AN OFF-AXIS CASSEGRAIN OPTIMAL-DESIGN FOR SHORT FOCAL LENGTH PARABOLIC SOLAR CONCENTRATORS

The present work addresses an off-axis Cassegrain optical concentratio n system. The specific primary collector analyzed, a short focal lengt h parabolic concentrator, is at the University of Florida's Energy Par k. A secondary hyperbolic reflective element was designed to redirect the solar radiation from the primary focal plane to an off-axis target on the polar axis of the primary concentrator. This ground level targ et will be required for planned experimental work. The analysis was pe rformed using a numerical ray tracing procedure that incorporates both random and systematic errors due to slope and surface irregularities, The optimization process varied secondary element she, curvature, and offset angle, and yielded information required for optimum design. As a single secondary element was found impractical, three elements were designed for use at various times of the year. The numerical analysis predicts that typically 70 to 75 percent of the solar flux incident o n the primary concentrator aperture was focused within a 0.5-meter rad ius. During the design, it was found that this type of compact concent ration system is a practical alternative. The optical system is also s hown to have advantages that are generally applicable for problems inv olving short focal length primary concentrators, or when the solar app aratus is to be placed outside the primary collector aperture.