Allan Rex Sandage

Sandage's distance-scale work generally favored a lower expansion rate than estimates promoted by astronomers such as Gérard de Vaucouleurs. The disagreement mattered because the Hubble constant determines estimates of the universe's expansion timescale and age.

After Hubble's death, Sandage carried forward observations of galaxies and the cosmic distance scale using major telescopes at Palomar and Mount Wilson. He refined the relationship between galaxy distance and recession velocity that underpins measurements of cosmic expansion.

Sandage studied quasi-stellar objects during the rapid development of quasar astronomy in the 1960s. His work included observations of unusually blue stellar-looking sources and helped expand investigation of active galactic nuclei.

Yes. Sandage used Hubble Space Telescope observations, including Cepheid variable measurements, to improve the extragalactic distance scale. This extended a research program he had pursued with ground-based observatories for decades.