LECS and MECS misalignment calibrations have been performed using observations carried out in summer 1996. As is discussed in the Appendix, during this period BeppoSAX pointings have suffered several attitude reconstruction problems which resulted in a non accurate (~20 arcsec) spacecraft attitude data.
The use of such data is most probably at the origin of the non perfect calibration of LECS and MECS misalignment matrices, as indicated also by the fact that the attitude inaccuracy is of the same order of the residual misalignment of BeppoSAX NFI instruments.
Moreover, LECS and MECS misalignment calibrations have been performed independently from two distinct teams and this makes very unlikely an error in the procedure used to compute the misalignment matrices.
Two different procedures can be adopted to correct the NFI misalignment calibration: 1) a new computation of LECS and MECS misalignment matrices and 2) a correction of the NFI event files coordinates.
While the first solution seems the most natural one, it requires a greater effort and new dedicated BeppoSAX pointings. Moreover, a complete reprocessing of all BeppoSAX observations would be needed to produce a new complete archive of LECS and MECS images.
We have therefore decided to adopt the second option. With this procedure is in fact possible to correct very quickly all the present NFI images, and to include the correction in the BeppoSAX data processing software, without any degradation of the scientific results obtained.