Energy losses of parabolic trough collector

I did a couple of calculations for parabolic trough and I reviewed also the manual a bit. But I'd like to understand the heat losses a bit better.
How is the "thermal energy absorbed" calculated? What is the relationship to the "total incident thermal energy"?
If I do the following calculation, there are other losses as result:
Total incident thermal energy
- Receiver thermal losses
- Thermal energy absorbed

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Other losses

What are these other losses? I guess they are all optical losses that mainly depend on the sun position. Please explain.
Thanks.