For the AXIS programme, a total of 36 XMM-Newton-Newton observations were selected for optical follow-up of X-ray sources, with a preference for those that were public early in the mission (mid 2000), or part of the SSC Guaranteed Time Program. The fields were selected having |b|>20°, total exposure time >15 ks, and excluding (optical and X-ray) bright or extended targets (except in two cases: A1837 and A399). All of these fields have been used for the study of cosmic variance, the angular correlation function and the determination of logN-logS in different bands (see details in Carrera F.J. et al. 2007, A&A 469, 27-46).
27 of the original 36 XMM-Newton-Newton observations from the AXIS programme were selected for optical follow-up of the X-ray sources with fluz > 2.E-14 erg/cm2/s. Two of those 27 fields (A2690 and MS2137) were later excluded from the main identification effort because they were far too south to be reached from the Nothern Hemisfere. 319 unique sources constitute the final XMS sample (see Barcons et al. 2007 A&A 476, 1191-1203) which is optically identified to completeness above 90%.
This SSAP service in the AXIS-SVO data centre hosts the optical spectra taken for the sources in the XMS sample.
More information about this survey can be found here.