To fully interpret the Ronchigram, a good automatic phase reduction algorithm is necessary. A new phase reduction algorithm, originally designed for interferometry test of large optics, is presented for the Ronchi test. Due to the common path property, only two Ronchigrams shifted by pi/2 are necessary for the reproducing phase. Accuracy can be better than one-thirtieth of the grating space. Methods are suggested for finding spherical aberration and astigmatism without integrating the phase using the Ronchi test. Tests of results using the new algorithm show good agreement with typical interferometry tests.