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.
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).
Presently, the field of astronomy is transitioning towards the analysis of extensive datasets and all-encompassing sky surveys, moving away from the traditional study of individual celestial bodies. The imaging terminal of large field of view sky survey telescopes is typically equipped with a splicing camera consisting of CCD/COMS detectors. In order to enhance the signal-to-noise ratio of imaging and mitigate dark current, the splicing CCD imaging surface must be enclosed within a vacuum Dewar and cooled to temperatures ranging from -80 to -100 ℃. This poses a significant challenge in facilitating the electrical signal feeding and readout of multiple detectors within a low-temperature vacuum environment. This study employs glass sintering and bonding techniques in conjunction with thermodynamic simulation analysis to achieve the transmission of more than 1600 signals through a constrained flange surface. Environmental adaptability is maintained through the thermal matching of materials, and the introduction of a stress relief groove mitigates the impact of flange surface deformation on bonding positions under vacuum conditions. Following multiple rounds of temperature loop test, it was determined that the leakage rate of the vacuum feedthrough component can be controlled to ≤ 1E-9 Pa • m ³/s. The high-density signal vacuum penetration scheme proposed in this study holds important implications for the design and signal extraction processes of multi-detector splicing focal plane cameras.
The near-infrared spectrum encompasses three primary atmospheric transmission windows corresponding to the wavelengths J, H, and Ks. This paper introduces a practical automated remote-control system based on Web technology, which utilizes NIR sky brightness measurements for calibrating astronomical observational data. The system enables remote control, automated observation and data visualization. EPICS serves as the connection layer between the microcontroller and the server, implementing the underlying event loop. The web server leverages the Tornado framework to efficiently manage automated observational tasks and performs a real-time correction of the Ks device's detector response using a blackbody radiative source. The system provides a client interface constructed with the Vue framework and utilizing the Element UI component library, allowing users to remotely control instruments, monitor and configure observation tasks, as well as visualize data using the Plotly charting tool via a web browser.
The Wide Field Survey Telescope (WFST) is a 2.5m diameter telescope proposed by the University of Science and Technology of China and the Purple Mountain Observatory. The telescope is located at the summit of the Saishiteng Mountain near Lenghu City. The WFST equipped with a mosaic camera on the primary focus plane that includes 9 scientific imaging CCDs, 8 wave-front CCDs, and 4 guiding CCDs. The CCDs are placed in vacuum Dewar, and electronic signals are transmitted through the PCB boards and the vacuum Dewar connectors to the readout electronics outside Dewar. We provided a low-noise readout solution and a high-speed reliable data upload solution for the mosaic CCD camera, and evaluate and tests the performance.
To detect exoplanets and study their formation and evolution, several exoplanet space missions, such as Kepler, TESS, GAIA, and CHEOPS, have been successfully developed and fully operated in space. However, China has not yet had its own exoplanet space mission. The Earth 2.0 (ET) space mission is being developed in China aiming at detecting and characterizing exoplanets, especially extra-terrestrial like planets. ET will carry six transit telescopes pointing to the same sky region and a gravitational microlensing telescope, with the goal of finding habitable Earth like planets (Earth 2.0s) around solar type stars and measure its occurrence rate. In order to detect Earth 2.0s, ultrahigh-precision photometry of ∼30 ppm is required, which places tight constrain on camera performance, such as high-speed readout, low readout noise, mosaic detectors, and radiation tolerance. As of now, a prototype camera utilizing a CCD250-82 detector from Teledyne e2v has been developed and its performance has been tested. At a readout rate of 2 M pixels/s, the readout noise of 10.96 e− RMS and the pixel response nonuniformity of 0.66% at 600 nm have been achieved. After receiving radiation doses of 5 krad (Si) and 13.43 krad (Si), the dark current of the CCD increased by 30% and 126%, respectively. The camera’s key performance meets the basic requirements for the ET space mission, except for its high cooling power consumption.
Scientific grade Charge Coupled Device(CCD) is widely used in astronomical telescope as imaging sensor at present. A CCD camera based on CCD driving and readout ASIC is designed using CCD47-20 in this paper. The CCD driving and readout ASIC chips include CCD drive chip BCDA2(Second Edition Bias Clock Driver ASIC) and CVRA2(Second Edition CCD Video Readout ASIC). BCDA2 provides multichannel clock and bias voltage for CCD. CVRA2 processes the output signal of CCD by correlated double sampling double slope integral. The processed signals will be sampled by external ADC. Based on the BCDA2 and CVRA2, the electronics system of the camera is designed including a Connecting Board, a pre-amp board and a master control board with CCD driver and CCD readout. The test result shows it’s working well.
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.
Earth 2.0 is a Chinese space satellite mission that uses the transit and microlensing methods to search for exoplanets, especially Earth-sized terrestrial planets, including habitable terrestrial planets around sun-like stars. The satellite will work in the halo orbit of the Sun-Earth L2 point for at least 4 years, and is expected to find about 20 Earth like planets in the 4-year observation period. In this paper a CCD camera prototype based on a 4kx4k CCD250 detector for early technology demonstration and high-accuracy photometric performance verification is introduced. The key performance indicators such as readout noise, gain, and linearity of the camera are tested, and the test results will be described.
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.
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.
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.
KEYWORDS: Charge-coupled devices, Imaging systems, CCD cameras, Data acquisition, Telescopes, Image quality, Control systems design, Clocks, Field programmable gate arrays, Power supplies
The Wide Field Survey Telescope (WFST) is a proposed 2.5m-aperture wide field survey telescope intended for dedicated wide field sciences in China. The focal-plane instrument is a mosaic CCD camera comprising 9 pieces of 9K×9K pixels CCD chips. In order to verify the WFST mosaic solution, we designed a 2×2 mosaic camera test system using CCD303- 88. The mechanical design of vacuum chamber, cryogenic refrigeration of CCD, mosaic CCD technique and multi CCD control electronic have been implemented on this system. We design a CCD controller capable of controlling two pieces of CCDs and a power supply module for the controller. The cryogenic refrigeration control is implemented with a refrigerator and temperature control electronics.
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.
The wide field survey telescope (WFST for short) is a new generation survey telescope located in Lenghuzhen, Qinghai Province in China, and has outstanding performance in sky survey. However, the feature demands a rigid flatness 20μm PV of the prime focus plane of the prime focus camera. The CCD290-99 flatness 15μm PV and -100°C working condition pose challenges to the CCD splicing. In order to verify the CCD mosaicing technology for WFST’s prime focus camera before the sensor arriving, we use the CCD 303-88 in our lab to set up the verification platform. In this article, we mainly introduce the recent research status of the platform.
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