Focus Changes Remember to adjust the telescope focus for different filter thicknesses. The visiting observer must calibrate the focus change for each of the filters, and manually change the telescope focus at each change. Do not change the MOS camera focus to adjust for different filters. The cassegrain upper-end focus value unit is 22 micron. At the end of an exposure check in the Feedback window that the message 'successful exposure' has been written, and keep track of the exposure number for your observing log, and later transfer to tape.
Atmospheric refraction depends on the zenithal angle and occurs for all instruments.
However there is one reason why it is more critical here. One scans many channels and it is not unusual to spend more than 5 hours on a same object. During this time, differential refraction may cause the objects to drift on the detector. To avoid this make sure that the guiding is done in a similar color as your observations. The first one is the most basic but requires the osbervers to do more homework.