Dark Energy Spectroscopic Instrument

Boston University is part of the Dark Energy Spectroscopic Instrument collaboration. DESI is a 5,000-fiber spectrograph on the Mayall 4-meter telescope at Kitt Peak National Observatory, and it has measured redshifts for more than 47 million galaxies and quasars — the largest three-dimensional map of the universe ever made.

What Boston University built

The 5,000 fibers sit in ten petals of 500. DESI’s focal surface curves, so no two sockets in a petal sit at the same angle: each one is precision machined to its own orientation, so that the fiber it holds points along the local normal to the surface.

Those petals were machined at Boston University. The fiber positioners were prototyped at BU as well, along with the jigging that held everything in register for the assembly carried out at the University of Michigan.

This is unglamorous engineering, and it sets a ceiling on everything downstream. A socket pointing a little off the local normal collects less light, and loses it before the spectrograph ever sees it. A positioner that cannot place its fiber to a few microns puts part of the wrong object — or part of the sky — onto a target subtending about one arcsecond. Neither error can be corrected later in software: the photons are simply not there. The precision of the map begins in a machine shop.

What the BU group measures

The growth of cosmic structure, the equation of state of dark energy, and the sum of the neutrino masses. The distances and ages behind those measurements can be worked out for any redshift with the cosmology calculator, which also gives the angular scale of the baryon acoustic oscillation ruler DESI measures.

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