Improve smearing help

[smk78]

A recent question from a user (reproduced below and answered by richardh) highlights a deficiency in the Smearing Functions section of the Help Documentation (in sm_help.rst): elaborate explantions of slit/pinhole smearing are given, but we don't actually explain what SasView does if a user loads data with a dQ component…

I have attempted to improve the introductory text but it needs more work and the input of someone who knows what SasView does.

From: Dabkowska, Aleksandra  
Sent: 03 January 2017 14:36
To: help@sasview.org
Subject: [Sasview-help] SASview: question about instrumental smearing
 
Hello, 

I am writing to ask about using SASview with dQ-containing data.
 
I have recently started to use SASview to analyze SANS data (of 
nanoparticles). The experiments were done in Munich and have 4 columns 
(Q, Intensity, dI and dQ).  However, when I set the fitting program to use 
instrumental smearing sometimes I get strange peaks in the fitted line 
(see attached). I have found the section on smearing in the manual but I 
am still struggling to understand where these peaks come from. I was 
wondering whether this is a problem with how I set up the model or my dQ 
data?   
 
Thank you for your help with this! 
 
Best regards, 
 
Aleksandra

For the benefit of the rest of the team – a summary of the answer that I 
have discussed with Aleksandra  -
 
Sasview was doing “as expected”. The data set was a concatenation of SANS 
data from several detector distances, where of course the worst Q 
resolution is next to the beam stop at each distance. (This is also 
noticeable in the residuals plot where there is poor overlap.) Sasview 
sensibly orders all the input points into increasing Q for nicer looking 
plots, however the “dQ data” can then vary considerably from point to 
point.  Using “no smearing” or “Custom pinhole smear” the fits look OK, 
but with “Use dQ data” smearing then “spikes” appear. 
 
Possible solutions include – remove the poor Q resolution data from 
shorter camera distances, or to fit the camera distances separately in a 
simultaneous fit.
 
Richard

[smk78]

In 8d2df05cc8c770af51d340dda4bdf2fe60d8a371/sasmodels:

Added note to smearing help. Closes #833