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Figure 1. ISOPHOT versus DIRBE surface brightness values. For each filter the beam, the heating power of the internal fine calibration source, and the standard deviation of the data points from a linear fit are given. At 150/xm the DIRBE values are computed via interpolation; in panel (a) the DIRBE 140/xm data are included in the interpolation, while in panel (b) they are not.

Figure 1. ISOPHOT versus DIRBE surface brightness values. For each filter the beam, the heating power of the internal fine calibration source, and the standard deviation of the data points from a linear fit are given. At 150/xm the DIRBE values are computed via interpolation; in panel (a) the DIRBE 140/xm data are included in the interpolation, while in panel (b) they are not.

with ISOPHOT a network of 30 dark fields covering the sky accessible by ISO (60° < e < 120°, where e is the distance from the sun). In these observations at least 7 and at most 13 filters were used in the 3.6-200 /xm wavelength range. The fields were carefully examined for low cirrus levels in the IRAS 100 /xm maps and do not contain infrared point sources. As far as possible, we also avoided stars visible on the Palomar Observatory Sky Survey in the beam. Absolute photometry mode was selected, in many cases including dark current measurements and checks for the zero level of the fine calibration source (FCS) measurements. All observations in a given filter were performed with the same aperture and FCS power.

The data was analysed with PIA V7.3.2 in the standard way, with no signal linearization. Surface brightness values were extracted from the DIRBE Weekly Sky Maps (nssdcftp.gsfc.nasa.gov/spacecraft_data/cobe/) for the dates/positions of the ISOPHOT observations, by taking the median of typically 66 pixels in a 1.5° radius field around the intended position. The ISOPHOT and DIRBE data sets were appropriately colour-corrected. To predict DIRBE values for the 7.3 and 150 mm ISOPHOT filters, interpola-

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