What is the most important part on the Hubble Space Telescope?

Answer 1

There are 2 key pieces of electronics aboard HST that if they were to fail would essentially end HST's service life:

Fine Guidance Sensors (FGS) are arguably the single most key pieces of electronics on HST. The FGS's are what point HST and keep it locked in position while observations are being made, a difficult task considering that HST orbits the Earth every 96 minutes, and is subject to extreme thermal stresses every time it passes through the solar terminator (the line between darkness and sunlight in orbit).

Science Instrument / Command & Data Handling Unit (SI C&DH) - The very first item I worked on during my years with the HST program, the SI C&DH converts all science data from HST observations prior to relaying to the ground via telemetry. Without this key unit, data from Hubble could not be transmitted to the Space Telescope Science Institute in Maryland (HST's control center), and HST's function would be useless. There are 2 SI C&DH units, one primary and one backup. One failed just prior to the last servicing mission; it was delayed so a spare unit (there are always at least 1 or 2 flight units available on the ground, or if necessary an engineering unit can be made flight-ready) could be readied and the Astronauts could prepare for replacing this key unit.

Answer 2

The primary mirror is the most important part of the Hubble Space Telescope. It collects incoming light from celestial objects and directs it to the instruments for analysis and imaging. The mirror's precision and quality are crucial for the telescope's ability to capture clear and detailed images of the universe.