Final answer:
The technology that revealed faint structures in planetary nebulae is the use of telescopes and photographic plates.
Step-by-step explanation:
The technology that revealed faint structures in planetary nebulae is the use of telescopes and photographic plates. In the early days, telescopes were not well-suited to observe faint structures like planetary nebulae, but astronomers made heroic efforts to photograph the spectra of these objects. Through the use of telescopes and photographic plates, astronomers managed to capture the faint light emitted by planetary nebulae and study their structures in detail.