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High resolution observations of the asteroids Iris and Juno have beenperformed by means of the UVES spectrograph at the ESO VLT to obtain theeffective accurac y of the spectrograph's radial velocity. The knowledge ofthis quantity has impo rtant bearings on studies searching for a variability ofthe fine structure cons tant carried on with this instrument. Asteroids providea precise radial velocit y reference at the level of 1 m/s which allowsinstrumental calibration and the recognition of small instrumental drifts andcalibration systematics. In particu lar, radial velocity drifts due to nonuniform slit illumination and slit optica l misalignment in the two UVESspectrograph arms can be investigated. The positi on of the solar spectrumreflected by the asteroids are compared with the solar wavelength positions orwith that of asteroid observations at other epochs or wi th the twilight toasses UVES instrumental accuracy . Radial velocities offsets in the range10--50 m/s are generally observed likely due to a non uniform slitillumination. However, no radial velocity patterns with wavelength are detectedand the two UVES arms provide consistent radial velocities. These resultssuggest that the detected alpha variability by Levshakov et al. (2007) deducedfrom a drift of -180 (+/- 85) m/s at z =1.84, between two sets of FeII linesfalling in the two UVES arms may be real or induced by other kinds ofsystematics than those investigated here. The proposed technique allows realtime quality check of the spectrograph and should be followed for very accuratemeasurements.

UVES radial velocity accuracy from asteroid observations. Implications for the fine structure constant variability