Scaling the multiple mirror telescope (MMT) to 15 meters — similarities and differences

With the benefit of experience gained from operation of the MMT, we are defining the characteristics of a much larger instrument. Scaling the MMT to 15m divides into two main aspects, one directly related to how one performs scientific observations, the other related to how one makes and keeps a telescope operating near its theoretical limits. In the observational area we discuss phasing for both interferometry and long wavelength imaging, matching images to solid state detectors, and multiple fiber and single object spectroscopy. In the more direct telescope area we discuss the mechanical characteristics of the scaled up structure, optimizing the diffraction pattern, optical and mechanical implementation of different foci, matching atmospheric demands on image quality, thermal control of the mirrors and structure, and mirror coating losses.