The camera I use is S1 IS. I need some help with manual settings for moon shots.

When I take moon pictures, I first try 1/500 to 1/800 in Shutter Priority Mode. I also try full Manual mode with shutter 1/500 to 1/800 and Aperture between F5.6 and F8 (I try various apertures between 5.6 to 8 with no luck).

The problem is that in Shutter Priority mode, the camera AF doesn't work. Focus never locks. So I try manual focus (in both Tv and M modes), set it to infinity. When I do that, moon looks nice and sharp on camera's LCD. But when I view it on computer, it looks blurry as in out of focus image.

Are there specific settings I should try? E.g. ISO xxx @ 1/xxx shutter @ Fx.x @ manual focus xx ft? Please help.