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

The "Nigel" spectrometer was operated at Dome A, Antarctica, over the 2009 Austral winter. As part of the PLATO (PLATeau Observatory) astronomical instrument suite, Nigel was designed as a site-testing instrument, to provide quantitative measurements of the day time, twilight and night time sky brightness. In particular, one of the main the aims of the experiment was to determine the airglow and aurora contribution to night sky brightness.The instrument itself is a fibre-fed spectrometer, and operates in optical wavelengths (between approximately 300-850 nm). It uses bare optical fibres pointed at the sky (each with a fields of view of approximately 25 degrees in diameter) and in three fixed directions (north at 40.0 degrees altitude; west at 71.5 degrees altitude; and the zenith). Each direction uses two fibres (one optimised for 'blue' wavelengths, and one optimised for 'red' wavelengths greater than 515 nm), giving the instrument a total of 6 fibres. Further information on the instrument is available at:http://mcba11.phys.unsw.edu.au/~plato-a/nigel.htmlDuring 2009, there were a total of 55,182 observations.Data is provided in a number of formats, including metadata, and both raw data files and reduced versions. A number of observations have been excluded from the reduced data sets (246 in 2009) because they were deemed to be corrupt (they are still, however, included in the raw archives).1) nigel-metadata.csv - The metadata .CSV file contains information about each observation, under the following columns:00: UNIX time epoch at beginning of observation [number of seconds since January 1st, 1970]01-06: Year, Month, Day, Hour, Minute, Second07: Day number of 200908: Exposure time [seconds]09: CCD temperature [degrees C]10: Solar altitude [degrees]11: Solar azimuth [degrees]12: Lunar altitude [degrees]13: Lunar azimuth [degrees]14: Lunar phase [percent] 2) nXXXX.tar.bz2 - a bzip2'd tar archive of raw data files, which contains all observations for which "XXXX" is the first four digits of the UNIX time, equivalent to approximately 11.5 days. Each archive contains individual raw FITS images, which are named nXXXXXXXXXX.fits, where XXXXXXXXXX is the UNIX time epoch at the beginning of the observation. The exposure time and CCD temperature of the observation are stored in the FITS image header.3) nigel-XY.fits - the "XY" images, stored in astronomy-standard .FITS images, represent all observations in a reduced format (optimally extracted from the raw images, with cosmic rays and hot pixels removed, and approximately spectrally calibrated). With one fibre per image, the "XY" suffix represents the fibre (X) and direction (Y), where X = B (blue) or R (red), and Y = N (north), W (west) or Z (zenith), For example, "BW" represents the "Blue West" fibre.Within the FITS image, the x-direction is the spectral direction, with each row in the y-direction corresponding to an individual observation. The rows match the ordering of the metadata .CSV file. For the "Blue" fibres:columns 0-14 are metadata (same columns as listed in (1))columns 15-906 are the spectra (each spectra is 892 pixels long)For the "Red" fibres:columns 0-14 are metadata (same columns as listed in (1))columns 15-610 are the spectra (each spectra is 596 pixels long)4) In addition to the data, a number of sample scripts and IPython Notebooks are provided to give a description of how to examine this data, including sample scripts:- nigel1-reduction.html - describes process of reducing raw images- nigel2-calibration.html - describes our preliminary flux calibration- nigel.py - script containing functions which can be used to aid analysis- nigel3-usage.html - describes sample usage of using "nigel.py"- datNigel/ - folder containing data (spectral response, hot pixels lists) for "nigel.py"References:Sims, G. et al. (2013), "Airglow and aurorae from Dome A, Antarctica", Publications of the Astronomical Society of the Pacific, 124, 637-649.Sims, G. et al. (2012), "Precipitable Water Vapor above Dome A, Antarctica, Determined from Diffuse Optical Sky Spectra", Publications of the Astronomical Society of the Pacific, 124, 74-83.Sims, G. et al. (2010), "Optical sky brightness at Dome A, Antarctica, from the Nigel experiment", Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 7733, 77334M.

