Think about a census. You could photograph every house in the country and produce a beautiful map, but without knocking on doors and asking questions, you'd know almost nothing about the people living in them. Astronomy finds itself in exactly this situation. Surveys like Euclid and the Vera Rubin Observatory will soon catalogue over 30 billion galaxies, an almost incomprehensible number. But converting those images into real scientific knowledge means measuring the light spectrum of each galaxy individually such as its redshift, its chemistry, its velocity. And that takes time, a lot of time. Installations like the Rubin Observatory will be cataloguing billions of galaxies (Credit : Vera Rubin Observatory) Right now, the best spectroscopic survey instrument on the planet is DESI, the Dark Energy Spectroscopic Instrument, operating in Arizona. It can capture the spectra of 5,000 objects simultaneously and has already built the largest 3D map of the universe ever made.…