Zhongwen Hu

Airglow and Aurorae at Dome A, Antarctica

Despite the absence of artificial light pollution at Antarctic plateau sites such as Dome A, other factors such as airglow, aurorae, and extended periods of twilight have the potential to adversely affect optical observations. We present a statistical analysis of the airglow and aurorae at Dome A using spectroscopic data from Nigel, an optical/near-IR spectrometer operating in the 300-850 nm range. These data complement photometric images from Gattini, a wide-field (90 degrees) CCD camera with B, V, and R filters, allowing the background sky brightness to be disentangled from the various airglow and auroral emission lines. The median auroral contribution to the B, V, and R photometric bands is found to be 22.9, 23.4, and 23.0 mag arcsec(-2), respectively. Auroral emissions most frequently occur between 10-23 hr local time, when up to 50% of observations are above airglow-level intensities. While infrequent, the strongest emissions detected occurred in the hours just prior to magnetic midnight. We are also able to quantify the amount of annual dark time available as a function of wavelength, as well as in the standard BV R photometric bands. On average, twilight ends when the Sun reaches a zenith distance of 102.6 degrees.

Precipitable Water Vapor above Dome A, Antarctica, Determined from Diffuse Optical Sky Spectra

The high altitudes and extremely low temperatures of the Antarctic plateau result in an exceedingly low atmospheric water vapor content. In this article we estimate the precipitable water vapor at Dome A using optical spectra of the diffuse solar irradiance in the zenith direction. The spectra were obtained from the Nigel spectrometer at Dome A during 2009. We find that the Nigel spectra contain sufficient information to extract daily average water vapor estimates, which agree with satellite observations to within +/- 0.22 mm. Finally, we use these water vapor estimates to model the optical and near-IR transmission (between 700 nm and 2.5 mu m) of Dome A to demonstrate significant advantages gained in this wavelength range and compare it with that of Paranal and Chajnantor, two midlatitude observatories.

Optical sky brightness at Dome A, Antarctica, from the Nigel experiment

Nigel is a fiber-fed UV/visible grating spectrograph with a thermoelectrically-cooled 256×1024 pixel CCD camera, designed to measure the twilight and night sky brightness from 300nm to 850 nm. Nigel has three pairs of fibers, each with a field-of-view with an angular diameter of 25 degrees, pointing in three fixed positions towards the sky. The bare fibers are exposed to the sky with no additional optics. The instrument was deployed at Dome A, Antarctica in January 2009 as part of the PLATO (PLATeau Observatory) robotic observatory. During the 2009 winter, Nigel made approximately six months of continuous observations of the sky, with typically 104 deadtime between exposures. The resulting spectra provide quantitative information on the sky brightness, the auroral contribution, and the water vapour content of the atmosphere. We present details of the design, construction and calibration of the Nigel spectrometer, as well some sample spectra from a preliminary analysis.

Optical Sky Brightness and Transparency during the Winter Season at Dome A Antarctica from the Gattini-All-Sky Camera

The summit of the Antarctic plateau, Dome A, is proving to be an excellent site for optical, NIR, and THz astronomical observations. GATTINI was a wide-field camera installed on the PLATO instrument module as part of the Chinese-led traverse to Dome A in January, 2009. We present here the measurements of sky brightness with the Gattini ultra-large field of view (90 deg x 90 deg) in the photometric B-, V-, and R-bands, cloud cover statistics measured during the 2009 winter season, and an estimate of the sky transparency. A cumulative probability distribution indicates that the darkest 10% of the nights at Dome A have sky brightness of S_B = 22.98, S_V = 21.86, and S_R = 21.68 mag arcsec^{-2}. These values were obtained around the year 2009 with minimum aurora, and they are comparable to the faintest sky brightness at Mauna Kea and the best sites of northern Chile. Since every filter includes strong auroral lines that effectively contaminate the sky brightness measurements, for instruments working around the auroral lines, either with custom filters or with high spectral resolution instruments, these values could be easily obtained on a more routine basis. In addition, we present example light curves for bright targets to emphasize the unprecedented observational window function available from this ground-based site. These light curves will be published in a future paper.

