Tom Wales wrote:
That filtre is made to be used also in light polluted skies so you should see it.
A complete answer would require a university degree course in Optics. Briefly: the filtre is made with multiple interference layers that should be used at the centre of the field of view. As soon as you move the stars progressively far from the centre of the field of view, the different interference layers of the OIII do interfere in a non optimal way, and that is the result you see.