The low sky coverage of ISO does not allow us to create an independent global model for the zodiacal light. Nevertheless, ISO can contribute to the study of the large scale structure of the zodiacal light in two ways. First, combining the new ISO measurements with the existing DIRBE all-sky maps, the ISO photometry can be used to cross-check and supplement the DIRBE data at many individual positions. This could refine our knowledge on the distribution of the extended sky brightness, possibly leading to a refinement of the DIRBE zodiacal light model, too. The advantages of the ISO data are their higher spatial resolution and the existence of observations within the September-December period when DIRBE was out of operation. The second way is to take advantage of ISO's good filter coverage in the 2.5-240/im wavelength range, and separate the main components of the infrared sky by their spectral characteristics. This spectral decomposition offers an independent way to determine the zodiacal light contribution in selected line-of-sights, and the results can be confronted with the zodiacal light extraction made by the DIRBE team. In the following subsections we investigate how realistic is it to combine the ISOPHOT and DIRBE databases; the spectral energy distribution of the zodiacal light will be discussed in Sect. 5.
2.1. Transformation between the ISOPHOT and DIRBE photometric systems
Surface brightness measurements with ISOPHOT and DIRBE may give different results on the same position due to differences in the photometric calibrations of the two instruments, and due to the fact that the ISOPHOT calibration has not been finalized yet. Combining DIRBE and ISOPHOT measurements requires therefore precise transformation relationships between the two photometric systems. In this subsection we create such relationships for 5 selected ISOPHOT filters, and check the accuracy of the transformation. We will use the ISOPHOT calibration files available in the PHT Interactive Analysis (PIA) package V7.3.2 . The transformation relations will have to be updated any time when a new version of the ISOPHOT calibration is released.
For the purpose of a systematic comparison of the two photometric systems we observed
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