Under a clear sky, the faint parts of a nebula quickly disappear into the background glow. The Askar C2 filter from the Colour Magic series only allows through two narrow pieces of the spectrum: the emission line of doubly ionized oxygen (OIII) and that of sulfur (SII). Street lighting and sky glow are largely blocked out.
A duo band filter is often used with a color camera, because you can capture two emission lines in one shot. It is a 2 inch filter, so it fits in a filter wheel for 2 inch filters or a filter tray that fits that size.
Combined with an H-alpha image, you can build a full narrowband image. Check beforehand that your filter holder or filter wheel actually accepts 2-inch filters, because not every holder does that. This filter is intended for the night sky and not for the sun.
For the H-alpha side, there is the D1 filter for H-alpha and OIII. You can find more filters for a clear sky at filters against light pollution.
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