Adding the sun to image based lighting environment

Thanks Lars and Santiago,
Yes you are right, it doesn't make any sense to have the sun at infinite
distance outside the sphere of finite distance. And yes, I'll keep it
simple, no optimization with mksource at this point ;o). And a third: Yes -
Radiance is tricky, therefore it is so good that this list is active!

However, when changing the 'sphere environment' from sphere to source I
cannot display my scene any longer. My source description is
env_glow source sphereenv
0 0
4 0 1 0 360
The simulation approach now follows exactly the one in the Image Based
Lighting tutorial from Debevec, p. 31, I get a white image
and the rvu message:
rvu: warning - compute error for colorpict "hdr_env"
rvu: Division by zero
rvu: Division by zero

Do you maybe also have tips on how I can solve this?

- Yes, I divided with zero in the .cal file. Including an if sentence
solved this problem - and now it works perfectly!
Thanks Santiago, from a smiling Anne.

It would however be nice if the location of the sun could be rotated without
changing the vector coordinates. I would like this to be possible because I
have calculated the sun position from the pixel coordinates of my original
.hdr image, which I afterwards have rotated. I can of course take into
account the rotation in the pixel coordinates, but if the other approach is
possible it would be easier for me ;o)