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This would normally be done with the grating to be used for observation -- some gratings are not flat so you cannot guarantee the focus from one grating to another.
The prompts in FOCUS (at the SETUP mode) are self-explanatory. Use a widened slit ( 300 m) to spread the image over several pixels, for accurate line centering. Follow the steps in Section 4.4 (pink pages).
A collimator auto focus routine then occurs in which five collimator steps are taken. A centre-of-gravity line centre is found in order to determine the Hartmann shifts (it was found that the cross-correlation technique depended too critically on the correct arithmetical signal levels). The optimum focus is determined automatically. Finally, type READY to return to normal CCD operations.