Hm… I did a post already before…
I think it’s an scaling issue. When the texture size is lower than the displayed area the texture will be interpolated which results in the usual soft image. Probably same with the mask which is interpolated and not scaled which results in grey values at the borders…
btw. I tried already with 3072x2304 textures, same result… I guess we’d need to use a different approach to map the whole 21k picture of the earth on one sphere.