Knockout mask settings?

In CS5 PS what settings do you use in the Refine Edge tool if you need to isolate the sky or background? I have tried smart radius and differing the manual radius but still get white edges on sharp transitions with the sky. This does not happen on all images and to fix the problem one pixel at a time is a royal PITB.

Thanks in advance.

Have you tried romove cromnatic aberation?

I'm working in CS3 so the settings may not be the same but I use Feather. Sharp transitions like the image you show are most prone to haloing. What I often do is select and copy the sky to a separate layer. You can expand the selection a bit and feather it before making the sky copy layer so that it bleeds slightly into the foreground without a sharp edge. Do your foreground sharpening on a Background copy below the sky layer. The sky layer should then hide the transition between the sharpened layer and the sky.

I usually feather the selection by one pixel and then expand (or contract) the selection by one or two pixels.If you selected the sky in this example, then I would probably expand the selection by two pixels.
That almost always does the trick. If not, then using your image above as an example, I'll select the stone monolith, invert the selection (but not apply a feather) so the sky and halo are selected, and then clone away the halo. The Clone tool will only work in the selected area, so you don't have to be ultra careful at all.

Thanks for all the help. I tried all the suggestions and none worked. It turned out to be a large blue luminance change in Raw to bring the sky down. It created a halo that wouldn't go away.