What was the required instrument for Hubble to successfully isolate and identify a faint Cepheid variable in Andromeda?

Answer

The 100-inch Hooker Telescope.

The task of spotting a single, flickering Cepheid star within the smudge of M31 required the unprecedented light-gathering power of the 100-inch Hooker Telescope located at the Mount Wilson Observatory.

What was the required instrument for Hubble to successfully isolate and identify a faint Cepheid variable in Andromeda?
measurementHubblegalaxyDistanceCepheid