Aims. PRONAOS is a stratospheric balloon-borne project dedicated to submillimeter astronomy. Sensitive to low emission gradients, PRONAOS allowed pointed observations in four photometric bands between 200 µm and 1.2 mm with an angular resolution of 2 to 3.5 . We present here the PRONAOS calibration strategy and the accuracy achieved. Methods. An absolute calibration scheme based on an Internal Calibration System provided original photometric data. The accurate determination of the spectral

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... dence of the measured fluxes is a key to the understanding of the cold universe. Results. We demonstrate that the PRONAOS absolute photometric accuracy is 8% (1-σ) and that the channel to channel relative accuracy is 6% (1-σ). This calibration is checked against observations of Saturn. We show that the PRONAOS calibration is in good agreement with the present knowledge of the Saturn submillimeter emission. PRONAOS observations also agrees with the observations of several compact and extended sources by the COBE/DIRBE or the ISOPHOT instruments. Appendix A is only available in electronic form at http://www.edpsciences.org However, these techniques increase the calibration and operational complexity. Second, the dimensions of optical components are not very large compared to the wavelength. Large diffraction effects are difficult to model or measure but need to be characterized for the determination of the optical efficiency and the far sidelobes. These two physical effects contribute to make submillimeter photometry among the least accurate astronomical measurements. The accuracy of absolute submillimeter calibrations is generally not better than 10%, and often much poorer, leading to poor photometric standards in this range. An important exception is the FIRAS instrument on the COBE satellite, which achieved an excellent absolute calibration accuracy of 3% (Fixsen et al. 1997) thanks to space conditions and the use of movable external blackbody filling the whole beam. However, FIRAS had a very small collecting optics diameter (less than 15 cm) giving a poor angular resolution (7 • ) and this method cannot be used for a large diameter telescope with better angular resolution, such as PRONAOS . PRONAOS ("PROgramme National d'AstrOnomie Submillimétrique") is a stratospheric balloon borne project Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.