The best I could do is appended here. In the end, after different 
experiments, I only changed the sky_sphere, replacing it by a sphere 
where I could control the finish. Assumed_gamma set to 1.0 of course. 
Norbert: I do not know how your sky_sphere  pigment works :-/

sphere {
   0, 1
   pigment {
     gradient <0, 1 ,0>
     color_map {
       blend_mode 3 blend_gamma 2.2
       [0.00 rgb <1.0, 1.0, 1.0>*0.9]
       [0.35 rgb <0.8, 0.8, 0.8>*0.3]
       [0.65 rgb <0.8, 0.8, 0.8>*0.3]
       [1.00 rgb <1.0, 1.0, 1.0>*0.9]
     }
     scale 2
     warp {turbulence 0.5}
   }
   finish {emission 1 diffuse 1.0 specular 0}
   scale 10e6
   translate -23.0*y
}

the blend_mode and blend_gamma commands come with version 3.7.1 and 
above (i.e 3.8). However, I do not see any difference by their use. I 
confess that I do not really understand their explanation in the wiki.

-- 
Thomas

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