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The HyD detectors are extremely sensitive, which means that it is very possible that illuminating specimens with the laser powers you are used to using with the PMTs will saturate them, causing them to shut down so they aren't damaged. If you regularly load saved settings from the instrument parameters or from saved data you these will be suitable for an all PMT system but may well saturate the HyDs. You should try the following steps before starting your scan to avoid inadvertently saturating the detectors:
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