Just out of interest, if the image is purely for the benefit of IDL calculations and maybe more blurred reflections, is there any benefit to using a lower res image, relative to te IDL processing overheads?

e.g. would there be any benefit, in certain scenes, to using two identical envsphere props. One with a lower res copy of your spherical panorama, set to be visible in ray tracing but invisible to camera... and the second with the hi res copy of the image, set to only be visible in camera?

Cheers