source: sasview/src/sas/calculator/media/resolution_calculator_help.rst @ 78f02c3

..resolution_calculator_help.rst

.. This is a port of the original SasView html help file to ReSTructured text
.. by S King, ISIS, during SasView CodeCamp-III in Feb 2015.

Q Resolution Estimator
======================

Description
-----------

This tool is to approximately estimate the resolution of Q based on the SAS 
instrumental parameter values assuming that the detector is flat and vertical 
to the incident beam direction.

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How To
------

1. Select the source and source type (Monochromatic or TOF). Note that the 
computational difference between the sources is only the gravitational 
contribution due to the mass.

2. Change the default values of the instrumental parameters as desired.

3. The input formats of wavelength and its spread (=FWHM/wavelength) depend on 
the source type.For monochromatic wave, the inputs are just one values as shown 
with the defaults.For TOF, the min and max values should be separated by "-" 
to describe the wavelength band range. Optionally, the input of the wavelength 
(NOT of the wavelength spread) could be extended by adding "; --" where the -- 
is the number of the bins for the numerical integration. Otherwise, the 
default value "10" bins will be used. The same number of bins will be used 
for the corresponding wavelength spread in either cases.

4. For TOF, the default wavelength spectrum is flat. The custom spectrum file 
(with 2 column text: wavelength(A) vs. intensity) can also be loaded by 
selecting "Add new" in the combobox.

5. Once set all the input values, click the compute button. Depending on 
computation loads the calculation time will vary.

6. 1D and 2D dQ will be displayed in the text-box at the bottom of the panel. 
Two dimensional resolution weight distribution (2D elliptical Gaussian 
function) will also be displayed in the plot panel even if the Q inputs are 
outside of the detector limit. The red lines indicate the limits of the 
detector (if a green lines appear (for TOF), it indicates the limits of the 
maximum q range for the largest wavelength due to the size of the detector). 
Note that the effect from the beam block is ignored, so in the small q region 
near the beam block 

[ie., q<2*pi*(beam block diameter) / (sample to detector distance) / lamda_min] 

the variance is slightly under estimated.

7. The summary can be accessed by clicking the 'light-bulb' icon at the bottom 
of the SasView main window.

.. image:: resolution_tutor.gif

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Theory
------

The scattering wave transfer vector is by definition

.. image:: q.gif

In the limit of the small angle, the variance of q in the first order 
approximation is

.. image:: sigma_q.gif

In summary, the geometric and gravitational contributions depending on the 
shape of each factors can be expressed as shown the table.

.. image:: sigma_table.gif

Finally, we use a Gaussian function to describe the 2D weighting distribution 
of the uncertainty in q.

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References
----------
D.F.R. Mildner and J.M. Carpenter, J. Appl. Cryst. 17, 249-256 (1984)

D.F.R. Mildner, J.M. Carpenter and D.L. Worcester, J. Appl. Cryst. 19, 311-319 
(1986)