Complex sentence: Ray decided to make the film on Bala, "the greatest Bharata Natyam dancer ever" according to him, to document her art for future generations with the "main value as archival".
Split into simpler sentences: Ray decided to make the film on Bala, "the greatest Bharata Natyam dancer ever" according to him, to document her art for future generations. The film was made with the "main value as archival".