Hi Sara

which detector do you use, is it under high vacuum!

I think the first step is to calibrate the instrument, then change the detector.

Good luck 

Gobara

Hi Gobarta

Yes I work in high vacuum ... Thank you for sharing your thoughts, I must ask for a calibration.

The value you are giving for Z, is it the stage (motor) position or the distance sample-pole piece? The first is a good value for a general orientation but does not help that much for the real positioning of the sample. It only tells you that you perhaps have no option anymore to move the stage and therefore the sample. In that case you need to mount the sample differently.

If it is the "focus" or sample-pole piece position you have to understand that this is only a predicted position of the beam focus based on your lens settings. In order to "sharpen" your sample image you change the settings of the lenses which moves the beam focus until your image becomes sharp. In light microscopy you do it vice versa. Since the focus is fixed you need to change the distance between sample and final lense. In SEM you don't do this usually since you can change the focus of the last lense. This focus-range depends on the the accelaration voltage. The  lower the kV the smaller the range (starting from the pole piece) and you have to go closer to the pole piece. If the system realizes that you want move the focus further, obviously in order to make a sharp image, it propably tells you that the required distance is out of limit. I am not sure whether this is the procedure used in your NOVA but it looks so. Simply use common imaging settings like 20kV. There you should have a focus which can vary between 0...~50mm or more.