“奈杰尔”（Nigel）光谱仪于2009年南极冬季期间在南极冰穹A（Dome A）开展观测工作。作为PLATO（高原天文台，PLATeau Observatory）天文仪器套件的组成部分，奈杰尔光谱仪被设计为一台现场测试仪器，用于定量测量日间、晨昏与夜间的天空亮度。本实验的核心目标之一，是确定气辉与极光对夜间天空亮度的贡献。

该仪器为光纤馈入式光谱仪，工作波段覆盖光学波段（约300~850纳米）。其采用裸光纤对准天空，每根光纤的视场直径约为25度，并设置了三个固定观测方向：北天顶高度角40.0度、西天顶高度角71.5度以及天顶方向。每个观测方向配备两根光纤：一根针对“蓝”波段优化，另一根针对515纳米以上的“红”波段优化，因此仪器共计配备6根光纤。有关该仪器的更多详细信息可访问：http://mcba11.phys.unsw.edu.au/~plato-a/nigel.html

2009年期间，本数据集共计包含55182次观测数据。数据以多种格式提供，包括元数据、原始数据文件和约化数据版本。部分观测数据因被判定为损坏，已从约化数据集中移除（2009年共计246条），但这些损坏数据仍保留在原始存档中。

1) "nigel-metadata.csv" - 元数据CSV文件包含每次观测的相关信息，各列说明如下：
00：观测开始时刻的UNIX时间纪元（即自1970年1月1日0时起的累计秒数）
01~06：年、月、日、时、分、秒
07：2009年的日序数
08：曝光时间（单位：秒）
09：CCD温度（单位：摄氏度）
10：太阳高度角（单位：度）
11：太阳方位角（单位：度）
12：月球高度角（单位：度）
13：月球方位角（单位：度）
14：月相（单位：百分比）

2) "nXXXX.tar.bz2" - 采用bzip2压缩的tar归档文件，包含以“XXXX”为UNIX时间前四位数字的所有观测数据，对应时长约11.5天。每个归档文件包含独立的原始FITS格式图像，文件命名格式为nXXXXXXXXXX.fits，其中XXXXXXXXXX为观测开始时刻的UNIX时间纪元。观测的曝光时间与CCD温度存储于FITS图像的头文件中。

3) "nigel-XY.fits" - 采用天文学标准FITS格式存储的“XY”格式图像，代表所有约化后的观测数据（已从原始图像中最优提取，去除了宇宙线与热像素，并完成了初步光谱定标）。每张图像对应一根光纤，“XY”后缀分别代表光纤类型（X）与观测方向（Y）：X为B（蓝通道，Blue）或R（红通道，Red），Y为N（北方向，North）、W（西方向，West）或Z（天顶方向，Zenith）。例如，“BW”代表“蓝通道西向”光纤。

在FITS图像中，x轴方向为光谱方向，y轴的每一行对应一次独立观测，行顺序与元数据CSV文件完全一致。对于蓝通道光纤：第0~14列为元数据（与1)中所列列项完全相同），第15~906列为光谱数据（每条光谱长度为892个像素）。对于红通道光纤：第0~14列为元数据（与1)中所列列项完全相同），第15~610列为光谱数据（每条光谱长度为596个像素）。

4) 除上述数据外，本数据集还提供了多个示例脚本与IPython Notebook（IPython Notebook），用于演示如何检视与分析该数据，具体包括：
- nigel1-reduction.html：介绍原始图像的约化处理流程
- nigel2-calibration.html：介绍本团队的初步流量定标工作
- nigel.py：包含可辅助数据分析的函数脚本
- nigel3-usage.html：演示"nigel.py"的典型使用方法
- datNigel/：存放"nigel.py"所需配套数据（光谱响应曲线、热像素列表等）的文件夹

参考文献：
Sims, G. 等 (2013)，《南极冰穹A的气辉与极光》，《太平洋天文学会汇刊》（Publications of the Astronomical Society of the Pacific），124卷，637-649页。
Sims, G. 等 (2012)，《基于漫射光学天空光谱测定南极冰穹A上空的可降水量水汽》，《太平洋天文学会汇刊》，124卷，74-83页。
Sims, G. 等 (2010)，《奈杰尔实验测得的南极冰穹A光学天空亮度》，《国际光学工程学会（SPIE，Society of Photo-Optical Instrumentation Engineers）会议论文集》，7733卷，77334M。