Unwraping hemisphere, but how?

[micheus]

By considering the texture you are using (with those lines) I believe the better option would be to use the Projection Camera option:

In this case I just marked for cut the helmet boundaries (two edge loops separating the in/out side); changed the view mode to Orthogonal and put the view along the Y axis. After created the UVMap it was needed a little of tweak in the opened mesh.