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Hi Ranna, Yes, it is possible that the tip could be contaminated and that the contamination is dominating the measurement. There are several methods for cleaning the probe. Most involve indenting a soft material such as gold or rubber with a high force. --Bede
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Just learned about this product from Photonic Cleaning Technology today ( http://www.photoniccleaning.com/ ) Natalia cleaned this very dirty grating in about 15min. Optical images below: Before: After: