Vera C. Rubin Observatory has passed a major construction milestone: the telescope structure is ready to be outfitted with a full-size replica of its 8.4-meter mirror and stand ins for its 3200-megapixel LSST Camera and other critical optical components. This major milestone means that Rubin Observatory is on track to help probe the mysteries of dark matter, study the fundamental nature of dark energy, document the dynamic Universe, and explore other grand challenges of cosmology.