Nebula Filters

City and suburban observers gained a new claim to the deep sky when nebula filters were developed in the late 1970s. These function on a straightforward principle. Emission nebulae give off light at narrow wavelengths that differ from those of sodium- and mercury-vapor streetlights. By using a multilayer interference filter, the spectrum of visible light can be cut finely enough to separate these wavelengths. The result is a much darker sky, somewhat dimmer stars and galaxies, and only slightly dimmer planetary and emission nebulae. This enhanced contrast can, in many circumstances, more than make up for the relatively small amount of light lost from the nebula, and so it stands out more clearly.

These filters do not bring country skies to the city, but they do help. One technique for detecting nebulae, especially tiny planetaries, is "blinking" with the filter. Hold it at the eye and move it rapidly in and out of the line of sight; a nebula will blink relative to the surrounding stars. Alternatively, blinking can be done by tilting the filter back and forth while looking through it, since it loses its effectiveness when at an angle.

Several nebula (or "light pollution") filter designs are available. They use somewhat different strategies for different types of objects and conditions.

The CCD

The biggest promise that technology holds out — for those who can afford it in both money and time — is the CCD camera. By 2000, CCD (charge-coupled device) cameras had taken over and vastly expanded high-end amateur astronomy, and their prices are declining every year. A CCD camera has two enormous strengths. First, the CCD chip is many times more sensitive to light than either your eye or photographic film. Second, it feeds a digitally recorded image from the telescope directly into your computer, where the image can be enhanced, analyzed, measured, and manipulated.

The most important manipulation is the ability to subtract away an extremely light-polluted background, as if by magic, with hardly any loss of data. An 8-inch telescope can now record 15th- or even 16th-magnitude stars in the worst city light pollution or moonlight. This is several times fainter than the same telescope can show stars to the eye under black, mountaintop conditions!

Drawbacks to CCDs include the very small field of view, the difficulty of aiming this field where you want, and problems of focusing. The equipment may be temperamental; the telescope mounting must be as rigid and controllable as for long-exposure astrophotography. And, of course, you're looking at a computer screen, not stars. It has been said that CCD astronomy is about working with equipment and computers, not skygazing.

The most important advance that CCDs represent is the science that can be done with the recorded images. For much of the 19th century, amateurs were almost on a par with professional astronomers in terms of the useful science they could do. Then amateurs fell very, very far behind — but now CCD cameras in dedicated hands are making up some of this lost ground. Amateurs are discovering asteroids in great numbers, performing professional-quality variable-star studies, detecting the 19th-magnitude optical afterglows of gamma-ray bursts near the limits of the observable universe, taking spectra of stars and galaxies, imaging the planets more finely than was once thought possible, and much more.

No machine, however, will ever replace the simplicity and delight of examining the stars directly, as a part of living nature.

Duck and Cover

"Light pollution" is the glow in the sky itself. It should not be confused with local lights that shine directly into the observer's eyes.

Local lights are more aggravating but easier to defeat. Many observers have cooperative neighbors who turn off outdoor lights on request. A good way to break the ice on this issue is to offer views through your telescope.

If you can't observe in the shade of trees or walls, you might rig a tarpaulin to shield your site. Max Wyssbrod lives in Lucerne, Switzerland, which he calls "the brightest country in Europe." His "cloth observatory" consists of four aluminum poles 10 feet long that fit into tubes cemented into the ground in a 10-foot square. The four walls are black cloth; guy ropes add stability. The whole rig, along with an 8-inch Schmidt-Cassegrain telescope, takes 15 minutes to set up.

Another strategy is to shield only your eye and the back end of the telescope. An old-fashioned photographer's black cloth or equivalent, or a cape that can be thrown up over your head, does the trick. Any telescope in bright local lights should also have a long dewcap or side shield to keep the light out of the tube. Eyepieces should have rubber eyecups.

"I use a black hood, and blinders I made from cardboard fitted to each side of my face," writes Charles Haun of Morristown, Tennessee. "This works quite well."

Hiding under cloth and wearing blinders may seem an ignominious way to experience the glories of the cosmos. But such is the garb that amateur astronomers shall increasingly wear as they march bravely into the future.