This group is dedicated to the pursuit of the imaging of deep sky astronomical objects with specialized "narrowband" filters.

Unlike standard RGB or LRGB imaging, narrowband filters let through just a very narrow range of light, usually from 3nm to 14nm in bandpass width. In RGB colour imaging, the red, green and blue filters each cover about one third of the visual spectrum, from over 700nm (red), to under 400nm (blue). The most commonly used narrowband filters include hydrogen alpha (Ha), ionized oxygen (OIII), and sulphur (SII), although other spectral lines are occasionally used.

This type of imaging has several interesting attributes. First, it allows for high-quality imaging from otherwise poor locations and times of the month. Regular light pollution, and even the presence of moonlight, has very littled effect on the wavelengths typically used. Secondly, since the other part of the "spectral continuum" is filtered out, stars tend to be smaller, and cameras do not bloom nearly as easily. This type of filtering requires long-duration subexposures, often extending up to an hour or more. Finally, since the assignation of the filters used to the red, green, and blue channels in a color image can be somewhat arbitrary, interesting color results can be obtained with different choices, or "palettes".

Objects that respond well to narrowband imaging include nebulae with a significant amount of emission activity - rather than that of reflected light. These objects "emit" the light associated with the type of atoms being "excited" by nearby stars. These objects include many nebulae in our galaxy, plus those in other nearby galaxies like M33 and to a lesser extent, M31 and M82.

The scope of interest here is both the optimal capture and subsequent processing challenges associated with this type of astrophotography! Your images are welcome here, as is the discussion of image capture techniques and equipment tips and tricks.