- *Width of the field (in degrees) along RA*

[**Height**]{} - *Height of the field (in degrees) along DEC*

[**Mag**]{} - *Limiting visual magnitude (V-band) in one minute exposure, taking into account the telescope diameter[^7]*

[**JD\_START**]{} - *Julian date of the first image in the archive*

[**JD\_END**]{} - *Julian date of the last image in the archive*

[**Nr. imgs**]{} - *Number of images in the archive indexed in the given period*

Instrument Etendue and Archive Etendue
--------------------------------------

To characterize the efficiency of telescopes and instruments in survey work, the astronomers use the term [*etendue*]{} ($A\Omega$) defined as the product of the telescope collecting area $A$ (expressed in square meters) and the surface on sky of the imaging camera $\Omega$ (in square degrees). To characterize the data mining efficiency of entire instrument archives, we propose the term [*archive etendue*]{} ($A\Omega\,A$) defined as the product of the etendue and the number of science images included in the given instrument archive.

Archive Collections
-------------------

We briefly present the six image archive collections included in [*Mega-Archive*]{} by 23 Feb 2019.\
[**The Canadian Astronomical Data Centre (CADC)**]{} was established in 1986 to store Hubble Space Telescope (HST) and later Canada France Hawaii Telescope (CFHT) data. 