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great progress in the understanding of physical properties of galaxies has been achieved with mid - infrared ( mir ) and far infrared ( fir ) observations using the iso satellite ( kessler et al . , 1996 ) and its isocam camera ( cesarsky et al . , 1996 ) .
deep optical surveys , such as the hubble deep field ( williams et al . , 1996 ) revealed a new population of distant sources at high - redshift ( @xmath9 ) either using ` drop - out ' techniques ( steidel et al . , 1996 ) or `
photometric redshift ' ( _ e.g. _ lanzetta et al . , 1996 , connolly et al . , 1997 )
yet optical surveys may miss a whole class of high - redshift dust - enshrouded galaxies ( blain & longair 1993 ) .
in particular the progenitors of today s e / s0 galaxies at @xmath102 may emit an important fraction of their total light in the nir , mir , fir and sub - millimeter regions ( _ e.g. _ franceschini et al . , 1994 ) .
indeed , galaxy formation models ( _ e.g. _ arimoto & yoshi 1987 , guiderdoni et al . ,
1997 ) predict that galaxies in their forming phase are heavily obscured by abundant gas and dust inside the system .
these issues have motivated very deep isocam observations of blank fields ( rowan - robinson et al .
, 1997 ; aussel et al . , 1997 ; aussel et al . , 1998 ;
dsert et al .
, 1998 ; taniguchi et al . , 1997 ) , as well as sub - millimeter observations ( smail , ivison and blain 1997 ; blain 1997 ; barger et al . , 1998 .
; hughes et al . , 1998 ) .
the isocam - hdf observations ( oliver et al . , 1997 ; aussel et al . ,
1998 ) showed that a non - evolving model at 15@xmath0 m is ruled out at 3@xmath11 .
analysing galaxy counts over a wide range of sensitivity , elbaz et al . , ( 1998 ) noted a change of slope in the counts observed at the mjy - level .
this variation can not be explained by a simple evolutionary model , but requires stronger star - formation activity for galaxies with @xmath12 .
very deep isocam observations have been conducted through gravitationally lensing clusters as part of a large programme executed in the iso science operations team guaranteed time ( the iso arcs programme ) .
the clusters used were a370 ( metcalfe et al .
, 1997 , 1999 ) , a2218 ( altieri et al . , 1998a ; metcalfe et al . , 1999 ) ,
ms2137 , cl2244 and a2390 .
further to a central goal of the arcs programme , fainter detection thresholds were reached in a given observation time , thanks to gravitationnal amplification .
we report here key results obtained for a2390 .    by pushing isocam to its ultimate limits with the help of gravitational lensing
, we performed ultra - deep observations through the core of probably currently the best studied lensing cluster : abell 2390 ( @xmath13 ) , to obtain a magnified view of the background sky .
this both increases the sensitivity of our mir maps and reduces the effects of source confusion .
these new observations supersede in area and depth the previous isocam observations of this cluster ( lmonon et al . , 1998 ) .    in section 2 the observations and data reduction techniques
are discussed .
photometry , source identification and counts are presented in section 3 .
results are discussed in section 4 .
througout this paper , we used @xmath14 , @xmath15 and @xmath16 km / s / mpc .
the esa guaranteed time observations reported here were allocated one full iso revolution ( science window of 16 hours ) , between december 26 and 29 , 1997 .
we used the best observational strategy for detection of faint sources , based upon in - flight experience .
in particular , the field was observed in 4 consecutive revolutions in 4 blocks of 4 hours . the allocated time was equally divided into the 2 broad - band isocam filters lw2 ( 5 - 8.5@xmath0 m ) , centered at 6.75@xmath0 m , and lw3 ( 12 - 18@xmath0 m ) , centered at 15 @xmath0 m .    the observational strategy , that reaches the ultimate sensitivity ( altieri et al . , 1998b ) ,
is performed by rastering the 32@xmath632-isocam detector array in microscanning mode . the pixel - field - of - view ( pfov ) of 3@xmath17   per pixel
was chosen to obtain high spatial resolution at the expense of signal , but obtaining a better sampling of the psf , crucial for source cross - identification .
the raster step size was 7@xmath17   ( 2.33 pixels ) , the minimum value to step out of the psf fwhm in two consecutive pointings .
the size of rasters was 10@xmath610 , so that , in the central part of each single raster , 100 different detector pixels sampled each sky pixel .
such redundancy was a key factor in isocam deep observations .
the final maps cover a field size of 2.6@xmath62.6.    .log of cam observation of the cluster - lens a2390 .
