…continuedObserving from the City 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 surroun