[ec392464] | 1 | .. resolution_calculator_help.rst |
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| 2 | |
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| 3 | .. This is a port of the original SasView html help file to ReSTructured text |
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| 4 | .. by S King, ISIS, during SasView CodeCamp-III in Feb 2015. |
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| 5 | |
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[a9dc4eb] | 6 | Q Resolution Estimator Tool |
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| 7 | =========================== |
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[ec392464] | 8 | |
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| 9 | Description |
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| 10 | ----------- |
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| 11 | |
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[5ed76f8] | 12 | This tool is approximately estimates the resolution of $Q$ from SAS instrumental |
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| 13 | parameter values assuming that the detector is flat and normal to the |
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[bc9a0e1] | 14 | incident beam. |
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[ec392464] | 15 | |
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| 16 | .. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ |
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| 17 | |
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[a9dc4eb] | 18 | Using the tool |
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| 19 | -------------- |
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[ec392464] | 20 | |
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[850c753] | 21 | 1) Select *SAS Resolution Estimator* from the *Tool* menu on the SasView toolbar. |
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[bc9a0e1] | 22 | |
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| 23 | 2) Select the source (Neutron or Photon) and source type (Monochromatic or TOF). |
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[ec392464] | 24 | |
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[5ed76f8] | 25 | *NOTE! The computational difference between the sources is only the |
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[bc9a0e1] | 26 | gravitational contribution due to the mass of the particles.* |
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[ec392464] | 27 | |
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[5ed76f8] | 28 | 3) Change the default values of the instrumental parameters as required. Be |
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[bc9a0e1] | 29 | careful to note that distances are specified in cm! |
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[ec392464] | 30 | |
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[5ed76f8] | 31 | 4) Enter values for the source wavelength(s), $\lambda$, and its spread (= $\text{FWHM}/\lambda$). |
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| 32 | |
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| 33 | For monochromatic sources, the inputs are just one value. For TOF sources, |
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| 34 | the minimum and maximum values should be separated by a '-' to specify a |
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[bc9a0e1] | 35 | range. |
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[5ed76f8] | 36 | |
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| 37 | Optionally, the wavelength (BUT NOT of the wavelength spread) can be extended |
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| 38 | by adding '; nn' where the 'nn' specifies the number of the bins for the |
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| 39 | numerical integration. The default value is nn = 10. The same number of bins |
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[bc9a0e1] | 40 | will be used for the corresponding wavelength spread. |
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[ec392464] | 41 | |
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[5ed76f8] | 42 | 5) For TOF, the default wavelength spectrum is flat. A custom spectral |
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| 43 | distribution file (2-column text: wavelength (|Ang|\) vs Intensity) can also |
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[bc9a0e1] | 44 | be loaded by selecting *Add new* in the combo box. |
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[ec392464] | 45 | |
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[5ed76f8] | 46 | 6) When ready, click the *Compute* button. Depending on the computation the |
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[bc9a0e1] | 47 | calculation time will vary. |
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[ec392464] | 48 | |
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[5ed76f8] | 49 | 7) 1D and 2D $dQ$ values will be displayed at the bottom of the panel, and a 2D |
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| 50 | resolution weight distribution (a 2D elliptical Gaussian function) will also |
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| 51 | be displayed in the plot panel even if the $Q$ inputs are outside of the |
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[bc9a0e1] | 52 | detector limit (the red lines indicate the limits of the detector). |
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[5ed76f8] | 53 | |
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| 54 | TOF only: green lines indicate the limits of the maximum $Q$ range accessible |
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[bc9a0e1] | 55 | for the longest wavelength due to the size of the detector. |
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[ec392464] | 56 | |
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[5ed76f8] | 57 | Note that the effect from the beam block/stop is ignored, so in the small $Q$ |
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| 58 | region near the beam block/stop |
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| 59 | |
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| 60 | [i.e., $Q < (2 \pi \cdot \text{beam block diameter}) / (\text{sample-to-detector distance} \cdot \lambda_\text{min})$] |
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[ec392464] | 61 | |
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[bc9a0e1] | 62 | the variance is slightly under estimated. |
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| 63 | |
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[5ed76f8] | 64 | 8) A summary of the calculation is written to the SasView *Console* at the |
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[bc9a0e1] | 65 | bottom of the main SasView window. |
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[ec392464] | 66 | |
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[6aad2e8] | 67 | .. image:: resolution_tutor.png |
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[ec392464] | 68 | |
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| 69 | .. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ |
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| 70 | |
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| 71 | Theory |
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| 72 | ------ |
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| 73 | |
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| 74 | The scattering wave transfer vector is by definition |
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| 75 | |
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[6aad2e8] | 76 | .. image:: q.png |
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[ec392464] | 77 | |
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[5ed76f8] | 78 | In the small-angle limit, the variance of $Q$ is to a first-order |
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[bc9a0e1] | 79 | approximation |
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[ec392464] | 80 | |
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[6aad2e8] | 81 | .. image:: sigma_q.png |
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[ec392464] | 82 | |
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[bc9a0e1] | 83 | The geometric and gravitational contributions can then be summarised as |
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[ec392464] | 84 | |
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[6aad2e8] | 85 | .. image:: sigma_table.png |
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[ec392464] | 86 | |
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[5ed76f8] | 87 | Finally, a Gaussian function is used to describe the 2D weighting distribution |
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| 88 | of the uncertainty in $Q$. |
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[ec392464] | 89 | |
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| 90 | .. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ |
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| 91 | |
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| 92 | References |
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| 93 | ---------- |
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| 94 | |
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[5ed76f8] | 95 | D.F.R. Mildner and J.M. Carpenter |
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[bc9a0e1] | 96 | *J. Appl. Cryst.* 17 (1984) 249-256 |
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| 97 | |
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[5ed76f8] | 98 | D.F.R. Mildner, J.M. Carpenter and D.L. Worcester |
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[bc9a0e1] | 99 | *J. Appl. Cryst.* 19 (1986) 311-319 |
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| 100 | |
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| 101 | .. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ |
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| 102 | |
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[a9dc4eb] | 103 | .. note:: This help document was last changed by Steve King, 01May2015 |
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