Changeset 3d40839 in sasmodels for doc/guide


Ignore:
Timestamp:
Oct 28, 2017 9:28:22 PM (7 years ago)
Author:
Paul Kienzle <pkienzle@…>
Branches:
master, core_shell_microgels, magnetic_model, ticket-1257-vesicle-product, ticket_1156, ticket_1265_superball, ticket_822_more_unit_tests
Children:
0db85af
Parents:
e964ab1
Message:

update orientation docs

File:
1 edited

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  • doc/guide/orientation/orientation.rst

    re964ab1 r3d40839  
    88or orientation in a magnetic field. 
    99 
    10 In general we first need to define the mean, or a reference orientation 
     10In general we first need to define the reference orientation 
    1111of the particles with respect to the incoming neutron or X-ray beam. This 
    1212is done using three angles: $\theta$ and $\phi$ define the orientation of 
     
    2929 
    3030    Definition of angles for oriented elliptical cylinder, where axis_ratio 
    31     b/a is shown >1, Note that rotation $\theta$, initially in the $xz$ 
    32     plane, is carried out first, then rotation $\phi$ about the $z$ axis, 
     31    b/a is shown >1, Note that rotation $\theta$, initially in the $x$-$z$ 
     32    plane, is carried out first, then rotation $\phi$ about the $z$-axis, 
    3333    finally rotation $\Psi$ is around the axis of the cylinder. The neutron 
    3434    or X-ray beam is along the $z$ axis. 
     
    4242Having established the mean direction of the particle we can then apply 
    4343angular orientation distributions. This is done by a numerical integration 
    44 over a range of angles in a similar way to polydispersity for particle size. 
     44over a range of angles in a similar way to particle size dispersity. 
    4545In the current version of sasview the orientational dispersity is defined 
    4646with respect to the axes of the particle. 
    4747 
    48 The $\theta$ and $\phi$ parameters to orient the cylinder only appear in the 
    49 model when fitting 2d data. On introducing "Orientational Distribution" in 
     48The $\theta$ and $\phi$ orientation parameters for the cylinder only appear 
     49when fitting 2d data. On introducing "Orientational Distribution" in 
    5050the angles, "distribution of theta" and "distribution of phi" parameters will 
    5151appear. These are actually rotations about the axes $\delta_1$ and $\delta_2$ 
    5252of the cylinder, the $b$ and $a$ axes of the cylinder cross section. (When 
    5353$\theta = \phi = 0$ these are parallel to the $Y$ and $X$ axes of the 
    54 instrument.) The third orientation distribution, in $\psi$, is about the $c$ 
     54instrument.) The third orientation distribution, in $\Psi$, is about the $c$ 
    5555axis of the particle. Some experimentation may be required to understand the 
    56562d patterns fully. A number of different shapes of distribution are 
     
    6565numerical integrations over one or more variables, so care should be taken, 
    6666especially with very large particles or more extreme aspect ratios. Users can 
    67 experiment with the values of Npts and Nsigs, the number of steps used in the 
     67experiment with the values of *Npts* and *Nsigs*, the number of steps used in the 
    6868integration and the range spanned in number of standard deviations. The 
    6969standard deviation is entered in units of degrees. For a rectangular 
    70 (uniform) distribution the full width should be $\pm\sqrt(3)$ ~ 1.73 standard 
     70(uniform) distribution the full width should be $\pm \sqrt(3)$ ~ 1.73 standard 
    7171deviations (this may be changed soon). 
    7272 
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