@xmath18 esa gt data , @xmath19 lmonon et al . , ( 1998 ) data . [ cols="<,<,<,^,^,^",options="header " , ]      thanks to our high - resolution images we have been able to unambiguously identify almost all the mir sources with counterparts in deep nir and optical ( hst / wfpc2 and ground - based ) images . the relative astrometric accuracy is found to be better than 1@xmath17   in both filters . in only a few cases
we suspect that two sources are blended .
there is one obvious case in the 7map , where the _ straight arc _ ( pell et al . , 1991 )
is blended with the nearby elliptical galaxy .
30 sources are detected at 7 .
half of them are easily identified as cluster member galaxies ( pell et al . , 1991 , leborgne et al . , 1992 , abraham et al . , 1996 ) .
the 5 - 8.5emission of the cluster galaxies corresponds to 4.5 - 6.9restframe emission . for e / s0 galaxies
it corresponds mostly to the rayleigh - jeans tail of their old stellar population as in the virgo cluster ( boselli et al . , 1998 ) .
two stars are identified , and at least ten sources are lensed distant galaxies .
these lensed sources are all detected at 15 .    at 15 ,
34 sources are detected in the central 2.25@xmath62.25 field .
only three sources are identified as cluster members : the cd galaxy ( lmonon et al . , 1998 , edge et al . , 1998 ) and two star - forming galaxies .
based upon spectroscopic or photometric redshifts , all the other sources are identified as faint lensed galaxies .
almost all sources for which we have spectroscopic redshifts are background objects . although , we can not rule out some of the targets being in the cluster , the probability is very small .
the detection of almost exclusively background sources in the cluster images demonstrates that at 15the cluster - core becomes _ transparent _ ( as in sub - mm / uv bands ) .
therefore the key feature is that the cluster - core acts as a natural gravitational telescope amplifying the flux of background sources , typically by a factor of 2 .      by correcting for the lens magnification and surface dilution effects , contamination by cluster galaxies , and non - uniform sensitivity of our maps
, we can derive number counts at 15to compare with _
blank _ sky counts ( e.g. in the hubble deep field and lockman hole ) .
the 7number counts are more difficult to derive due to the larger contamination by the cluster and because of the small number statistics .
the number density of sources is high with respect to the size of the fwhm ( @xmath20 6@xmath17   diameter at 15 ) , but the psf is well sampled on the final maps , and its shape can be used to separate the sources ; only two 15sources lie at the location of pairs of suspected high - z galaxies .
our counts are not significantly affected by confusion .
the occasional blending of the sources has not been taken into account , but the surface area occupied by bright sources is subtracted for the computation of the surface density of the fainter ones ( ie . other faint sources could be hidden by brighter ones ) .
we used the completeness of the detection at 15given in 
this correction is negligible to the 7counts and was not applied .    due to the non - uniform sensitivity of our maps ( because of observation strategy ) and the lensing effect ,
the object density per flux bin was computed using gain - dependant surface areas , and only the central 2@xmath62 area was taken into account for the faintest fluxes .
a detailed lensing model of a2390 has been produced by kneib et al . , ( 1998 ) .
the lensing acts in two ways on the background population of galaxies : + _ i ) _ an amplification of the source brightness , typically by a factor of 2 , but up to 10 near the caustic lines .
+ _ ii ) _ a surface dilation effect of the area probed , which itself depends on the redshift ; the space dilation is stronger towards the centre ( core of the cluster ) and increases with source - plane redshift .    to estimate these factors we used : the spectroscopic redshift for 7 objects ( pell et al . , 1991 , bzecourt & soucail 1997 ) , and
for the rest we use the best redshift estimate obtained with photometric redshift techniques ( pell , private communication ) , and/or lensing inversion techniques ( kneib et al . , 1998 ) . by analysing the case with all background galaxies at a mean redshift @xmath21
, we checked the dependence of the results on redshift uncertainties .
the source counts , corrected for cluster contamination and lensing effects , in both the lw2 and lw3 bands , are presented in figs . 3 and 4 .
we have used only 5@xmath11 sources ( i.e. @xmath2060 @xmath0jy before lensing amplification correction ) . according to hogg & turner ( 1998 ) ,
this is sufficient to avoid the positive flux - estimate eddington bias which occurs for faint source counts .    at 7 ,
we find that the source density is greater than that of taniguchi et al . , ( 1997 ) by a factor of 2 .
we suggest that their observations are incomplete below 50@xmath0jy , whereas our 7map is 80% complete down to 30@xmath0jy .    at 15the number counts
are compatible with the results of aussel et al .
