Off-axis sources

The analysis presented in the previous sections has pointed out that both LECS and MECS images are shifted in RA and Dec by an amount of ~20 arcsec, which corresponds to a rotation around the X and Y axes of the spacecraft reference frame.

The non perfect calibration of the misalignment matrices may also concern the rotation around the Z axis, the one co-aligned with the NFI instruments. Since this corresponds to a rotation of the X-ray images, the analysis of LECS and MECS on-axis sources is not useful to investigate this issue.

The residual misalignment around the Z axis is expected to have a similar amplitude to those around the X and Y axes (~20-30 arcsec). As the following simple argument shows, the effect of the Z misalignment on the image coordinates is negligible. If we consider a 30 arcsec rotation of the image around its center the effect on the coordinates of a source located at an off-axis angle of 30 arcmin is a very small shift (~0.3 arcsec).

We have verified this conclusion comparing the BeppoSAX X-ray and optical positions of a small sample of off-axis sources. In all cases we found that there is no need of a rotation of the image and that the RA and Dec coordinate correction described by Equations (1) and (2) corrects very well also the positions of off-axis sources. An example of a MECS3 image before and after the correction is shown in Figure 6.
 

Figure 6: Left: MECS3 image of the Seyfert galaxy Mrk 766 (target) and the BL Lac object ON 325 (NW source) before the RA and Dec correction described by Equations (1) and (2). The small circles indicate the optical positions of the two sources. Right: MECS3 image after the correction. The improved accuracy of the X-ray position of both sources is evident.