View source for /Background handling

Ideas for discussion were introduced with the following slides:
[[File:Background ideas.pdf|thumb]]

Notes:
* How do you know which type of (multiple coh/incoh.) scattering exists? And then which model for background to use accordingly? Are there approximations?
* always use multiple terms of the DWBA approximation? Where to draw the boundary?
* should we aim at always including a ray tracing approach (like McStas) to consider multiple scattering effects? (see union components McStas): https://mads-bertelsen.github.io/tutorial/Union_tutorial_1_processes_and_materials.html 
* this all would strongly affect fitting of GISANS - we guess that this is one reason that no fitting of GISANS exists at the moment?
* typical approach of subtracting background doesnt work if it comes from "the sample itself, i.e., by multiple scattering in the sample"
* can this be "tested" how much the sample affects the background, are there test samples for comparison?
* why not the "typical approach": you subtract the instrument background (has to be well known), and all other "background" has to be from the sample and has to be simulated/fitted? this has then to be known for all wavelength bands and incident angles
* comparison to QENS where signals are always weak - how is that handled? Can what is known from there be taken over? QENS: Start with approximations for the model, and this then has to be refined. Also there the precise knowledge of the sample and the estimations of multiple scattering involved have to be taken into account!
* measure multiple states / dispersions / other observables to decrease ratio (unkown parameters)/(measured parameters)
* how to judge if the simulation (even if fitting perfectly to the data) is the physical correct one? For the question of which parameters are influencing the cross section mostly using bayesian fitting: see papers from Josh (for reflectometry): https://journals.iucr.org/j/issues/2021/04/00/ge5096/ge5096sup1.pdf


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group 1

Notes:
Alexandros: going to a corner and measuring bkg

Philipp: this corner you go is below/or above the horizon?
Alexandros: below. 

Philipp: below the horizon, you have negative intensities. the corner will be different below or above the horizon. 

Alexandros: yes, because you have something that absorbs and scatters at the same time. 

Philipp: better to take it below the horizon instrument background.

Alexandros: problem we had usually with bare substrate, we had no other way to estimate bkg.

JF: the incoherent signal you get is not as constant in angle as you expect. 

SJ: in GISANS, closer to footprint, after 2 cm is all incoherent.
JF: and inelastic
SJ: instruments masuring reflectometry and GISANS, first you scan through angle (and identify critical edge).
then, use this approach to get average SLD of the sample. problem with this approach would be that no SANS instrument can use this approach. put sample in, first reflectometry (get SLD) and afterwards then GISANS.



intensity and wavelength 

Henrich: a SANS machine can do everything a reflectometry machine can do.
Philipp: can cover some angular range to see total reflection.

JF: Yes, but with poor resolution, for subtle variations of SLD this maybe not possible. we try to measure reflectivity curves when we do GISANS measurements.

SJ: is always worth it.