Hello Greg,
thanks for you help. I made some progress on this and have new questions 
The image linked below shows three scenarios I tried.
The left image shows heliostats reflecting the sun [source, at 0, 0, 1, ring with 0.5�] onto the receiver. This works fine so far.
But I also tried other materials that I would have expected to work.
The middle image uses a white glossy plastic with specularity 1.0 and roughness 0.0 . I would have expected to see the reflection on the receiver, too.
Same for the right picture, here I used a non-transmitting dielectric.
I would have expected that the complete radiation is reflected.
Do you have an idea what might causethis behaviour or what I might have missed?
Kind regards,
Joe