interval, needed for comparison with next day crawling). Each instrument archive is searched at once, checking only the most recent data (ingested in the collection during the previous day). Most problems (possible mal-functions due to servers or internet connection) could be traced and corrected next day by the admin who checks the logs listing the operations with all instrumental archives crawled in all collections, but in practice such system errors very rarely happen.

Archive Comparisons
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As of March 2019, the [*Mega-Archive*]{} includes 111 instruments, listed in Table 1. The columns list several parameters, namely the [*archive etendue*]{} ($A\Omega\,A$), the field of view of the instrument (FOV), the archival date interval, and the total number of raw science images indexed to date. Using these data, we could compare the archives and assess their use for data mining.\
Figure 1 plots the histogram counting the total number of archives versus the number of images in each archive. The first three instruments are actually three surveys, namely CTIO1.3m (mostly 2MASS), VISTA and SDSS-DR9, followed by CTIO0.9m, INT-WFC and WHT-LIRIS.\
Based on the etendue ($A\Omega$), the most powerful survey facilities in the present [*Mega-Archive*]{} are Subaru-HSC, Blanco-DECam, VISTA-VIRCAM, Subaru-SuprimeCam, VLT-VIMOS and CFHT-MegaCam. These 