1. A method of forming a parabolic trough from an elongated strip of sheet aluminum or aluminum alloy comprises applying forces inducing equal bending moments to each lengthwise edge of the strip until the sheet has achieved a predetermined parabolic cross-section having a rim angle of at least 70.degree., said forces being the sole forces applied to the strip during bending, and heating the strip to a temperature above 400.degree.F. to substantially stress-relieve the metal while maintaining the strip in parabolic configuration.
2. The method of claim 1 wherein the heating occurs at a substantially constant temperature of from about 400.degree.F. to about 600.degree.F.
3. The method of claim 1 wherein the sheet metal is bent to a parabolic cross-section having a higher rim angle than the desired rim angle, the heating step is effected under conditions of time and temperature to only partially stress-relieve the metal, and the metal is cooled and relaxes to the desired rim angle.
4. The method of claim 3 wherein the heating conditions are controlled to permit relaxation of not greater than six percent of aperture of the trough.
5. The method of claim 1 wherein the parabolic cross-section of the trough has a rim angle of from about 70.degree. to about 115.degree..
6. The method of claim 1 wherein the parabolic cross-section of the trough has a rim angle of from about 90.degree. to about 105.degree..
7. The method of claim 1 wherein the trough has an aperture of from about 6" to about 24" and a length of from about 1' to about 12 '.
8. The method of claim 1 which comprises forming a flange along each long edge of the strip, and attaching bending apparatus to each flange prior to applying the bending moments.
9. The method of claim 1 wherein the strip is heated for less than about one hour.