Program: GN-2020B-Q-409

One of the most exciting results in modern cosmology has been the discovery of the acceleration of the Universe. It has been interpreted as the consequence of either Dark Energy or the modification of general relativity. The need to better understand the nature of the acceleration has led to the development of ESA's Euclid and NASA's WFIRST space telescopes (to be launched in 2022 and ~2025 respectively). Tight spectro-photometric requirements to better than 1.5% across the full survey are vital for the surveys' principal science goals as well as legacy science. Extreme care must be taken to control systematic errors and biases. We propose to use GMOS spectroscopy in poor weather conditions to establish a common set of 5 stable, high S/N white-dwarf spectro-photometric standards for both missions, including 2 extremely red and stable extra-galactic sources. They are located near the ecliptic north pole and are always visible for both spacecraft. Confirmed white dwarfs and the characterized two red sources will then be subject to follow-up HST grism spectroscopy to establish them as absolute spectro-photometric flux calibrators. This proposal aims to verify just a single WD before the field becomes unaccessible for 4 months.