詞彙
Glossary
光害 Light Pollution
光害(又稱光污染)是一種因過度使用人為室外照明，而對自然環境和生態等引起的負面影響。人為室外照明包括街燈、霓虹招牌、廣告版照明等。設計不良的室外照明將光線直接照射或反射上天空，大氣中的雲、霧、懸浮粒子等污染物會將這些光線散射開，導致夜空被照亮，可見星星的數目因為夜空的對比下降而減少。
Light Pollution is a form of environmental degradation in which excessive artificial outdoor lightings, such as street lamps, neon signs, and illuminated signboards, affect the natural environment and the ecosystem. The wasteful light emitted directly by or reflected from the poorly-design artificial sources upwards can be scattered by cloud, fog, and pollutants like suspended particulates in the atmosphere. The night sky is brightened and number of stars visible is reduced due to the decrease brightness contrast of the night sky.
夜空光度 Night Sky Brightness
光害令光線直接照射或反射上天空，大氣中的雲、霧、懸浮粒子等污染物會將這些光線散射開，導致夜空被照亮。夜空的光度反映光害的情況。我們的香港光害調查利用夜空光度測量錶(SQM)，量度夜空的光度，單位是等每平方角秒。
Light are emitted directly or reflected to the sky due to light pollution. Light can be scattered by cloud, fog, or pollutants like suspended particulates in the atmosphere. The night sky is brightened as a result. The night sky brightness can thus reflect the condition of light pollution. Our Hong Kong Light Pollution Survey uses the Sky Quality Meters (SQM) to measure the night sky brightness. The unit of night sky brightness is magnitude per arcsecond square.
環境及自然保育基金 Environment and Conservation Fund, ECF
環境及自然保育基金是根據《環境及自然保育基金條例》（第450章），由政府環保署成立。目的是資助與環保和自然保育有關的教育、研究、技術示範和其他項目及活動，以及社區廢物回收項目。該基金自1994年成立以來，合共資助超過1,000個項目，批出撥款約2億元。
(基金網頁： http://www.ecf.gov.hk/ )
The Environment and Conservation Fund (ECF) was established by the Environment Protect Department of the HKSAR Government according to The Environment and Conservation Fund Ordinance (Cap. 450). The ECF provides funding support for educational, research, technology demonstration, and other projects and activities in relation to environmental and conservation matters, as well as community waste recovery projects. Since its establishment in 1994, the ECF has supported over 1,000 projects, with total commitment of about $200 million.
(ECF website：http://www.ecf.gov.hk/ )
夜空光度測量錶 Sky Quality Meter, SQM
夜空光度測量錶是一個便宜(售價約港幣七百元)、體積細小(約只有一盒紙牌般大)、使用方法簡單和可靠(準確度約±10% 或±0.10等每平方角秒)的夜空光度測量工具。觀測者需在遠離人為燈光的地方，手持SQM在頭頂附近的位置，將SQM接收器的一面向天頂，夜空光度讀數會在數秒鐘內顯示出來。
(SQM供應商Unihedron網頁：http://unihedron.com/projects/darksky/ )
The Sky Quality Meter (SQM) is a cheap (price at about HK$700), portable (sized about a deck of playing cards), user-friendly, and reliable (accuracy ±10% or ±0.1 magnitude per arcsecond square) derive used for measuring night sky brightness. Holding the SQM near the head, with the sensor facing the sky, and standing far away from artificial lightings, the value of sky can be measured within seconds.
(website of the SQM supplier, Unihedron: http://unihedron.com/projects/darksky/ )
散射 Scattering
散射是光線被其穿透介質中一個或多個局部不平均範圍干擾，而偏離直線軌道的一種物理現象。散射常見於天文學上，例如日間天空是藍色，是因為大氣粒子對來自太陽的藍光散射得最利害。恆星閃爍的現象(像童謠「一閃一閃小星星……」)，部分原因是星光被大氣粒子散射。
Scattering is a physical process where the path of light deviates from a straight trajectory by one or more localized non-uniformities in the medium through which it passes. Scattering is common in astronomy. For example, the sky is blue is due to the stronger scattering of the blue sunlight by atmospheric particles. The twinkling of stars (like in the nursery rhyme “Twinkle Twinkle Little Star……”) is also partly due to the scattering of star light within the atmosphere.
