WFST telescope is a large-aperture sky survey telescope jointly built by the University of Science and Technology of China and Purple Mountain Observatory. It is currently the most powerful optical time-domain sky survey telescope in the northern hemisphere. Its scientific goals are diverse, and the urgency and observation window period of different scientific tasks are different. During the observation period, the control of equipment requires accuracy, real-time, efficiency and safety. Therefore, we design and implement the OCS (Observatory Control System) of WFST which performs multi-layer abstraction on each hardware device and puts more attention on the operation of the observation process and the scheduling of observation tasks. In order to ensure the safety of equipment during the observation process, the OCS introduces a weather alarm system and fault diagnosis system. At the same time, we design an observation strategy system in the OCS to adjust the observation plan based on task priority, equipment information and meteorological information to maximize the efficiency of sky survey.
We present the current design of WFOS, a wide-field UV/optical (0.31-1.0 µm) imaging spectrograph planned for first-light on the TMT International Observatory 30 m telescope. WFOS is optimized for high sensitivity across the entire optical waveband for low-to-moderate resolution (R ∼ 1500-5000) long-slit and multi-slit spectroscopy of very faint targets over a contiguous field of view of 8′ .3×3 ′ .0 at the f/15 Nasmyth focus of TMT. A key design goal for WFOS is stability and repeatability in all observing modes, made possible by its gravity-invariant opto-mechanical structure, with a vertical rotation axis and all reconfigurable components moving only in planes defined by tiered optical benches parallel to the Nasmyth platform. WFOS’s optics include a linear ADC correcting a 9′ diameter field, including both the science FoV and 4 patrolling acquisition, guiding, and wavefront sensing camera systems; a novel 2-mirror reflective collimator allowing the science FoV to be centered on the telescope optical axis; a dichroic beamsplitter dividing the collimated beam into 2 wavelength-optimized spectrometer channels (blue: 0.31-0.56 µm; red: 0.54-1.04 µm); selectable transmissive dispersers (VPH and/or VBG) with remotely configurable grating tilt (angle of incidence) and camera articulation that enable optimization of diffraction efficiency and wavelength coverage in each channel; all-refractive, wavelength-optimized f/2 spectrograph cameras, and UV/blue and red-optimized detector systems. The predicted instrumental through put of WFOS for spectroscopy averages > 56% over the full 0.31-1 µm range, from the ADC to the detector. When combined with the 30 m TMT aperture, WFOS will realize a factor of ∼20 gain in sensitivity compared to the current state of the art on 8-10 m-class telescopes.
The wide field survey telescope (WFST) is a 2.5 meter optical telescope that is currently under construction in China. Designed with a large field of view (FOV) of 3 degrees in diameter and equipped with a 0.75 gigapixel mosaic CCD camera, the telescope will be mainly used for high sensitivity time-domain imaging surveys across the northern sky. The optical design for WFST features an advanced primary-focus assembly (PFA) housing five corrector lenses, an atmospheric dispersion corrector, filters of six bands, and the CCD camera. Stray light rejection performance is crucial for WFST to achieve an optimal sensitivity and maximize its scientific outputs. The primary-focus geometry of WFST helps to reduce the celestial background compared with a Cassegrain geometry, but the wide FOV imposes additional difficulty in stray light control and suppression. In this paper, the stray light behavior of WFST is carefully modeled by establishing a detailed opto-mechanical model of the telescope, assigning proper surface properties, and launching ray tracing simulations for a variety of scenarios. Important stray light paths including ghost effect and first-order scatterings are identified. Stray light mitigation measures including baffle and mask designs are proposed and optimized based on the stray light analysis results, which show promising suppression capability.
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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