Why can't I properly merge different positions?

From my experience, when data don't merge properly after flat-file corrections there are mainly two reasons:

• FLUORESCENCE. We ahve already discussed about it and normally it shouws up as differences between individual channels or groups of channels i.e. the patterns don't match in several places. This does not seem the case, in my opinion (but do you know exactly what's inside Aspirin?)

• Background scattering from the air. When you move the detector the walls of the housing make a different shadow on the modules and at small angles if you have no beamstopper it could even be backscattering from the housing hit by the beam in the different positions. The only way of improving it is to properly place the beamstopper and to avoid air scattering before the sample e.g. by using a "nose" from the end of your flight tube to very close to the sample. Of course the problem is stronger at lower energies (more air scattering) and normally with the detector positioned at low angles (scattering from the housing, shadowing of the "forward" scattered beam). When you see this problem you could try to take data e.g. at 20-25 degrees instead of 5-10 degrees and see if it's still there. It could also be that the geometry of your housing with the flat window and shorter path inside the housing amplifies the problem, so that you should take special care of it with respect to the SLS where we have just a feww cm before entering the housing and then half a meter inside it.

• Your sample changes over time e.g. in case of radiation damage and long exposure times (usually several seconds).