  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 ESA's 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.
