We have developed at CERN a cheap method for manufacturing accurate parabolic mirrors. These mirrors were needed for imaging to a ring the light generated by the Cherenkov radiation in the Ring Imaging Cherenkov detector (RICH) proposed by the DELPHI Experiment  at CERN. These mirrors are numerous (300). They are cut from a parabola 800 mm in diameter and have focal lengths ranging from 370 to 400 mm. The focal point needs to have a diameter smaller than 1.2 mm. They should have a good reflectivity in the UV region above 80% at 160 nm. A very good surface quality was required (an average roughness below 1 nm). In order to comply with these requirements and to produce these mirrors as cheaply as possible the conventional heat forming technique had to be thoroughly restudied and improved.