The hydrogen-alpha filter is a narrowband light filter that blocks all other light and only allows hydrogen-alpha emmissions past it to the camera sensor.
It's ideal for imaging nebulas like the Bubble Nebula that emit h-alpha light. Because the filter blocks so much other light, the incoming h-alpha image is pretty dim. To compensate for this, a typical h-alpha frame is usually ten to twenty minutes long. A comparable RGB frame would only take two to five minutes.
The h-alpha data on this image was actually a bit short. The individual h-alpha frames are only eight minutes long. Later I found that ten to fifteen minutes worked better with this scope and camera combination. This is also the first usable image where I used the field flattener on the NP101is. It did a very good job of correcting oddly shape stars in the corners and the edges.