Saki's Cosmosphere, Level 4
A part of the same Cosmosphere as:
This one isn't optional, however, and you will end up getting this through the interaction of Sarapatra and Saki on stage, performing. So, technically, while the Cosmosphere is an optional part of the game to some degree, this specific scene isn't, and if you are progressing through them at a steady pace, you will get this soon enough.