Thank you for your replies
Glen, the filter ring says "Orion extra narrowband imaging filter" so I assume it is an imaging filter. Also apologies, I used word "ultra narrowband" instead of "extra narrowband" in my original post. It has been corrected now in the original post.
The seller said that the filter is, and I am quoting:
"12nm?"
Note the question mark at the end so I don't think they were sure. Internet suggests that "extra narrowband" is 7nm.
Marc, I was not aware that Ha (narrowband) will exacerbate the tilt/collimation problems. I suspect that there is tilt in my system but it never bothered me before. I am using a refractor so collimation is not an issue.
I have found
this page which shows how collimation can affect the focus. If tilt has similar effect, it can explain the crescent shape and the double-lobe shape when out of focus.
I will try playing with this when it stops raining and also try the SII filter again which I thought worked OK. I don't trust my memory from events after 2am
