The bright horizontal line through the middle is the light of the galaxy we observed that night. Redder colors are on the left, and bluer colors are on the right. This is a spectrogram of a galaxy in the constellation Hercules, taken by the 200 inch telescope at Palomar. Unfortunately, city lights play havoc with spectrographs. Spectroscopy is probably the most valuable tool in the astronomers' toolbox. By noting the redshift of the lines (how far to the red side of the spectrum they are shifted), astronomers can determine how fast the object is moving. By studying the strengths of these lines, astronomers can deduce the chemical composition and temperatures of the objects they observe. Each line is a unique indicator of the presence of a certain chemical. When you take a spectrum of fluorescing objects like galaxies, you see that the spectrum is not smooth, but made up of a number of lines. Often, astronomers want to take the spectra of an object, splitting the light from the telescope into its component colors. The second way city lights interfere with astronomy is much more insidious. This is where you want the light anyway, and by shielding the light you will both reduce light pollution and reduce the wattage of bulb required for the same amount of ground lighting, thus saving energy as well! You can help reduce this problem by making sure that lights around your street are shielded so that most of the light is pointed downwards. When we try to take a picture of a very dim object, sometimes the glow from the sky is too bright to ever see the object clearly. Now, the sky does not glow as brightly at night as it does during the day, but the increase in sky glow caused by cities is enough to make it difficult to see dim objects in the sky.
This sets the sky aglow, in much the same way that the sun sets the sky aglow during the day. The first is simply that unshielded lights send their light in all directions, including straight up. There are two ways that light pollution interferes with our ability to study the sky. What are the effects of "light pollution" on our ability to view the stars? Especially, how are the new city lights helping or hurting the situation? Do sodium vapor lamps really help? What is it about mercury vapor lamps that hurt our ability to see through the atmoshere? How bad is it today from existing observatories to get good viewing conditions as compared to say the last 50 years? Is there any effort to correct this problem? Is there corrective lens that can be used to help as was done with the Hubble telescope? What do you see as the future of land based, visible light telescopes concerning this specific problem?