Someone will have to translate their writing for me. DM
Hi,
Basically too much trust/confidence in a very strict tolerances machine (tester) that turned out to be incorrectly assembled. Problem was detected with lesser machine but ignored (confirmation bias).
hand-polishing was also used .
Quotes below
https://en.wikipedia.org/wiki/Hubble_Space_Telescope#Flawed_mirror
its mirror needed to be polished to an accuracy of 10 nanometers (0.4 microinches,
0.01micron), or about 1/65 of the wavelength of red light.
Although it was probably the most precisely figured optical mirror ever made, smooth to about 10 nm (
0.01 micron, 0.4 μin), at the perimeter it was too flat by about 2,200 nanometers (
2.2 micrometers; 87 microinches).
The Allen Commission found that a reflective null corrector, a testing device used to achieve a properly shaped non-spherical mirror, had been
incorrectly assembled—one lens was out of position by 1.3 mm (0.051 in).
A few final tests, using the conventional null correctors, correctly reported spherical aberration. But these results were dismissed, thus missing the opportunity to catch the error, because the reflective null corrector was considered more accurate.
Instead, the fact that the mirror had been ground so precisely to the wrong shape led to the design of new optical components with exactly the same error but in the opposite sense, to be added to the telescope at the servicing mission, effectively acting as "spectacles" to correct the spherical aberration.
Hubble - Construction, The Hubble Mirror, Blurred Vision, Hubbles Contact Lens
Finally, highly skilled opticians using manually controlled tools and delicate hand-polishing techniques removed an additional 0.03 of an inch
( 762 micron ) from the mirror's surface
Hubble Space Telescope Optics System | NASA