ST has what is known as a Cassegrain design whereby the light picked up by the primary mirror is reflected forward onto a secondary mirror and then bounced back through a hole in the primary mirror  where it is focused onto the instrument detectors.

The HST has a range of detectors and filters that share the plane of focus:

ACS (Advanced Camera for Surveys) is a wide field, high resolution imager for a wide band of wavelengths from UVt o IR.   Its has many filters, a polarizer, a coronagraph and slitless spectrometry capabilities.   It was installed into HST in the Shuttle servicing mission in March 2002.

STScI ACS homepage 

The ST-ECF ACS slitless spectrometry project

WFPC2 (Wide Field and Planetary Camera 2) is a near-UV to far red general purpose CCD imager with broad and narrow band filters, polarizers and linear ramp filters.

WFPC2 homepage at STScI

NICMOS (Near Infrared Camera and Multi-Object Spectrograph) is a near-IR (0.8-2.2micron) imager, installed in February 1997, with three cameras (256x256 HgCdTe detectors) and coronagraphic, polarimetric and slitless grism spectrographic modes. It ran out of cryogen in January 1999, but has been revived in the SM3B servicing mission in March 2002.  

NICMOS homepage at STScI  

 NICMOS page at ST-ECF.

STIS (Space Telescope Imaging Spectrograph) is a long slit UV and optical imager and spectrograph, installed in February 1997, with resolutions 1000, 10000, 30000 & 11000 in the UV (photon counting MAMA detectors) and 800 & 7000 in the optical (CCD detector)

STIS stopped science operations on 2004-Aug-03, due to the failure of a power supply .  It awaits repair in the next servicing mission.

STIS homepage at STScI

STIS homepage at GSFC.

FGS (Fine Guidance Sensor) is an interferometric guidance system which can be used for high precision astrometry (measurements of stars)

FGS homepage at STScI