星等 Magnitude, mag
星等(或稱視星等、簡稱等) 是描述天體光度的天文單位。愈光的星體，星等數字便愈小(甚至負數)。星等具對數關係，每六等之差相等於整整一百倍的光度差。因此，一等之差相等於大約2.512倍光度差。例如天琴座中最光亮的恆星-織女星是零等，而太陽約是負二十六等，滿月時光度約為負十二等。在極佳或極黑的環境下，肉眼可看見暗至五至六等的恆星，全天總共約二千至五千顆恆星。但此情況永遠不會在香港出現。
The magnitude (or apparent magnitude), abbreviated as mag, is an astronomical unit describing the brightness of a celestial object. The brighter the object, the smaller (even sometime negative) is the magnitude. Magnitude is in logarithmic scale. A difference in 6 magnitudes is defined to imply a brightness ratio of exactly 100. Therefore, a magnituide difference of 1 mag implies a brightness ratio of ~2.512. Vega, the brightest star in the constellation Lyra, for example, is a 0 mag star. The Sun is about -26 mag while the full Moon is about -12 mag. Under an ideal or extermely dark condition, our eyes can see up to 5or 6 mag stars, meaning a total of 2,000 to 5,000 stars could be visible on the celestical sphere. However, this standard is almost never achievable in Hong Kong.
兩個天體的光度I1、I2和星等m1、m2的關係為：
The relation between the intensities I1 and I2 and magnitudes m1 and m2 of two celestial objects is:
m1 – m2 = –2.5 log10 ( I1 / I2)
 
(Credit: Hong Kong Space Museum)
極限星等 Limiting Magnitude
天文儀器的極限星等是指，該儀器能探測得到的最暗淡天體的星等。在沒有儀器的協助下，肉眼能觀測得到最暗淡的星等，稱目視極限星等。對大部分人來說，目視極限星等約為五至六等。
The limiting magnitude of an astronomical instrument is the magnitude of dimmest celestial object that can be observed by that instrument. The limiting magnitude of unaided eyes is called the limiting visual magnitude and is ~ 5–6 mag for most people.
角秒 Arc Second, arcsec
角秒是天空長度的單位。一個全圓有三百六十度，一度有六十角分，而一角分可再細分為六十角秒，一角秒即3,600分之一度。滿月的視直徑約為三十角分或半度。
Arc second (arcsec) is the unit of length in the celestial sphere. There are 360 degrees on the full celestial circle, and there are 60 arc minutes (arcmin) in 1 degree, and 60 arc seconds (arcsec) in 1 arcmin. Therefore 1 arcsec = 1/3,600 degree. The diameter of the full Moon in the sky is roughly 30 arcmin, or 0.5 degree.
平方角秒 Arc Second Square, arcsec2
平方角秒是天空面積的單位。一平方角秒相等於一角秒乘一角秒的大小。
Arc second square (arcsec2) is the unit of area of the sky in the celestial sphere. The size of 1 arcsec2 equals to that made by a square patch of 1 arcsec × 1 arcsec.
等每平方角秒 Magnitude per Arc Second Square, mag/arcsec2
等每平方角秒是量度夜空光度的國際單位。如果甲地方的夜空光度是20等每平方角秒，則可以理解為甲地方夜空的光度，相等於一個20等的天體，填滿一角秒乘一角秒大的天空。如果乙地方的夜空光度是19等每平方角秒，即乙地方的夜空比甲地方的光約2.512倍(請參考「星等」一項)。
Magnitude per arc second square (mag/arcsec2) is the international unit used for measuring the night sky brightness. If the night sky of site A is at 20 mag/arcsec2, the brightness of sky there is equivalent to a celestial object of 20 mag filling up a patch of sky of area 1 arcsec x 1 arcsec. If the night sky of site B is at 19 mag/arcsec2, then the sky in site B is 2.512 times brighter than that in site A (Please refer to the entry on magnitude).
