When Hubble was built, there was a defect in the mirror that caused the images to be blurry. Astronauts then went to install extra optics on the telescope to fix the blurriness. What did these extra optics look like? Are there any pictures of them before and after they got installed onto Hubble?
The original "fix" was an package called COSTAR (Corrective Optics Space Telescope Axial Replacement). Externally it just looked like a big silver box.
(Image credit - NASA)
In order to install it, one of the 4 original axial instrument boxes - containing the High Speed Photometer - had to be removed so that COSTAR could take its place.
During the same servicing mission (STS-61) another instrument, the Wide Field and Planetary Camera (WFPC) was replaced with WFPC 2 which had corrections built in. COSTAR corrected the optics for the remaining axial instruments, the Goddard High Resolution Spectrograph, the Faint Object Spectrograph, and the Faint Object Camera.
Future instruments that were replaced included built-in corrections, so COSTAR was eventually removed and replaced by the Cosmic Origins Spectrograph, regaining that axial slot for an instrument.
So COSTAR now resides in the National Air and Space Museum. There you can see the deployed mirrors that were inserted into the light path to correct the aberration.
(Image credit - NASM)
This schematic shows how the mirrors were deployed from the big silver box.
Here is an orientation rendering from the NASA visualization tool DOUG, showing COSTAR in an axial instrument slot and the WFPC 2 "popped out".