sources. The CVF covers a spectral range from 5 to 16.5$\mu$m with a 1.5 PFOV and a spectral resolution of  50. Each integration step was composed of 12 images with 5.04 second integration time and during the CVF scan the wavelength step varied between 0.05 and 1.11$\mu$m. Details on the observing parameters are summarized in Table \[param\].

The data were analyzed with the CAM Interactive Analysis software (CIA[^2]). A dark model taking into account the observing time parameters was subtracted. Cosmic ray contamination was removed by applying a wavelet transform method [@Starck1997]. Corrections of detector memory effects were done applying the Fouks-Schubert’s method [@Coulais2000]. The flat field correction was performed using the library of calibration data. Finally, individual exposures were combined using shift techniques in order to correct the effect of jittering due to the satellite motions (amplitude $\sim$1). A deconvolution using multiscale resolution techniques [@Starck1999] was subsequently applied to estimate the physical size of the quasi-point like sources responsible for the infrared emission in our data (see section \[sec:results\]).

The details of the analysis of the ISOPHOT-S data of the three galaxies, which we also include in this paper for reasons of comparison, are published by @Rigopoulou1999.

Based on 