As I know, this angle is used to calculate the maximum concentration

ratio. I think the book mentions that somewhere.

Dear Amin

When tracking the sun, you need to check the movement of the sun with respect to the direction of focus of your parabolic dish. One normally has to set an angle at which your tracker need to make a move, but this angle should be less that the half angle of the view of the sun, otherwise the tracker starts to miss the sun.

Check the tracking strategy in this book we wrote on solar tracking mechanisms and micro-controller tracking control with micro-processors:

 https://www.researchgate.net/publication/263085113_Solar_Tracking_High_precision_solar_position_algorithms_programs_software_and_source-code_for_computing_the_solar_vector_solar_coordinates_sun_angles_in_Microprocessor_PLC_Arduino_PIC_and_PC-based_sun

Trust this helps, regards

Gerro

Book Solar Tracking: High precision solar position algorithms, pr...

Dear Mr. Al-khazzar. Thank you very much!

Dear Mr. Prinsloo, I do appreciate your concern in my question. I will definitely read this book.

Generally this parameter have to taken into consideration in the optical analysis and in the heat flux distribution over the absorber. Practically the solar iradiation comes as a cone and no as single rays. So, a cone comes to the reflector and it is reflected to the absorber. The final result is one point in the reflector to send solar energy to a region in the absorber. If you make an anlytical optical analysis, the dimensions of the absorber have to be adequate in order to calture the majority of the reflected solar rays. The intercept factor is the parater which determines which part of these solar rays reach to the absorber. This parameter is about 95% in the newer collectors. The reason for prefering an absorber which not captures theapproximately 100% of the solar rays, is the higher heat loses in the case of a "huge" absorber. 

Please share me the refrence of this book..