In solar tower plants, where a rotationally symmetric field of heliostats surrounds the tower, an axisymmetric secondary concentrator such as a compound parabolic concentrator (CPC) or a tailored concentrator or a cone is the obvious choice. For locations at higher latitudes, however, the reflecting area of the heliostats may be used more efficiently if the field of heliostats is located opposite to the sun as seen from the tower. Then the field is asymmetric with regard to the tower. In the case of an asymmetric field, an axisymmetric concentrator necessarily has a concentration significantly lower than the upper limit. Furthermore, the area on the ground from which a tilted axisymmetric concentrator accepts radiation is an ellipse, including also heliostats very distant to the tower producing a large image of the sun. For these reasons we investigate asymmetric secondaries. From the shape of the edge ray reflectors constructed for rays in the central south-north plane we conclude that a skew cone reflector might be appropriate for the field, and optimize its free parameters by means of ray tracing. Asymmetric concentrators may increase the concentration by up to 25% at the same efficiency compared to optimized axisymmetric CPC or cone reflectors.