Data Reduction Frequently Asked Questions

The IFU slice object traces show a not negligible shift. How can I improve the corresponding object traces alignment?


From time to time the IFU slice traces do not overlap. In such a case the alignment of the IFU slices may be improved by providing (in the sof) the optional static table tagged as IFU_CFG_TAB_<ARM> (ARM = UVB, VIS, NIR).

If the offset between slices is constant with wavelength the corresponding coefficients S_LOW_OFF or S_UPP_OFF need to be corrected in this table. The correction is defined by the shift one wants to correct multiplied by the size of the spatial bin one has chosen to reduce the data (by default 0.16/0.16/0.21 arcsec/pixel for UVB/VIS/NIR data, respectively).

If the offset between the slices varies with wavelength also the other coefficients provided in that table need to be changed. The coefficients controlling the slope are W_UPP_COEF1 and W_LOW_COEF1. The coefficients controlling the curvature are W_UPP_COEF2 and W_LOW_COEF2.

If the user has to change these parameters we recommend to first optimise the offset, then the slopes (judging if it needs to be increased or decreased and, in order to make small variations, starting from the second significant digit of the default value) and similarly eventually changing the curvature coefficients.

This correction can be determined by aligning the traces of the telluric standard observation usually taken in the same night. The table optimized this way should then be used as input of the (usually low signal to noise) object data reduction.