( 1998 ) in the hdf .
however , with the help of gravitational lensing we are able to extend the counts in both bands down to 30@xmath0jy . putting all background sources at @xmath22
only slightly decreases the faintest counts , at the faintest end , because a few suspectedly high - z faint sources would be less amplified .
this shows the small dependency of our derived counts on the models .
the very small positive magnification bias in the lensing - uncorrected counts is consistent with the measured slope of the source counts , while the general shift of the points towards higher flux follows from the loss of the lensing gain .
the 15 @xmath0 m counts show a steadily increasing excess ( by more than a factor of 10 ) with respect to the prediction of a no - evolution model ( dotted line , franceschini et al . ,
this confirms the steeper count slope below 1  mjy found on the lockman hole ( elbaz et al . , 1998 ) and is in good agreement with the iso hdf counts ( aussel et al . , 1998 ) .
the counts are a factor 2 - 3 higher than the boundaries of the counts coming from an early analysis of the background fluctuations in the isocam 15map of the hdf ( oliver et al . , 1997 ) .
in particular , we do not seem to detect any sign of flattening of the counts at the faintest levels , as expected from evolutionary models ( franceschini et al .
, 1997 , oliver et al . , 1997 ) .
the slope stays close to -1.5@xmath29 down to 30 @xmath0jy .
this source density at faint levels favours extreme evolution models , needed to fit the counts at brighter fluxes , as shown by elbaz et al .
( 1998 ) .    integrating the number counts over the whole flux range and extrapolating the counts we find that respectively @xmath30 and @xmath31 wm@xmath4sr@xmath8 is emitted at 15and 7 .
this implies that a larger fraction of the uv / optical background is re - radiated in the mir than in the local universe .
the absolute astrometry of mir sources is difficult in deep surveys because of the lack of any obvious optical - nir counterparts . in the a2390 field ,
unambiguous cross - identification of more than 90% of the sources was possible thanks to a large density of sources and a good sampling of the psf .
a number of these sources are well correlated with faint galaxies in the visible , some of them having very red colors in the nir .
15isocam deep imaging is a good way to select star - forming galaxies and dusty agns which are not easy to identify in uv / optical surveys .
a more detailed analysis of the sed of these mir detected galaxies will be necessary to determine the nature of these sources and give an estimate of their sfr .
our observations confirm that abundant star formation activity occurs in very dusty environments at @xmath32 .
great caution must therefore be taken to infer global star formation activity based only on uv - continuum or optical luminosities of high - z galaxies .
the global star formation history can be traced fully only if the effect of dust is taken into account in a consistent way .
( _ e.g. _ blain et al . , 1998 ) .
further , detailed analysis of our selected sample is needed to unveil the nature of these mir galaxies .
we present results of ultra - deep isocam observations through a cluster - lens at 7@xmath0 m and 15@xmath0 m with the infrared space observatory ( iso ) satellite . 
 these observations reveal a large number of luminous mid - infrared ( mir ) sources . 
 cross - identification in the optical and near - infrared ( nir ) wavebands shows that about half of the 7@xmath0 m sources are cluster galaxies . 
 the other 7@xmath1 m and almost all 15@xmath0 m sources are identified as lensed distant galaxies . 
 thanks to the gravitational amplification they constitute the faintest mir detected sources , allowing us to extend the number counts in both the 7 and 15bands . 
 in particular , we find that the 15counts have a steep slope @xmath2 and are large , with n@xmath3 arcmin@xmath4 . 
 these numbers rule out non - evolutionary models and favour very strong evolution . down to our counts limit 
 , we found that the resolved 7and 15 background radiation intensity is respectively ( 2@xmath50.5)@xmath610@xmath7 and ( 5@xmath51)@xmath610@xmath7 wm@xmath4sr@xmath8 .