Its extremely difficult and dangerous to corectly align the mirror facets in a segmented solar reflector. Before starting construction, an array builder needs to know the formulas needed to get the mirrors at the proper angle to illuminate the receiver. Here's some basic geometry and trig that shows you how to do it.
Step 1: Linear Optics
....and you always wondered why they taught you that stuff!
Step 2: Plug and Crank
Two equations in two unknowns, solve it and use it as a spreadsheet formula.
The PDF file explains the math and gives you the formula: