I read that the largest lens diameter of the LSST is 1.55m. Up to now, I knew that the largest lens was 1.25m for a stationary lens at the Paris 1900 Exhibition. The largest movable lens with similar atmospheric pressure environmental characteristics is the Yerkes telescope with 1.02m. There is the Swedish 1m Solar telescope with an aperture of 1.1m, stopped down to 98cm, but I don't take this as a reference point for the 1.55m LSST telescope and my question below because, as the Swedish Solar telescope information on their website explain, Solar telescopes are designed to handle the pressure differences inside the sealed tubes with either a vacuum or Helium inside. What I am interested in are similar pressure levels all around the lens, which I assume is valid for the LSST 1.55m and Yerkes 1.02m lenses.
OK, I am wondering how it was achieved to build the lens of the LSST so much larger than previous lenses which were hindered to be made larger by gravitational saging due to their own weight. What advances were made in glass material or lens support to jump from ~1m for a movable lens to ~1.5m without introducing obstruction?