A detailed procedure for aligning the slit unit can be found in document INT-IAC-PRC004. As the INTEGRAL slit unit is similar to that of AUTOFIB2, the slit focus procedure is identical for both instruments.
Note, the WYFFOS optical design results in some field curvature. Hence it is not possible to obtain a similar focus for all parts of the chip, therefore a compromise value for the focus must be reached. Figure 11 shows an example of the de-focusing on the CCD and its effect on the measured FWHM.
Figure 11: Top: FWHM (in pixels) of arc lines over the CCD at the best focus position. Bottom: Best focus value (encoder units) over the CCD. These were obtained with the bundle sb2 (200 micron fibres) and the R1200B grating.
When the INTEGRAL slit assembly is in proper alignment within WYFFOS all science bundles have the same focus. Further details may be found in INT-IAC-TRP-009.