Repeated imaging of selected areas of sky is used to discover the supernovae, then followup observations give their peak brightness, which is converted into a quantity known as luminosity distance (see distance measures in cosmology for details).
Repeated imaging of selected areas of sky is used to discover the supernovae, then followup observations give their peak brightness, and spectral lines of their light can be used to determine their redshift. The peak brightness is then converted into a quantity known as luminosity distance (see distance measures in cosmology for details).