The `Hipparcos magnitude', designated as , is defined by the pass-band of the Hipparcos main detection chain, which ranges from 340 to 850 nm. This broad-band system yields magnitudes close to the visual magnitude , but shows residuals with respect to especially for red stars. For the prediction of the magnitude with the specified accuracy (necessary for the optimum allocation of satellite observing time, which varies strongly with magnitude especially towards the fainter stars), it was necessary to obtain both a magnitude and a colour index for each programme star. Transformation formulae were established to obtain from the magnitudes and colours available or newly observed in various photoelectric systems (Grenon 1988b). magnitudes given in the Input Catalogue have been derived using on-ground calibration of the detection chain and preliminary transformation formulae (which will be refined for the derivation of the magnitudes from the satellite data). An approximate relation valid for O-F stars and G-K giants in terms of the Johnson magnitude and colour index is:
For variable stars, further information is given in field 26, and in fields 39-44.