A solar collector including a linear receiver and an array of linear mirrors positioned to concentrate sunlight onto the linear receiver, with all of the mirrors in the array tracking solar motion in a direction perpendicular to the longitudinal axis of the receiver, a fraction of the mirrors tracking solar motion in a direction parallel to the longitudinal axis of the receiver and a two- axis steering mechanism adapted to combine the linear images formed by each linear mirror into a single linear image that remains focused on the linear receiver as the mirrors rotate about the two axes, with the linear receiver incorporating a secondary concentrator, and concentrated sunlight heating an atmospheric-pressure gas-phase heat- transfer fluid.