Right from the very beginning Hubble has been designed to be a new type of mission - a permanent space-based observatory that could be regularly visited by the Space Shuttle and serviced. At the same time astronauts would be able to replace or upgrade outdated science instruments.
On board Hubble all the instruments are placed in the so-called radial instrument bay (in red) and axial bay (in yellow).
Hubble's complement of science instruments currently includes one camera, three imaging spectrographs, and fine guidance sensors (primarily used for astrometric observations).
Two new instruments are planned to be installed during the Servicing Mission 4: the Cosmic Origins Spectrograph (COS) and the Wide Field camera 3 (WFC3). The latter will replace the WFPC2, and COS will be installed in the instrument bay currently occupied by COSTAR, as this set of corrector mirrors for the first generation instruments is no longer required.
Hubble's position above the Earth's atmosphere means that these science instruments can produce high resolution images of astronomical objects. Ground-based telescopes can seldom provide resolution better than 0.5-1.0 arc-seconds, except for very short times under the very best observing conditions. Hubble's resolution is about 5-10 times better, or 0.05-0.1 arc-seconds.
|WFC3 ||Wide Field Camera 3 (installed during SM4)|
|COS ||Cosmic Origins Spectrograph (installed during SM4)|
|ACS ||Advanced Camera for Surveys (installed during SM3B)|
|STIS ||Space Telescope Imaging Spectrograph (installed during SM2)|
|NICMOS ||Near Infrared Camera and Multi-Object Spectrograph (installed during SM2)|
|FGS ||Fine Guidance Sensors (astrometry) (part of the original complement; individual units replaced during SM2, SM3A and SM4)|
|WFPC2 ||Wide Field and Planetary Camera 2 (installed during SM1; removed during SM4)|
|COSTAR ||Corrective Optics Space Telescope Axial Replacement (installed during SM1; removed during SM3B)|
|FOC ||Faint Object Camera (one of the original complement of instruments; replaced by ACS during SM3B)|
|FOS ||Faint Object Spectrograph (one of the original complement of instruments; replaced by NICMOS during SM2)|
|GHRS ||Goddard High Resolution Spectrograph (one of the original complement of instruments; replaced by STIS during SM2)|
|HSP ||High Speed Photometer(one of the original complement of instruments; replaced by COSTAR during SM1)|
|WFPC1 ||Wide Field and Planetary Camera 1 (one of the original complement of instruments; replaced by WFPC2 during SM1)|