Construction of the Southern African Large Telescope (SALT) was largely completed by the end of 2005 and since then it has been in intensive commissioning. This has now almost been completed except for the telescope's image quality which shows optical aberrations, chiefly a focus gradient across the focal plane, along with astigmatism and other less significant aberrations. This paper describes the optical systems engineering investigation that has been conducted since early 2006 to diagnose the problem. A rigorous approach has been followed which has entailed breaking down the system into the major sub-systems and subjecting them to testing on an individual basis. Significant progress has been achieved with many components of the optical system shown to be operating correctly. The fault has been isolated to a major optical sub-system. We present the results obtained so far, and discuss what remains to be done.