Analysis of very large multi-reflector antennas like that of RATAN-600 radio telescope presents a formidable computational challenge. In this work, we develop and apply a comprehensive modeling approach based on the near- and far-field multilevel physical optics (MLPO) algorithms. The proposed combined near- and far-field MLPO has been applied for the analysis of the RATAN-600 antenna and shown to produce as accurate results as the regular PO-PTD, with a computation time proportional to N^2logN, with N being the problem electrical size.