Don’t ever put any equipment with fans on your air table!
We just spent several days trying to track down the source of an image non-uniformity on our confocal. This was an intermittent problem that would manifest itself either as horizontal lines in the image of varying intensity, or sinusoidal image distortions. It occured after moving the microscope, and we first thought it was electrical interference. We spent an hour or two recabling the microscope without any luck. Next we checked the laser launch and its table for vibration, made sure the air table was floated properly … no luck, again. Finally, we realized that the arc lamp that used to be on its own shelf was now on the table with the confocal. Removing the arc lamp solved the problem – it turned out that vibration from the ventilation fan was coupled through the table to the confocal.
We’ve seen this problem with cameras as well, which is harder to solve since taking the camera off the microscope is not an option. Typically the solution is replacing the camera fan.