Photosphere is the the lair of the Demons for the first half of the game. Zeikfried and Alhazad brought Mother's coccoon here. Presumably what the Demons arrived to Filgaia in, as it has flight capabilities.
The Photosphere has a shielding device known as the "Soldelita" (which gives Photosphere its name) which blocks any access from intruders, and vaporising them in the process.
The interior of the Photosphere is full of technological traps and advanced security systems, during the day Rudy would acquire a new skill the place has several rooms sealed with doors of electronic locks .
During the passage Rudy discovers a new ARM, the Twin Orbs.Throughout the day we have followed Mother since her birth, The Quarter Kingths spend most of their time working out plans in the room with Mother's body being restructured.
After the fall of Belselk Mother gives the order that Boomerang would join the Quarter Knights, a decision that disappoints Zed, who waited anxious for an opportunity, and that goes against the will of Zeikfried that has antipathy for Metal Demon.
Asgard the Earth Golem destroys the Soldelita with the help of the Rune Drives. The Tear Drop powers the place on first entry, until Zeikfried (disguised) allows the trio to reclaim it. This lowers the surveillance and weakens Mother's powers.
Following Mother's defeat, the Photosphere sinks into the ocean and decays. Zeikfried is sent back to the underwater helm of Mother's throne room following the Gate Generator incident, and Mother's remains consume him.