The Backup Program of the Dark Energy Spectroscopic Instrument's Milky Way Survey

TL;DR

The Dark Energy Spectroscopic Instrument's Milky Way Backup Program efficiently collects spectra of millions of stars, extending stellar observations to brighter and more southern sources using minimal DESI observing time.

Contribution

This paper introduces the MWBP, a novel survey leveraging twilight and poor weather to expand stellar spectroscopic data beyond the main DESI survey.

Findings

Obtained spectra of ~7 million stars as of March 2025.

Covered over 21,000 square degrees of the sky.

Utilized less than 9% of DESI's observing time.

Abstract

The Milky Way Backup Program (MWBP), a survey currently underway with the Dark Energy Spectroscopic Instrument (DESI) on the Nicholas U. Mayall 4-m Telescope, works at the margins of the DESI Main surveys to obtain spectra of millions of additional stars from the Gaia catalog. Efficiently utilizing twilight times (<18 deg) and poor weather conditions, the MWBP extends the range of stellar sources studied to both brighter magnitudes and lower Galactic latitude and declination than the stars studied in DESI's Main Milky Way Survey. While the MWBP prioritizes candidate giant stars selected from the Gaia catalog (using color and parallax criteria), it also includes an unbiased sample of bright stars (i.e., 11.2 < G < 16 mag) as well as fainter sources (to G < 19 mag). As of March 1, 2025, the survey had obtained spectra of ~7 million stars, approximately 1.2 million of which are included in the DESI Data Release 1. The full survey, when completed, will cover an area of more than 21,000 square degrees and include approximately 10 million Gaia sources, roughly equal to the number of stellar spectra obtained through the DESI Main Survey, while only utilizing <9% of all DESI observing time.