AN ASTIGMATIC CORRECTED TARGET-ALIGNED HELIOSTAT FOR HIGH-CONCENTRATION

Conventional heliostats suffer from astigmatism for non-normal inciden ce. For tangential rays the focal length is shortened while for sagitt al rays it is longer than the nominal focal length. Due to this astigm atism it is impossible to produce a sharp image of the sun, and the ra ys will be spread over a larger area. In order to correct this the hel iostat should have different curvature radii along the sagittal and ta ngential direction in the heliostat plane just like a non axial part o f a paraboloid. In conventional heliostats, where the first axis, fixe d with respect to the ground, is vertical while the second, fixed with respect to the reflector surface, is horizontal such an astigmatism c orrection is not practical because the sagittal and tangential directi ons rotate with respect to the reflector. We suggest an alternative mo unt where the first axis is oriented towards the target. The second ax is, perpendicular to the first and tangent to the reflector, coincides with the tangential direction. With this mounting sagittal and tangen tial direction are fixed with respect to the reflector during operatio n. Therefore a partial astigmatism compensation is possible. We calcul ate the optimum correction and show the performance of the heliostat. We also show predicted yearly average concentrations.