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--- processing:backgroundgeneral [2019/09/04 18:50] – smerkel
+++ processing:backgroundgeneral [2019/09/05 14:55] (current) – smerkel
@@ Line 23: / Line 23: @@
 The diffraction from the //sample grains//, that give rise to well-defined diffraction spots are removed and **//do not contribute//** to the median image!
-Creating an average image can be done with ''timelessMeanFileSeries'' which is part of the [[software:timelesstools|TIMEleSS tools]]. Creating the median can be done with ''median.py'' which comes with [[software:fabian|Fabian]].
+Creating an average image can be done with [[xray_data:averagemanyedf|timelessMeanFileSeries]] which is part of the [[software:timelesstools|TIMEleSS tools]]. Creating the median can be done with ''median.py'' which comes with [[software:fabian|Fabian]]. You can also calculate median and average images with [[software:imagemath|ImageMath]] but we have not used it in a while. There is an issue with file formats.
+The median image is less affected by outliers than the average and is probably a better representation of your background. For some purposes you might still need to use the average image.
+===== Testing and subtracting the background =====
+The most efficient way is to test your background with [[software:fabian|Fabian]]. You can
+  * load an example diffraction image,
+  * select ''Image'' > ''Correction'' > ''Background image''
+  * select ''Image'' > ''Correction'' > ''Subtract background''
+You will be able to visualize the effect you the background subtraction and whether your evaluation is satisfactory.
+Typically, subtracting the ω-median from a single ω-step diffraction image will be efficient at removing
+  * noise, from the detector, the source, etc
+  * diffraction from anything within the path of the x-ray beam, which is not your sample, as long as it is in the form of a powder,
+  * diffraction from the smaller sample grains (i.e. a fine matrix), that does not show up as well-defined diffraction spots but rather continuous diffraction rings.
+It will not work, however, for complex environments such as diamond anvil cells for which the diamond anvils give rise to strong and well defined diamond diffraction spots.
-The median image is less affected by outliers than the average. Therefore, if you have strong diamond peaks in some of the images, you will probably still see them also on the average image while there are none in the median one. Because of that, for subtracting the background the median image should be preferred. However, for other purposes you might still need the average image.
 ===== When to subtract the background? =====