We present the first results of mid-infrared (MIR) ultra-deep observations towards the lensing cluster Abell 2390 using the ISOCAM infrared camera on-board ESAs Infrared Space Observatory (ISO) satellite. They reveal a large number of luminous MIR sources. Optical and near-infrared (NIR) cross-identification suggests that almost all 15 microns sources and about half of the 7 microns are identified with distant lensed galaxies. Thanks to the gravitational amplification these sources constitute the faintest MIR sources detected. We confirm that the number counts derived at 15 microns show a clear excess of sources with respect to the predictions of a no-evolution model. The possible extension of the NGST instrumentation from the near-IR (1-5 microns) to the thermal infrared, up to 20 microns (as suggested by the NGST task group report, October 1997) would permit study of this new population of dust-enshrouded AGN/starburst galaxies detected by ISOCAM, up to very high redshifts and with vastly improved spatial resolution. The existence of this population demonstrats that the discrimination of dust contributions, possible in the MIR, must be an important consideration in reaching an understanding of the Universe at high redshift. Therefore we stress that the access of NGST to the thermal infrared would increase tremendously its scientific potential to study the early universe.
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