Objectives can hit the vessel holder when they are focused too high in the Z axis (up and down). This is a particularly a problem with the EVOS FL Auto Imaging System during instrument start-up, when the stage moves during system initiation, or when changing objectives. Coverslip-corrected objectives tend to be wider and flatter at the top of the barrel, which means that they are more likely to run into the edges of the vessel holder, particularly if you are imaging at the edges of the sample container. In those cases, use of that objective for those areas of the container may not be possible. If the objective if jammed by the vessel holder, then carefully unscrew the thumbscrews of the vessel holder and lift it straight off the stage, then move the objective downward in focus and toward the center of the stage. It is a good idea to have a shut-down procedure in your lab that includes moving the objectives to the lowest magnification and focusing downward with course focus prior to turning off the instrument for the day.
An objective can be damaged by scraping against the vessel holder. If this happens, take out the objective and examine it carefully for damage, particularly on the lens.
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