The geometric concentration factor A/B for an optimally designed trough type solar collector with flat mirrors and an acceptance angle d is shown to equal sin(d + (2n+l)a)/sin(d + a), where a = mirror angle and n = number of reflections. The number n is given by n = Int((90-d)/(2a)) and the mirror width R by R/B = (A/B - 1)/(2 sin a). It is shown that The theoretic maximum concentration factor of such a trough equals 1/sin d, i. e. the theoretic maximum concentration factor of any trough. The formulae have been used to design different "cornet" type solar concentrators, and one of these is described. Collector performance taking mirror reflectivity and absorber glazing transmittivity into account has been calculated for rays incident perpendicular to the aperture. Preliminary results from a study of concave absorber glazing transmittivity are presented and suggest lower losses than flat glazing in several instances.