Winter sky brightness and cloud cover at Dome A, Antarctica

At the summit of the Antarctic plateau, Dome A offers an intriguing location for nfuture large scale optical astronomical observatories. The Gattini Dome A project was created nto measure the optical sky brightness and large area cloud cover of the winter-time sky above nthis high altitude Antarctic site. The wide field camera and multi-filter system was installed non the PLATO instrument module as part of the Chinese-led traverse to Dome A in January n2008. This automated wide field camera consists of an Apogee U4000 interline CCD coupled to na Nikon fisheye lens enclosed in a heated container with glass window. The system contains a nfilter mechanism providing a suite of standard astronomical photometric filters (Bessell B, V, nR) and a long-pass red filter for the detection and monitoring of airglow emission. The system noperated continuously throughout the 2009, and 2011 winter seasons and part-way through the n2010 season, recording long exposure images sequentially for each filter. We have in hand one ncomplete winter-time dataset (2009) returned via a manned traverse. We present here the first nmeasurements of sky brightness in the photometric V band, cloud cover statistics measured so nfar and an estimate of the extinction.

The infrared imaging spectrograph (IRIS) for TMT: design of image slicer

The InfraRed Imaging Spectrograph (IRIS) is one of three first light science instruments for the Thirty Meter Telescope (TMT). It will provide dedicated function of imaging and integral field spectroscopic observations in parallel with the assistance of a Narrow Field InfraRed Adaptive Optics System (NFIRAOS). The IRIS imager delivers celestial light to a dual-channel Integral Field Spectrograph (IFS) through a pair of pick-off mirrors in the central field. The IFS creates multi-functional ability to explore the universe in IR (0.84 – 2.4um) with moderate spectral resolution of R=4,000/8,000 and four spaxel scales of 4, 9, 25, 50 milli-arc-seconds (mas). An image slicer serves one of the two spectral channels as its Integral Field Unit (IFU) in two coarse spaxel scales of 25 and 50mas over the continuous science fields of 2.2x1.125 arc-seconds (arcsec) and 4.4x2.25 arcsec respectively. It splits the field to 88 unit systems, and then re-images at two parallel slits in order to take full advantage of the detector (4Kx4K @ 15um). This paper describes a novel all-reflective design of image slicer, which uses a new ‘brick stage’ layout to stagger the adjacent mirrors and deliver image quality close to diffraction limit. The quasi-telecentric optical design gives more friendly interfaces with pre-optics and spectrograph than the conceptual design. Here, more technical issues are discussed to guide the further study on optical performance and fabrication feasibility.

Airglow and Aurorae from Dome A, Antarctica

Despite the absence of artificial light pollution at Antarctic plateau sites such as Dome A, other factors such as airglow, aurorae and extended periods of twilight have the potential to adversely affect optical observations. We present a statistical analysis of the airglow and aurorae at Dome A using spectroscopic data from Nigel, an optical/near-IR spectrometer operating in the 300–850 nm range. The median auroral contribution to the B , V and R photometric bands is found to be 22.9, 23.4 and 23.0 mag arcsec −2 respectively. We are also able to quantify the amount of annual dark time available as a function of wavelength; on average twilight ends when the Sun reaches a zenith distance of 102.6°.

First look at HRCAM images from Dome A, Antarctica

HRCAM (High Resolution CAMera) is a Canon 50D 15-megapixel digital SLR camera equipped with a Sigma 4.5 mm f/2.8 fish-eye lens. It was installed at Dome A on the Antarctic plateau in January 2010 and photographs the sky every 15 minutes. Primarily functioning as a site-testing instrument, data obtained from HRCAM provide valuable statistics on cloud cover, sky transparency and the distribution and frequency of auroral activity. We present a first look at data from HRCAM during 2010, including an overview of how we intend to reduce the images. We also demonstrate the potential of stellar photometry by using linear combinations of the in-built Canon RGB filters to convert instrumental magnitudes into the photometric BVR bands.