Ha 20 min x 8 = 2.67 hr
This image of the Lagoon Nebula was taken with a monochrome astronomical CCD camera through a narrowband Hydrogen-alpha filter that isolates the red light of interstellar hydrogen emission at 656 nm.
Light at this deep red wavelength is prominent in interstellar space, but is blocked by the sensors of most DSLR cameras because it is so close to the infrared.
To capture this important astronomical wavelength you can modify the filters in a DSLR camera, or use a monochrome sensor as I did here.
This Ha light was combined with RGB images (separately taken through red, green, or blue filters) to bring out more detail in the color image of this nebula elsewhere in this gallery.