Now look at the middle part of Figure 11.5, showing the transmission of various filters. Some of them are quite effective at cutting through the glow of city lights.
The "poor man's nebula filter" is didymium glass, which blocks the brightest emissions of sodium-vapor (orange) streetlights. Didymium glass looks bluish by daylight and purplish by tungsten light. It contains a mixture of praseodymium and neodymium. Glassblowers look through didymium glass to view glass melting inside a sodium flame.
Today, didymium glass is sold in camera stores as "color enhancing" or "intensifier" filters. Nature photographers use it to brighten colors because it blocks out a region of the spectrum where the sensitivities of the red and green layers of color film overlap.
The highest-quality didymium filter that I know of is the Hoya Red Intensifier (formerly just Intensifier). As Figure 11.7 shows, it has a useful effect, but even the multi-coated version is not invulnerable to reflections if there is a bright enough star in the field. Avoid Hoya's Blue Intensifier and Green Intensifier, which are less transparent at the desired wavelengths. Hoya's Portrait or Skintone Intensifier is a weaker kind of didymium glass with less blockage of the unwanted wavelengths.
Interference filters are made, not by adding dye or impurities to glass, but by depositing multiple thin, partly reflective coatings on the surface. The thickness and spacing of the coatings resonate with particular wavelengths of light. In this way it is possible to select which parts of the spectrum to transmit or block.
For DSLR photography, a "broadband nebula," "galaxy," or "deep-sky" filter is what you want (see the middle of Figure 11.5). The goal is to block light pollution while transmitting as much light as possible of the rest of the spectrum. The
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