理想的天文觀測研究地方，夜空應該黑暗至22等每平方角秒。國際天文聯會(IAU)建議理想的天文觀測地方，波長在550nm附近的夜空天頂光度，應不光於21.6等每平方角秒。
(參考Derek McNally, The Vanishing Universe: adverse environmental impacts on astronomy, Cambridge University Press 1994)
For an good site for astrionomical research, the night sky should be as dark as 22 mag/arcsec2. The International Astronomical Union (IAU) suggests that the zenith night sky brightness near 550nm wavelength at a good observatory site should not excess 21.6 mag/arcsec2.
(reference: Derek McNally, The Vanishing Universe: adverse environmental impacts on astronomy, Cambridge University Press 1994)
適應黑暗環境的肉眼 Dark Adapted Eyes
人眼的虹膜會根據環境的光暗而收縮或放鬆，從而調節在其中央的瞳孔大小。情況就如調節相機的光圈大小。當環境光亮時，虹膜會收縮令穿過瞳孔的光線減少，保護視網膜。相反，當環境黑暗時，虹膜會放鬆令進入視網膜的光線增加。要完全適應黑暗環境，雙眼要連續在黑暗的環境下十至二十分鐘。這時雙眼瞳孔會完全放大，直徑約6至7mm，眼睛收集的光線最多，亦最適合觀星。一旦有強烈的光線進入眼球，虹膜會立即收縮。所以富經驗的天文愛好者觀星時會用暗淡的紅光電筒照明，避免強光影響眼睛。
Iris of an eye will contract or relax according to the environmental light intensity, thus changing the size of the pupil at the center. The situation is similar to the changing of aperture in a camera. In order to protect the retina, the iris will contract when the environment is bright and the light entering the retina will decrease. In the opposite, in a dark environment the iris will relax and the light entering the retina will increase. To achieve dark adapted eyes, one should first spend 10–20 minutes in a dark location. The pupil will then be fully expanded to a size of 6–7mm in diameter. At that time, the eyes can receive a maximum amount of light and the condition is most suitable for stargazing. The iris will contract immediately upon presence of strong light. Therefore experienced stargazers prefer weak red light torches which least affect one’s eyes.
目視光度測量器 Visual Photometer
(Credit: F. Della Prugna, 1999)
結構簡單的目視光度測量器，準確度達百分之十(0.1星等)。它由兩條並排、內裡全黑的管子組成。其中一支管密封，另一支向天的一邊是個圓形窗口，向觀測者眼睛的一邊是個可調整大小的四方形缺口。缺口可以由一粒測準了的螺絲準確地調整由全關至大約一平方毫米大。觀測者利用其中一隻已適應黑暗環境的眼睛，透過缺口全開的測量器觀測夜空上大小約二十平方度的範圍，漸漸收緊螺絲至夜空緊可見。愈光的夜空，缺口收得愈細，夜空的光度可由缺口的面積計算出。受人眼的感光能力限制，目視光度測量器最暗可量度約27等每平方角秒的夜空。
Visual photometer is a simple instrument with measuring accuracy of ~10% (0.1 mag). It consists of two tubes coupled together with fully blackened interior. One of the tubes is closed while the other has a circular opening at the end facing the sky and an adjustable, square diaphragm at the other end, just in front of the observer's eye. The size of the diaphragm can be adjusted from being totally closed to about one square millimeter in size by a calibrated precision screw. When the observer sees through this device with the diaphragm totally open, a circular patch of sky of about 20 square degrees can be viewed by a dark-adapted eye. The observer adjusts the diaphragm until the patch of the sky seen through the photometer is barely visible. The brighter the sky, the smaller is the diaphragm opening to reach this condition. The sky brightness can then be inferred from the area of the opening. Limited by the light sensitivity of a human eye, the dimmest sky that can be measured by the Visual Photometer is ~ 27 mag/arcsec2.
光密度計 Densitometer
光密度計是一種量度底片黑暗程度的工具，由一個光源和光電電子元件組成，電子元件測量底片的光學密度。
A densitometer is a device that measures the degree of darkness of a photographic film. The densitometer consists of a light source aimed at a photoelectric cell which determines the optical density of a sample.
