選用有利天文觀測的光源 Choosing Astronomical-friendly Lighting Sources

從被光害污染的夜空光譜,天文學家發現NaD(鈉的其中一種化學形)和Hg(汞、即水銀)的發射線,而它們源自人為的高壓鈉燈和含水銀的光管。由於這些發射線處於可見光光譜以內,對地面光學天文觀測甚有影響。不會對天文觀測造成影響的光源包括低壓鈉燈,它的發射線集中在可見光光譜的一小部分,天文學家容易將此發射線用濾鏡等方法除去,有利天文觀測。

From the light-polluted sky spectra, astronomers discovered NaD (a chemical form of sodium) and Hg (element mercury) emission lines. They originated from artificial high pressure sodium lamps and mercury-contained fluorescent lamp. These emission lines located within the range of visible light and affect ground-base optical astronomical observations. Low pressure sodium lamps are better for astronomical observations. It is because the emission lines of low pressure sodium lamps occupy a small portion in visible spectrum only. Astronomers can easily filter out its lines by applying filters, etc.

(鈉燈 Sodium lamps)

(From 'La Palma night-sky brightness', Isaac Newton Group. Credits: Chris R. Benn)

右圖是在北大西洋拉帕爾馬島,由威廉·赫歇爾望遠鏡在1991年3月無月的晚上拍攝的夜空光譜。由此夜空光譜,不難察覺夜空的數個光源,例如天然的氧原子發射線 (以 OI 標示) 及氧分子發射線 (以 O2 標示) 。光譜中部分發射線源於光害,最明顯的有鈉D (NaD) 及汞 (即水銀) (Hg) 發射線。U、B、V、R、I是指電磁光譜的五個波段。

Spectrum of a moonless La-Palma sky taken with the William Herschel telescope on March 1991. Several features of airglow can be seen in this spectrum of the night sky, such as the emission from neutral Oxygen atom (labeled by OI), and Oxygen molecule (O2). Several lines are caused by light pollution, most notably the emission lines from sodium-D (NaD) and mercury (Hg). The range of the U, B, V, R, and I filter passbands are also shown.