Which critical component was either missing or installed in the wrong position within the faulty measurement instrument?

Answer

relay lens

A critical component, a relay lens, was either left out entirely or installed in the wrong position in the null corrector, causing all subsequent measurements of the mirror shape to be flawed.

Which critical component was either missing or installed in the wrong position within the faulty measurement instrument?

#Videos

FreeSchool Presents: Repairs to the Hubble Telescope - YouTube

HubbleSpacetelescopeopticsMirror