The Antarctic Survey Telescope-AST3 consists of three optical telescopes with 680mm primary mirror and 8 square degree field of view, mainly for observations of supernovas and extrasolar planets searching from Antarctic Dome A. The first two AST3 telescopes (AST3-1 and AST3-2) were successfully installed on Dome A by Chinese expedition team in Jan. 2012 and Jan. 2015 separately. Multi-anti-frost methods were designed for AST3-2 and the automatic observations are keeping on from March 2016. The best limited magnitude is 19.4m with exposure time 60s in G band. The third AST3 will have switchable interface for both optical camera and near infrared camera optimized for k dark band survey. Now the telescope is under development in NIAOT and the K-band camera is under development in AAO.
We present a new application of frame transfer Charge-Coupled Device (CCD) on measuring astronomical seeing. If a telescope is equipped with a shutterless, frame transfer CCD camera, a bright star will generate a trail during the frame transfer phase. Because the transfer is very fast, the trail is a series of short exposures (about 1 ms) of the target star. Therefore the centroid is jittery due to atmospheric turbulence, and the amplitude can be utilized to derive astronomical seeing. We present the preliminary results from STA1600FT CCD on the second Antarctic Survey Telescope (AST3) tested in China. The trail seeing moderately agrees with the simultaneous DIMM seeing.
We have developed a specialized software package, called ast3suite, to achieve the remote control and automatic sky survey for AST3 (Antarctic Survey Telescope) from scratch. It includes several daemon servers and many basic commands. Each program does only one single task, and they work together to make AST3 a robotic telescope. A survey script calls basic commands to carry out automatic sky survey. Ast3suite was carefully tested in Mohe, China in 2013 and has been used at Dome, Antarctica in 2015 and 2016 with the real hardware for practical sky survey. Both test results and practical using showed that ast3suite had worked very well without any manual auxiliary as we expected.
We have successfully operated the AST3 telescope remotely as well as robotically for time-domain sky survey in 2015 and 2016. We have set up a real-time system to support the operation of the unattended telescope, monitoring the status of all instruments as well as the weather conditions. The weather tower also provides valuable information of the site at the highest plateau in Antarctica, demonstrating the extremely stable atmosphere above the ground and implying excellent seeing at Dome A.
Twilight/night sky images are often used for flat-fielding CCD images, but the brightness gradient in twilight/ night sky causes problems of accurate flat-field correction in astronomical images for wide-field telescopes. Using data from the Antarctic Survey Telescope (AST3), we found that when the sky brightness gradient is minimum and stable, there is still a gradient of 1% across AST3’s field-of-view of 4.3 square degrees. We tested various approaches to remove the varying gradients in individual flat-field images. Our final optimal method can reduce the spatially dependent errors caused by the gradient to the negligible level. We also suggest a guideline of flat-fielding using twilight/night sky images for wide-field robotic autonomous telescopes.
We have developed a new method to correct dark current at relatively high temperatures for Charge-Coupled Device (CCD) images when dark frames cannot be obtained on the telescope. For images taken with the Antarctic Survey Telescopes (AST3) in 2012, due to the low cooling efficiency, the median CCD temperature was -46°C, resulting in a high dark current level of about 3e−/pix/sec, even comparable to the sky brightness (10e−/pix/sec). If not corrected, the nonuniformity of the dark current could even overweight the photon noise of the sky background. However, dark frames could not be obtained during the observing season because the camera was operated in frame-transfer mode without a shutter, and the telescope was unattended in winter. Here we present an alternative, but simple and effective method to derive the dark current frame from the scientific images. Then we can scale this dark frame to the temperature at which the scientific images were taken, and apply the dark frame corrections to the scientific images. We have applied this method to the AST3 data, and demonstrated that it can reduce the noise to a level roughly as low as the photon noise of the sky brightness, solving the high noise problem and improving the photometric precision. This method will also be helpful for other projects that suffer from similar issues.
The photon transfer curve (PTC, variance vs. signal level) is a commonly used and effective tool in characterizing CCD performance. It is theoretically linear in the range where photon shot noise dominates, and its slope is utilized to derive the gain of the CCD. However, recent researches on different CCDs have revealed that the variance progressively drops at high signal levels, while the linearity shown by signal versus exposure time is still excellent and unaffected. On the other hand, bright stars are found to exhibit fatter point spread function (PSF). Both nonlinear PTC and the brighter-fatter effect are regarded as the result of spreading of charges between pixels, an interaction progress increasing with signal level. In this work we investigate the nonlinear PTC based on the images with a STA1600FT CCD camera, whose PTC starts to become nonlinear at about 1/3 full well. To explain the phenomenon, we present a model to characterize the charge-sharing PSF. This signal-dependent PSF can be derived from flat-field frames, and allow us to quantify the effects on photometry and measured shape of stars. This effect is essentially critical for projects requiring accurate photometry and shape parameters.
The AST3 project consists of three large field of view survey telescopes with 680mm primary mirror, mainly for observations of supernovas and extrasolar planets searching from Antarctic Dome A where is very likely to be the best astronomical site on earth for astronomical observations from optical wavelength to thermal infrared and beyond, according to the four years site testing works by CCAA, UNSW and PRIC. The first AST3 was mounted on Dome A in Jan. 2012 and automatically run from March to May 2012. Based on the onsite winterization performance of the first AST3, some improvements such as the usage of high resolution encoders, defrosting method, better thermal control and easier onsite assembly et al were done for the second one. The winterization observation of AST3-2 in Mohe was carried on from Nov. 2013 to Apr. 2014, where is the most northern and coldest part of China with the lowest temperature around -50°. The technical modifications and testing observation results will be given in this paper. The third AST3 will be optimized from optical to thermal infrared aiming diffraction limited imaging with K band. Thus the whole AST3 project will be a good test bench for the development of future larger aperture optical/infrared Antarctic telescopes such as the proposed 2.5m Kunlun Dark Universe Survey Telescope project.
A 10k x 10k single-chip CCD camera was installed on the first Antarctic Survey Telescope (AST3-1) at Dome A,
Antarctica in January 2012. The pixel size is 9 μm, corresponding to 1 arcsec on the focal plane. The CCD runs
without shutter but in frame transfer mode, and is cooled by thermoelectric cooler (TEC) to take advantage
of the low air temperature at Dome A. We tested the performance of the camera in detail, including the gain,
linearity, readout noise, dark current, charge transfer efficiency, etc. As this camera is designed to work at Dome
A, where the lowest air temperature could go down to −80°C in winter, we tested to cool not only the CCD
chip but also the controller which usually is operated at normal temperatures for ground-based telescopes. We
found that the performance of the camera changes a little when the controller is cooled.
The first of the trio Antarctic Survey Telescopes (AST3) has been deployed to Dome A, Antarctica in January
2012. This largest optical survey telescope in Antarctica is equipped with a 10k × 10k CCD. The huge amount of
data, limited satellite communication bandwidth, low temperature, low pressure and limited energy supply all
place challenges to the control and operation of the telescope. We have developed both the hardware and software
systems to operate the unattended telescope and carry out the survey automatically. Our systems include the
main survey control, data storage, real-time pipeline, and database, for all of which we have dealt with various
technical difficulties. These include developing customized computer systems and data storage arrays working at
the harsh environment, temperature control for the disk arrays, automatic and fast data reduction in real-time,
and building robust database system.