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Imaging with narrowband filters has become very popular. It is an ideal tool to observe structure in emission nebula because it produces very high contrast, and is a wonderful means to enjoy our hobby even when the moon is up.
The Astrodon OIII & SII filters have become the preferred choice of imagers for the following important reasons:
• Parfocal with all Astrodon LRGB, C, and H-alpha filters eliminating the need for refocusing.
• A narrow 5 nm spectral bandwidth (full width at half maximum transmittance) consdered a "sweet spot" among commercially available filters to achieve the highest signal in the shortest time while keeping stars reasonably small.
• Incredible blocking of light outside the bandpass to better than 0.0004% to achieve very high S/N and high contrast, even with a bright moon nearby.