NICMOS ("Near-infrared Camera and Multi-object Spectrometer") replaced the earlier GHRS, but has not been used since 2008 due to an unresolved malfunction.
The PCS ("Pointing Control System") is made up of instruments
of the following types:
FGS ("Fine Guidance Sensors") can also be used for astrometry.
The optical telescope assembly (OTS) is basically the telescope
itself up to the instruments, including the mirrors, baffles and
optical shields, locations for instruments, and the frame supporting
these in place.
HST was launched in 1990, serviced and upgraded
by astronauts (replacing instruments) a number of times,
the final occasion in 2009.
It remains in operation in 2025 with hopes of continuing
for a decade or more.
Initial instruments:
As launched, the optics were flawed and a system to compensate
called COSTAR (Corrective Optics Space Telescope Axial Replacement)
was developed and installed in 1993.
COSTAR required an instrument bay and HSP was removed to provide that space.
Each of HST's current instruments has a correcting mechanism built in
and COSTAR was removed in 2009.