710 that the pv unit 726 b is referred to as subject to double coverage at the complete crossover distance 706 c a consequence of the partial double coverage on the pv unit 726 b between the initial crossover distance 706 b and the complete crossover distance 706 c and the complete double coverage at the complete crossover distance 706 c is that a quasi parabolic concentrator is much more robust in the face of surface defects such as post installation localized damage blemishes dirt or interfering matter such as bird feces than a known parabolic concentrator subject to the same surface defect for the same reason the quasi parabolic concentrator is more robust than the known parabolic concentrator in the face of pre installation damage and or any other surface defect which could arise during manufacture assembly or during shipment referring now to fig8 a qualitative illustration of relative intensity across a pv unit 826 a is discussed it is known that a known parabolic concentrator reflecting incident radiation from a distant object illuminates a flat pv unit with a circularly symmetric pattern having an intensity maximum near the center outside of any shadow and monotonically decreasing to a minimum at the edges as such in fig8 the reflected radiation from point p 1 on the parabola 811 passing through the focus 803 and falling on the pv unit 826 a at spot q 1 has a higher intensity than that from point p 2 on the parabola 811 passing through the focus 803 and falling on the pv unit 826 a at spot q 2 an equivalent point to p 1 on the left partial portion of the.