The original platform didn't include the Gregorian dome. The auxiliary cables were added in the mid 90s during a major upgrade that included the dome, adding a

The original platform didn't include the Gregorian dome. The auxiliary cables were added in the mid 90s during a major upgrade that included the dome, adding a considerable amount of weight. The choices were made based on funding and ability. The main platform, towers, and reflector was designed and constructed in 1960, long before our computing capabilities could have even conceived of being able to accurately deform the surface. Heck, the reason the dome is so massive is that it holds a secondary and tertiary reflector to convert the focal line (an artifact of the primary reflector being a spheroid instead of a parabola) into a focal point. Also, the 30-ton platform in FAST is the main limitation preventing FAST from being able to perform planetary radar science. It is too small and cannot support the weight necessary for a radar system of sufficient capacity.