Cirrus spectra of low surface brightness regions

We have studied the galactic cirrus in low surface brightness regions using ISOPHOT raster maps at 90, 150 and 180um. Observations are used to determine dust emission spectra and dust temperatures. The data extend to longer wavelengths than IRAS observation. Compared with DIRBE data the resolution and the sensitivity are better and this makes it possible to study faint cirrus emission in individual fields. We will discuss the calibration of the observations and present results of a comparison between ISOPHOT and DIRBE surface brightness values. The correspondence was found to be better than 30%. At 90um the ISOPHOT surface brightnesses tend to be slightly higher than the DIRBE values while at longer wavelength the situation is reversed. Surface brightness variations caused by cirrus fluctuations make it possible to determine the spectrum of the dust emission. Cirrus spectra were obtained for six fields with surface brightnesses in the range 1-2 MJy/sr after the Zodiacal light and the contribution of extragalactic sources have been subtracted. Assuming f^2 emissivity law the dust temperatures are in the range 18-20 K. Temperature variations can be seen even within individual fields. These values are higher than values from DIRBE but this can be explained by differences in the calibration.