I'm using Geoserver in conjunction with the google maps API to add a raster layer onto a map. It loads fine, however there's some strange problem with opacity at the edge of the layer where you can see a boundary. Have you any idea how to fix this?

(The original is at obstest.heliohost.org/map2.html.)
I'm totally new to gis and to Geoserver so sorry if this is an obvious question!
Thanks.

