@Salsbury. I have been testing an aspheric component without a custom null, so I am getting retrace error in my results and I need to calibrate the error out.
What method is preferred for calibrating retrace error? I have read regarding a random steel ball calibration process, and I have done this in the past. However, I have recently read two papers stating that a ray trace method , or a point characteristic function (PCF) method  may be preferable. However, my concern with both of these methods is that they seem more suited for a custom or ‘home-built’ interferometer, as opposed to a commercial system which I am using. Any thoughts would be much appreciated!
 Dong Liu, Yongying Yang, Chao Tian, Yongjie Luo, and Lin Wang, “Practical methods for retrace error correction in nonnull aspheric testing,” Opt. Express 17 , 7025-7035 (2009)
 He Yiwei, Xi Hou, Quan Haiyang, and Weihong Song, “Retrace error reconstruction based on point characteristic function,” Opt. Express 23 , 28216-28223 (2015)