The heliostats at the Sandia National Solar Thermal Test Facility (NSTTF) have 25 facets each. The relative angle of these heliostats are set to maximize solar flux during tests, using an off-axis canting prescription. This is differs from the simpler on-axis canting prescription that is often assumed, which specifies a symmetric paraboloid with focal length equal to the slant range distance from the heliostat to the tower. Instead, off-axis canting specifies each facet’s canting angle to direct the reflected beam to the target point at a specific target time (for NSTTF, solar noon on the spring equinox). For heliostats off the field’s north-south axis, this leads to rotated astigmatism, as seen in the above example for heliostat 5E09. To support research, metrology testing, and education, OpenCSP_Data includes the off-axis canting prescriptions for every NSTTF heliostat, and also the on-axis prescriptions for comparison. OpenCSP_Code contains algorithms to compute both.