天文光度分析學 Astronomical Photometry
天文光度分析學是量度天體光度的一門技巧。常用於量度變星和超新星等瞬變天體的光度變化。天文學家會利用不同波段範圍的濾鏡，選取特定波段範圍(如紅外線、藍色等)的光線作觀測和量度。
Astronomical Photometry is a technique of measuring the amount of light emitted by a celestial object. It is widely done to study the variation in brightness of transient objects such as variable stars and supernovae. Astronomers use filters of different passbands to filter specific wavelength passbands (e.g. IR, blue, etc) for observations and measurements.
孔徑測光法 Aperture Photometry
孔徑測光法是天文光度分析學的其中一個技巧。由天體發出的光線，由在天體旁稱「孔徑」影像範圍內的光度總算出。孔徑大多是圓形的。所用孔徑的大小視乎天體的特性、天體所在環境(是稠密或被孤立)等因素決定。合適的孔徑大小，應包含天體所有的光線，但愈細愈好#。
#最常用的孔徑直徑是影像的點擴散函數(PSF)的1.4倍。
Aperture photometry is one of the techniques of astronomical photometry. The light emitted by a celestial object is measured by integrating the amount of light recorded around the object over a region, called the aperture, of the image. Usually a circular aperture is used. The size of the aperture used depends on many factors, including the nature of the object and the environment of the object (crowded or isolated object).
The optimal aperture is that the aperture with size large enough to enclose most of the flux, but otherwise is as small as possible#.
#The commonly used aperture has a diameter of 1.4 times Full-Width-Half-Maximum (FWHM) of the Point Spread Function (PSF) of the image.
UBVRI濾鏡的波段範圍
The passbands of UBVRI filters.
(Credit: Data from M. Bessell UBVRI passbands, Graphic from CSIRO Australia)
UBVRI波段範圍 UBVRI passbands
UBVRI是指紫外線(Ultra-violet)、藍(Blue)、可見光(Visual)、紅(Red)、紅外線(Infra-red)五種波段範圍。UBVRI濾鏡就是只讓指定波段範圍光線通過的濾鏡，常用於光度分析學。
UBVRI stands for five passbands: Ultra-violet, Blue, Visual, Red, and Infra-red. The UBVRI filters are filters designed for specific UBVRI passbands. These filters are widely applied in photomety.
標準星 Standard Stars
由於每座望遠鏡和測光感應器(如CCD相機)的結構和表現都有不同，為了比較從不同儀器量度出的結果，我們需要透過量度部分標準星的光度。標準星是特選的一系列恆星，它們在不同波段的星等都被準確地量度，而且光度十分穩定。天文學家會定期拍攝和量度標準星場，與公佈的星等作比較，用作日後的光度分析學顏色轉換之用。一個最常被專業天文學家使用的，是由天文學家Arlo U. Landolt發表的標準星表。這個星表包含562顆在天球赤道旁的標準星的UBVRI星等和位置(參考： http://adsabs.harvard.edu/abs/1983AJ.....88..439L )。標準星亦有作為測定天體光譜數據，這些標準星稱光譜及測光標準星 (請參考「光譜」一項) 。
Since each telescope and light sensor (such as CCD) is unique in structure and performance, we have to measure the light emitted from some standard stars so that measurements made by different instruments can be compared. Standard stars are a group of selected sets of stars with well-known and stable magnitudes in different passbands. Astronomers will take standard star fields regularly, and then measure their magnitudes and compare those with the published magnitudes. Results are reserved for color-transformations later on. A commonly used standard star list for professional astronomers is the one by astronomer Arlo U. Landolt, in which positions and UBVRI magnitudes of 526 standard stars near the celestial equator are listed (reference: http://adsabs.harvard.edu/abs/1983AJ.....88..439L ). Another type of standard stars, called spectrophotometric standards, are used for taking spectrum of celestial objects (refer to the entry Spectrum).
