Launched in 1990, the NASA/ESA Hubble Space Telescope remains the premier UV and visible light telescope in orbit. With well over 1.6 million observations from 10 different scientific instruments, the ESA Hubble Science Archive is a treasure trove of astronomical data to be exploited.
European Space Agency, Geisler et al., 2001, 'The Metallicity Distribution of the Globular Cluster Systems of Giant Elliptical Galaxies', 1.0, European Space Agency, https://doi.org/10.5270/esa-yxanysq