The Wide Field Survey Telescope (WFST) is a large optical image survey telescope. In order to verify some technologies used in development of camera of WFST, a scientific imaging prototype camera using CCD290-99 is designed. CCD has to work at a low temperature to reduce its dark current. Aiming at the low temperature request of the scientific CCD detector, a high precision temperature control system is designed. The system includes a refrigerator, cooling strips, temperature control electronics and its cooling scheme. Based on the thermal simulation, the temperature control system with high stability is designed and established. The test shows the system can cool the CCD to -100℃ in 3 hours and the temperature stability is less than 0.1℃, which meets requirements of the WFST camera.
With the development of astronomy, more requirements of low noise and high frame rate are put forward for observation instruments. With the advancement of CMOS technology, scientific CMOS camera can provide excellent quantum efficiency, noise performance, and higher frame rate compared to CCD camera. Based on this, we have designed a scientific CMOS camera with 2K*2K pixels and 11um pixel size. The frame rate of the camera could reach 48fps, with very low readout noise. The structural design, electronic design, cooling method and data processing of the camera will be introduced in this paper.
The wide field survey telescope (WFST) is a new generation survey telescope that is being built in China. Its optical design is a primary-focus system, and its camera is a mosaic charge-coupled device (CCD) camera composed of nine 9 K × 9 K CCD290-99 chips for scientific imaging. A verification platform to test the CCD290-99 chips is designed. The test platform includes a light source system, CCD controller, vacuum Dewar, and refrigerator for cooling the CCD. The CCD controller is a prototype design of the WFST camera that has a high performance, including low readout noise, flexible readout rate configuration, low power dissipation, etc. The digital double correlated sample method is used for video sampling of the CCD’s 16 channels. The specifications of the CCD detector system using a CCD290, such as gain, noise linearity, and crosstalk, are tested using this platform. The test results show that the CCD test platform meets the requirement of the CCD test and the design of CCD controller meets the scientific imaging requirements for the WFST camera.
KEYWORDS: Black bodies, Calibration, Near infrared, Infrared imaging, Infrared radiation, Temperature sensors, Infrared detectors, Control systems, Coating, Temperature metrology
The Antarctic Plateau is one of the best places for infrared and submillimeter observations in the world, which has the advantages of high altitude, low water vapor and low atmospheric thermal radiation. It is indispensable for the design of instruments to know the environment of the observatory site in advance, especially the infrared sky background brightness. It determines the ultimate magnitude of infrared observation of the equipment, which is an important reference to evaluate whether a candidate site is suitable for constructing corresponding equipment. We have designed a NIR sky brightness monitor (NISBM) based on InGaAs photodiode, which is used to monitor the J, H and Ks bands of sky background brightness at the Dome A. In the Ks band the signal is sensitive to thermal radiation and temperature fluctuations. So, it needs to be calibrated in real time by a surface source blackbody. According to this requirement, we have designed a surface source blackbody that has the property of low temperature resistance, high emissivity, and high temperature uniformity. The device has a compact structure. The control system and the radiation surface are packaged in the same square house, which is suitable for outfield installation and calibration with low ambient temperature.
Many specs of scientific CMOS cameras characterize the performance of camera, which can help developers analyze the quality of cameras. In order to test the performance of sCMOS cameras, we designed and built a test platform for performance test of sCMOS cameras which has been developed. The test platform includes a group of test devices and a automatic test software system. The software system is designed based on remote controllable WEB technology and EPICS-based real-time control framework, making the test platform flexible and convenient. According to the features of sCMOS camera, in the test platform four test procedures are designed to test various specs of sCMOS camera, including FPN test, dark current test, gain, noise, linear error, full well capacity test and dead pixel test. Users can perform automatic tests on camera through web UI, including the control of test platform device, data acquisition and data processing. At the same time, the test platform also provides users with various functions such as test data query and test report generation.
Scientific Complementary Metal-Oxide Semiconductor (sCMOS) image sensor has higher readout speed, higher resolution, lower readout noise than traditional Charged Coupled Device (CCDs). Since the orbital debris observation has the demand for high speed imaging system, we designed and built a sCMOS camera, and developed the corresponding operational software system. The operational software contains three lays: a software development kit (SDK), Common Language Runtime(CLR) library and an operational software with a Graphic User Interface (GUI) named PXViewer. Each of them were tested and benchmarked. Several data acquisition modes including photo, timer, continuously capture and video are implemented for different observation scenarios. Users can get fully control and operation of the sCMOS camera through the software system, including cooling, data acquisition and configuration. During the benchmark, the sCMOS camera is able to capture image of 4128*4096 pixels at 7.8 frame per second (fps), and 2064*2048 pixels at 30 fps.
With the increase of human activities in space, a large number of space artifacts have been generated around the Earth which called Near Earth Objects (NEO), most of which are space debris. CMOS image sensor can achieve very high frame rate by electronical shutter and suitable for NEO observation with its fast moving. For space objects observation, key technologies of a large-format and high-rate scientific CMOS camera were studied, including low-noise readout and low-interference refrigeration technology, real-time processing algorithm, high-speed data transmission technology, system integration technology and high precision timing technology , etc. A 4K*4K pixel scientific CMOS camera is introduced in this paper with 24fps rate in full frame mode and high timing accuracy of exposure synchronization with 10ns, which has great advantages for the initial orbit positioning of the space objects. The overall size of the camera is 143mm * 160mm * 168mm. The readout noise of the camera is about 4.4e-. At present, the camera has been installed and running at Xinglong Observatory.
