Could someone explain what the 'first order' diffraction peak is?
It depends on the context. Generally it simply means the first diffraction peak, from the http://en.wikipedia.org/wiki/Diffraction_formalism" [Broken].
If its the 'diffraction peak, to first order' that can mean something different. That could be the value of the first diffraction peak, according to a 'first order' approximation.
Thanks for your help! I also liked the article you referred to.
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