The in-flight characteristies of ISOPHOT-S are presented, in particular spectral response function, beam profile, dark current, straylight suppression, and wavelength calibration. Photometric calibration of point-like and extended sources is described. Sensitivity limits and the actual error budget are dis- cussed. The validation of the pipeline

As part of the ISO calibration programme, we observed the mid-infrared spectrum of the Zodiacal Light at low and high ecliptie latitudes, using the spectrophotometer ISOPHOT-S. In the 6 um to 12 um range the average spectrum can be reproduced with a T = 264 K blackbody. By combining this blackbody spectrum with photometry from COBE / DIRBE, we

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NLTE effects in iron lines are carefully investigated for the sunspot umbral model of Maltby et. al. (1986). Our model atom is realistic: it has hundreds of levels including many high-excited ones among which infrared transitions take place. The self-consistent solution of the kinetic and radiative transfer equations is obtained using recently