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The shuttle Discovery launched the Hubble Space Telescope (HST) on 24 April 1990. It is an f/24 telescope with a 2.4m primary mirror. Its five science instruments can obtain astronomical observations from the far UV to the IR. Every three years, it is visited by a team of astronauts for a Servicing Mission in which some necessary tune-ups, change of instruments and orbital boosters are performed.

At present, the on-board instruments are three cameras (FOC, NICMOS and WFPC2), one spectrograph (STIS) and three FGSs. These instruments are located in radial and axial bays on the satellite. During the first servicing mission in 1993, the Wide Field and Planetary Camera (WF/PC) and the HSP were replaced by the WFPC2 and the pseudo-instrument COSTAR (Corrective Optics Space Telescope Axial Replacement), respectively. COSTAR actually is an optical bench with a series of arms to which mirrors are attached to intercept the beam and correct the spherical aberration of the primary mirror (only FOC still uses COSTAR, all the new instruments correct internally for the aberration). During the second servicing mission in February of 1997 the astronauts replaced the FOS and GHRS with NICMOS and STIS. The FOC will be replaced during the next servicing mission at the end of 1999 with the ACS. Finally, during the 2002 servicing mission, COS will be installed in the COSTAR bay.

To point the telescope a pair of Guide Stars are observed with two FGSs. In the fine lock mode, the FGSs keep the telescope fix (with an RMS of 0.007") during the observation. The FGS that is not used for pointing can be used for astrometric observations.

WFPC2 has four CCDs, shaped as an L with a 2.5' length. With this camera, it is possible to make images between 1200 and 11000Å and a limiting magnitude of V ~ 28 (during "normal" operations, campaigns like the Hubble Deep Field have pushed the limit to I ~ 30). The FOC can take high resolution images in the UV with a field of view of 14'' x 14'' and a limiting magnitude V ~ 226.5. With a larger field and sensitivity STIS can also take UV images and spectra. STIS can also take images and two-dimen= sional spectra from 1150Å to 1.1µm. NICMOS has three cameras that can obtain images and spectra in the near IR from 0.8µm to 2.5µm and a limiting magnitude of K ~ 21 in a typical one orbit integration.

The FOS obtained low resolution spectra with its two detectors, one of which had a range further in the "blue" than the other. The GHRS was able to obtain high resolution spectra with R between 2000 and 80000, the latter with an echelle.

The following sections briefly describe the most used instruments. In the case of the WFPC2 also the calibration pipeline is described; while in the STIS section a description of the data path is included. More information and detailed descriptions can be found in the HST Data Handbook and in the WWW instrument pages. These extremely brief notes constitute a very succinct description of each instrument and its calibration (correct at the time they were written, but whose exactness will change when more information is known about the instruments and their data). The reader is urged to consult the instrument pages at and the HST data Handbook before conducting any serious work with HST data.

HST data is more complex than ground-based, in particular the data from NICMOS and STIS, as explained below, is more complicated than for the other instruments. It was decided, then, to spend more time explaining the "simple" HST data to let the readers become familiar with it and summarize the characteristics of these more complex datasets.

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