A solar thermal concentrator system is proposed comprising a cylindrical heat-pipe receiver and a variable-focus-parabolic-trough (VFPT) reflector in which the focal length varies as a function of the vertical displacement of the incidence point relative to the horizontal centerline of the receiver. The light ray paths within the concentrator system are analyzed using a skew-ray tracing approach. A method is then proposed for optimizing the geometry of the concentrator system in such a way as to optimize the uniformity of the irradiance distribution on the heat-pipe surface. The validity of the proposed optimization method is demonstrated by means of ZEMAX/Solid-Works-Flow simulations. It is shown that the optimized VFPT concentrator yields a significant improvement in both the irradiance uniformity and the heating efficiency compared to conventional cylindrical-trough and parabolic-trough concentrators. (C) 2012 Elsevier Ltd. All rights reserved.