  Context: Current instrument developments at the largest telescopes worldwide
have provisions for Multi-Conjugated Adaptive Optics (MCAO) modules. The large
field of view and more uniform correction provided by these systems is not only
highly beneficial for photometric studies but also for astrometric analysis of,
e.g., large dense clusters and exoplanet detection and characterization. The
Multi-conjugated Adaptive optics Demonstrator (MAD) is the first such
instrument and was temporarily installed and tested at the ESO/VLT in 2007. We
analyzed two globular cluster data sets in terms of achievable astrometric
precision. Data were obtained in the layer-oriented correction mode, one in
full MCAO correction mode with two layers corrected (NGC 6388) and the other
applying ground-layer correction only (47 Tuc). Aims: We aim at analyzing the
first available MCAO imaging data in the layer-oriented mode obtained with the
MAD instrument in terms of astrometric precision and stability. Methods: We
calculated Strehl maps for each frame in both data sets. Distortion corrections
were performed and the astrometric precision was analyzed by calculating mean
stellar positions over all frames and by investigation of the positional
residuals present in each frame after transformation to a
master-coordinate-frame. Results: The mean positional precision for stars
between K = 14-18 mag is ~1.2 mas in the full MCAO correction mode data of the
cluster NGC 6388. The precision measured in the GLAO data (47 Tuc) reaches ~1.0
mas for stars corresponding to 2MASS K magnitudes between 9 and 12. The
observations were such that stars in these magnitude ranges correspond to the
same detector flux range. The jitter movement used to scan a larger field of
view introduced additional distortions in the frames, leading to a degradation
of the achievable precision.
