Models for SFX (CGI is more properly used for Computerized SFX) are shot one frame at a time on special cameras that can precisely align each frame so that they don't produce jitter when projected as a motion picture. Lucas produced a film about this, we had a seminar at EK about it, and several magazines at that time had lengthy articles about this. Sorry that you guys missed this. One mag was American Cinematographer.

BTW, if the model is moving rapidly, then single frame is impractical and so the moving ships were shot in real time, but the large animals on Hoth were shot single frame.