The infrared sky brightness level is an important parameter for infrared astronomical observation from the ground. It is necessary to obtain the infrared sky brightness level at an observatory site to evaluate the feasibility of infrared telescopes and instruments. In order to evaluate the possibility of developing infrared astronomical observations at several sites in China, the design of a continuous-scanning near-infrared sky brightness monitor (CNISBM), measuring 2.5 to 5 μm infrared sky brightness based on an InSb detector and a linear variable filter, is proposed. The optics and the detector were put in a vacuumed cryogenic dewar to reduce the background emission. The CNISBM has been tested by measuring the flux intensity of the observing window in the L-band. The results show that the sensitivity of CNISBM satisfies the requirements of the observations of 2.5- to 5-μm near-infrared sky brightness.
The 2.5~5um infrared band is an important waveband in infrared astronomy research. Infrared sky brightness monitoring is an important part of ground-based infrared astronomical observations. The measurement of infrared sky brightness and the characteristics of the infrared observation conditions of an area, especially the average intensity and variation parameters of infrared radiation will provide an important reference for future design of infrared telescopes and other observation instruments. We designed a sky brightness spectrograph for 2.5-5um continuous infrared spectroscopy using an InSb detector and conduct a test measurement of the sky brightness radiation intensity with L band whose center wavelength is 3.77um.
The Antarctica Plateau with high altitude, low water vapor and low thermal emission from the atmosphere is known as one of the best sites on the earth for conducting astronomical observations from the near infrared to the sub-millimeter. Many optical astronomical telescopes are proposed by Chinese astronomical society at present, such as Kunlun Dark Universe Survey Telescope (KDUST), 6.5-meter optical telescopes and 12-meter optical and infrared telescopes. Accurate estimation of the sky background brightness of proposed sites provides the scientific basis for instruments design and observatory site selection. Based on this requirement, a near-infrared sky brightness monitor (NISBM) based on InGaAs photoelectric diode is designed by using the method of chopper modulation and digital lock-in amplifier in the near infrared band of J, H, Ks. The adaptability of the monitor under extremely low temperature conditions in Antarctica is promoted by taking advantage of PID heating and fault detection system. Considering the weak signal of Ks band in Antarctica, a surface blackbody is equipped for real-time calibration. For the adverse circumstances to human, an EPICS and Web based Remote Control Software is implemented for unattended operation. The NISBM has been successfully installed in Dome A, Antarctica on January 2019.
KEYWORDS: Sensors, Observatories, Calibration, Black bodies, Near infrared, Indium gallium arsenide, Temperature metrology, Signal to noise ratio, Infrared radiation, Electronics
The Ngari (Ali) observatory is located in Ngari, Tibet, a region known as “the roof of the roof of the world.” The observatory benefits from abundant photometric nights, low perceptible water vapor, high transmittance, and good seeing. Due to these advantages, it promises to be one of the best locations in the world at which to make infrared and submillimeter observations. However, no data on the sky background radiation at this location are available, impacting the planning of future facilities at the observatory. To remedy this deficiency, a near-infrared sky brightness monitor (NISBM) has been designed to obtain data in the J, H, and Ks bands. This monitor is based on an InGaAs photoelectric diode and uses chopper modulation and digital lock-in amplifier processing, which considerably enhance its signal-to-noise ratio, detectivity, and data acquisition speed. An independent device has been designed for each band (J, H, and Ks) and calibrated in the laboratory. The NISBM was installed at the Ngari observatory in July 2017 and has obtained the first NIR sky brightness data for that location.
Tibet is known as the third pole of the earth. The Ngari (Ali) observatory in Tibet is a good site, and promising to be one of the best place for infrared and submillimeter observations in the world. However, there is no data available for sky background brightness in such place. In the near infrared band of J, H, Ks, a NIR sky brightness monitor (NISBM) is designed based on InGaAs photoelectric diode. By using the method of chopper modulation and digital lock-in amplifier processing, the SNR (Signal Noise Ratio), detectivity and the data acquisition speed of the device is greatly improved. The NISBM has been installed in Ngari observatory in July of 2017 and obtained the first data of NIR sky brightness at Ngari observatory.
Infrared sky background level is an important parameter of grounded infrared astronomy observations, which should be firstly measured in a good infrared observatory site, and only the site with low background level is suitable for infrared observations. Infrared sky background level can provide background data for the design of related infrared instruments. However, there is no such data available for major sites in China. Based on the requirement, In order to supplement the current site survey data and guide the design of future infrared instruments, a multiband near-infrared sky brightness monitor (MNISBM) based on an InSb sensor is designed in this paper. The MNISBM consists of optical system, mechanical structure and control system, detector and cooler, high gain readout electronic system, operational software. It is completed and carried out an experimental measurement in the laboratory. The result shows that the sensitivity of the MNISBM meets the requirements of the measurement of near-infrared sky background level.
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