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Some time back I learned that the information captured on film is limited by the background level of the sky. To maximize what the film obtains, it is enough to expose so that the sky background just exceeds the base "fog level" of the film. There should be a visible difference between the exposure of the sky and a completely unexposed frame. Conversely, once you have reached this level, there is not much to be gained by exposing any further, since the ratio between the astrophoto subject and its background will be held constant by the behavior of film operating in its straight-line (density) region above its toe. This explains why pictures taken at dark sites are better than in light-polluted areas- the contrast is higher. If it is so dark that the sky doesn't show on the film yet, the exposure can be increased and the signal improved even further, right up to where the sky eventually does leave a trace of exposure on the emulsion. It seems natural to expect that the various "light pollution filters" available on the market should improve your pictures, after all, that is what they are designed for. After hearing favorable reports for one of them, I bought a Tokai (now IDAS) LPS (Light Pollution Suppression) filter from Hutech and have had a few occasions to try it. The technology implements thin film interference effects, in which careful arrangements of sub-wavelength thicknesses of specific-delectric materials can create a set of "notches" in the transmission characteristic of the filter. The notches are matched up with the dominant emission lines of artificial lighting. Many of these emission lines are in the red end of the visible spectrum and so there is an overall reduction of energy there, and the filter has a cyanish appearance. But since the background sky in a scene is basically the illumination of air and water vapor by terrestial light sources, it should become less visible through one of these filters. I have not done a formal test of this filter, but I have recently used it on a nighttime scene using reversal film and was reminded of one of the characteristics of thin film technology, namely that the wavelength interference behavior is also dependent on viewing angle. The careful cancellation of specific wavelengths is designed for light passing through the filter normal to its surface. At other angles the effective wavelength increases, and the notches will "blue-shift" (shorter wavelengths will have an effectively longer wavelength and fall into the tuned notch of the thin film layers). The result can be quite dramatic, see the pictures below. The shift of the notches from dominating the red end of the spectrum toward the shorter wavelengths is readily apparent.
Fortunately, these filters are not usually used in wide angle imaging. The pictures above have about a 150-degree angle of view. The magenta color fringe was also visible on pictures I took with a 50mm lens with its 90-ish angle of view. For most astrophotos the angles are limited to a only a few degrees, usually much less, and the blue-shift effect is inconsequential So how about the center of each picture? Can we tell how well the LPS filter worked?
I'm not sure I can make any conclusions strongly in favor of the LPS for this particular application. I had hoped to find additional faint stars showing up in the LPS exposure, but if they are present, they are not obviously so. A followup experiment might compare similar exposure times but opening the lens slightly when the filter is in place. It could be that the nature of the light pollution in this region (Altamont pass, near Livermore CA) is broadband in nature, from incandescent or high pressure lamps that emit energy everywhere in the spectrum, not just at the specific wavelengths so carefully removed by the LPS filter. More applicable comparisons of this filter have been made by others; here's one by Jerry Lodriguss that gives a better idea of what it can do in deep sky astrophotography: http://www.astropix.com/HTML/I_ASTROP/I11/I11.HTM
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