The Horsehead Nebula (also Barnard 33) is a dark nebula in Orion, just to the south of Alnitak, the easternmost star of Orion’s Belt. It is part of the much larger Orion molecular cloud complex. The hydrogen in the cloud is being ionized by the proximity of Alnitak, providing a very strong Ha signal, while another other gas cloud, closer to us and not ionized, shadows the bright background, providing for dark nebulas.
This dataset has excellent response in both Ha and SII, while the OIII signal is extremely weak. In RGB the response is good, although the integration time is on the short side.
The RGB data is difficult to process. Here is however how it can be done.