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.
The conventional response band of Indium Gallium Arsenide (InGaAs) detectors ranges from 0.9 to 1.7μm. The J atmospheric window (1.25μm) in infrared astronomy falls at the center of the response band of InGaAs detectors, making them widely used in this spectral region for infrared astronomy. Three representative Chinese-made Indium Gallium Arsenide focal plane arrays (InGaAs FPAs) were selected, and corresponding interface circuits were designed to match the testing system. Key performance indicators such as dark current, gain, and readout noise were tested.
In order to enhance the observational ability in infrared wavelength, a K-band MCT(Hg-Cd-Te) mosaic camera is constructed for a near-infrared telescope. In this paper methods of low temperature control and vibration measurement of the infrared camera are introduced. In order to suppress the dark current of the MCT detector of the camera, the detector is refrigerated to below 80K using a tube cryocooler in a vacuum chamber. To suppress the scatter light from lens of optics, the lens in front of the detectors should be cooled to below 150K. And in order to avoid damage of the cold optical lens caused by low temperature, two-stage temperature control is carried out on the base-plate of detectors and cold optics. Furthermore, the running cryocooler will cause small vibration of the detector, which will reduce the imaging quality of the camera. An anti-vibration method is designed to reduce the influence of the vibration. A three-axis vibration measurement of the detector is introduced, which provides data basis for the analysis of the influence of vibration on imaging and image correction.
Chinese scientists plan to build an 80 centimeter caliber near-infrared astronomical telescope in Antarctica, consisting of one telescope and two terminal devices. The K-band astronomical imaging system is one of the main terminals. We designed and characterized a 1x3 K-band near-infrared mosaic camera and the MCT detectors form Shanghai Institute of Technical Physics. The single pixel array of this camera is 640x512, with a single pixel size of 15 μm. Three detectors arranged in a mosaic configuration generated a focal plane pixel array of 1920x512. To ensure the optimal performance of the K-band astronomical imaging system, we used Dewar sealing, vacuum maintenance, and thermoacoustic refrigerator to cool the detector to 80K and the optical lens and components to 150K. In addition, a low-noise power supply design and electronic devices with low readout noise are adopted to ensure the minimum dark current and low readout noise of the mosaic infrared camera. The flatness of the mosaic detector is less than 20 μm . Complies with the design specifications for K-band astronomical imaging system.
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.
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