光譜 Spectrum
日常生活中說的「光」，其實是電磁幅射。可見光有不同的顏色，各具有不同的頻率。物體的光譜是物體光線的頻率分佈圖。彩虹是自然界中最為人所熟悉的光譜：太陽的白光可視為由低(紅色)到高(紫色)頻率的連續光譜。天文學家拍攝天體的光譜，去了解天體的特性(例如溫度、化學成份和移動速度等)。觀察夜空的光譜，更可以離析出人為和自然的光害來源。
(參考： http://outreach.atnf.csiro.au/education/senior/astrophysics/spectra_astro_types.html )
What we commonly refer to as light is actually a electromagnetic radiation. This radiation can have different frequencies, which we can observe as different colors in the light visible to our eyes. The spectrum of an object represents the distribution of these different frequencies of light for the object. The familiar spectrum that exists in nature is the rainbow: the white sunlight can be shown to have a spectrum of a continuous range of colors from the short (red) to high (purple) frequency. Astronomers take spectra of celestial objects to study their properties (e.g., temperature, chemical composition, and motion). By observing the night sky spectra, we may be able to identify whether the light is from artifical or natural sourses.
(reference: http://outreach.atnf.csiro.au/education/senior/astrophysics/spectra_astro_types.html )
光譜學 Spectroscopy
拍攝物體光譜的科學。天文學家用光譜儀獲得天體的光譜。光譜儀常包括棱鏡和光柵。
The science of taking spectrum of objects. Astronomers used a tool called spectrometer to obtain spectra of celestial objects. Commonly used spectrometers include prisms and diffraction gratings.
左：截光型燈具的設計；右：非截光型燈具的設計
Left: The design of Full-Cut-Off Lighting;
Right: The design of non-Full-Cut-Off Lighting.
截光型燈具 Full-Cut-Off Lighting, Down Lighting
截光型燈具是一種將光線向下聚焦在0至65度範圍內的儀器(0度指垂直向下；90度指水平；180度指向天頂)。這樣的設計盡量消除向天照耀的光線，故減少光害。
The “Full-Cut-Off Lighting” or “Down Lighting” type of lighting equipments focuses the light to a downward light cone with light only spanning from 0 to 65 degree (0 degree means directly downward, 90 degree means horizontal light, 180 degree means light pointing upwards). This type of lighting design minimizes the light emitting directly upward towards the sky and hence reduces light pollution.
低壓鈉燈&高壓鈉燈 Low Pressure Sodium Lamps & High Pressure Sodium Lamps
低壓和高壓鈉燈常用作街燈和保安照明等室外用途，屬高強度氣體放電燈，其發光元件是一顆置於耐高溫燈管(弧光管)內的電弧放電器。低壓鈉燈(又稱氧化鈉SOX或LPS)的中央有兩支U型硼矽酸鹽玻璃管，內含固體鈉及小量的氖氣和氬氣。外殼是真空玻璃管，電鍍了一層氧化銦錫膜，反射紅外線，只讓可見光通過。電流將鈉氣化，發出黃光，主要譜線是589.3 nm。高壓鈉燈(又稱SON或HPS)結構與低壓鈉燈相似，但體積較少及包含水銀等元素，且具較高的氣壓，弧光管含氧化鋁，發出波長在589nm位置的鈉(Na)「D」發射線(黃光)。
Low pressure sodium and high pressure sodium lamps are widely used for outdoor lighting such as street lights and security lighting. They are high-intensity discharge lamps those produce light by means of an electric arc housed inside fused tube. Low pressure sodium lamps (or SOX / LPS) has two inner borosilicate glass U-pipes that hold solid sodium and a small amount of neon and argon gas. The outer envelope is a vacuum glass tube coated with an infrared reflecting layer of indium tin oxide which allows the visible light wavelengths out and reflect the infrared back. Electric charge vaporizes sodium metal and emitt yellow mainly at 589.3 nm wavelength. High pressure sodium lamp (or SON / HPS) is similar with LPS in structurte, but contains additional elements such as mercury and smaller in size. Its high pressure sodium arc is typically made of aluminum oxide with higher pressure. HPS gives yellow light in 589nm wavelength (sodium “D” lines).
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