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Imaging with narrowband filters, specifically hydrogen- or H-alpha, 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. Lastly, it is a wonderful way of taking images of the moon, which requires milli-second exposures with normal filters.
The Astrodon H-a filter has become the preferred choice of imagers for the following important reasons:
• Parfocal with all Astrodon LRGB, C, OIII and SII filters eliminating the need for refocusing.
• An incredible 90-92% transmittance at the H-a emission wavelength to reduce your total imaging time.
• 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.
• A single substrate to eliminate internal reflections that cause the halos around bright stars seen in many other H-a filters.
• 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.