Correcting for sample misalignment

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When mounting your sample on a spectrometer, it can often be the case that it is slightly misaligned compared the to the 'perfect' alignment assumed when generating the SQW file (the u and v vectors provided in gen_sqw and accumulate_sqw). It is straightforward to correct such misalignment once enough data have been accumulated by comparing the positions of Bragg peaks compared to what they should be. The alignment correction is thus a process to be done in several steps - first the misalignment must be determined and checked, and then the correction must be applied to the data.

Step 1 - determine the true positions of Bragg peaks

bragg_positions

First you should find several non-parallel strong Bragg peaks in your data. Next run the following routine, which generates radial and transverse cuts around specified Bragg peaks and determines the deviation from the expected values.

 
[rlu0, widths, wcut, wpeak] = bragg_positions (sqw_file, bp, ...
                radial_cut_length, radial_bin_width, radial_thickness,...
                trans_cut_length, trans_bin_width, trans_thickness, energy_window,<keyword options>)

The inputs are

sqw_file - the uncorrected data file

bp - an n-by-3 array specifying the notional Bragg positions (i.e. integer hkl)

radial_cut_length - length of cuts along the Q of the Bragg peak

radial_bin_width - bin (step) size along the radial cut

radial_thickness - integration thickness along the axes perpendicular to the radial cut direction

trans_cut_length - length of cuts along the Q orthogonal to that of the Bragg peak

trans_bin_width - bin (step) size along the transverse cuts

trans_thickness - integration thickness along the two perpendicular directions to the transverse cuts

energy_window - Energy integration window around elastic line (meV). Choose according to the instrument resolution. A good value is 2 x full-width half-height. Note that this is the full energy window, e.g. for -1meV to +1 meV, set energy_window=2

The following keyword options are available:

For binning choose either 'bin_absolute', which denotes that the radial and transverse cut lengths, bin sizes, and thicknesses are in inverse Angstroms [Default]; or choose 'bin_relative', which denotes that cut lengths, bin sizes and thicknesses are fractions of |Q| for radial cuts and degrees for transverse cuts.

For fitting options choose 'outer', which determines peak position from centre of peak half-height by finding the peak width moving inwards from the limits of the data - useful if there is known to be a single peak in the data as it is more robust to too finely binned data. [Default]; 'inner' determines the peak position from centre of peak half height by finding the peak width moving outwards from peak maximum; 'gaussian' fits a Gaussian on a linear background.

The outputs are:

rlu0 - the actual peak positions as an n-by-3 matrix of h,k,l as indexed with the current lattice parameters.

widths - an array of size n-by-3 containing the FWHH in Ang^-1 of the peaks along each of the three projection axes

wcut - an array of cuts, size n-by-3, along three orthogonal directions through each Bragg point from which the peak positions were determined. (Note that this can be passed to bragg_positions_view together with wpeak to view the output. [Note: the cuts are IX_dataset_1d objects and can be plotted using the plot functions for these methods.]

wpeak - an array of spectra, size n-by-3, that summarise the peak analysis. Pass to bragg_positions_view together with wcut to view the output. [Note: for aficionados: the cuts are IX_dataset_1d objects and can also be plotted using the plot functions for these objects.]

Step 2 - check the Bragg positions fits worked properly

You can make plots of the cuts and fits of your notional Bragg peaks to check that the program has correctly fitted everything, using outputs from the bragg_positions describe above.

bragg_positions_view(wcut,wpeak)

You will be prompted in the Matlab command window as to which plot and fit you wish to view. Press 'q' to exit this interactive mode.

Step 3 - calculate the misalignment correction

Using the outputs of bragg_positions, together with certain optional keyword arguments, you can determine a transformation matrix that goes from the original misaligned frame to the correct aligned frame.

[rlu_corr,alatt,angdeg] = refine_crystal(rlu0, alatt, angdeg, bp,<keyword options>);

The inputs are;

rlu0 - the actual peak positions as an n-by-3 matrix of h,k,l as indexed with the current lattice parameters (see above)

alatt, angdeg - the lattice parameters and angles used in the original sqw file.

bp - the notional (integer) Bragg peaks corresponding to rlu0

The keyword options for defining exactly what is and is not-corrected for are as follows:

fix_lattice - Fix all lattice parameters [a,b,c,alpha,beta,gamma], i.e. only allow crystal orientation to be refined

fix_alatt - Fix [a,b,c] but allow lattice angles alpha, beta and gamma to be refined together with the crystal orientation

fix_angdeg - Fix [alpha,beta,gamma] but allow the lattice parameters [a,b,c] to be refined together with crystal orientation

fix_alatt_ratio Fix the ratio of the lattice parameters as given by the values in the inputs, but allow the overall scale of the lattice to be refined together with crystal orientation

fix_orient - Fix the crystal orientation i.e. only refine the lattice parameters

NB: To achieve finer control of the refinement of the lattice parameters: instead of fix_lattice, fix_angdeg, etc. use the following:

free_alatt - Array length 3 of zeros or ones, 1=free, 0=fixed

e.g. 'free_alatt',[0,1,0],... allows only lattice parameter b to vary

free_angdeg - Array length 3 of zeros or ones, 1=free, 0=fixed.

e.g. 'free_angdeg',[1,1,0],... fixes lattice angle gamma buts allows alpha and beta to vary

The outputs are:

rlu_corr - Conversion matrix to relate notional rlu to true rlu, accounting for the the refined crystal lattice parameters and orientation qhkl(i) = rlu_corr(i,j) * qhkl_0(j)

alatt - Refined lattice parameters [a,b,c] (Angstroms)

angdeg - Refined lattice angles [alpha,beta,gamma] (degrees)

rotmat - Rotation matrix that relates crystal Cartesian coordinate frame of the refined lattice and orientation as a rotation of the initial crystal frame. Coordinates in the two frames are related by v(i)= rotmat(i,j)*v0(j)

distance - Distances between peak positions and points given by true indexes, in input argument rlu, in the refined crystal lattice. (Ang^-1)

rotangle - Angle of rotation corresponding to rotmat (to give a measure of the misorientation) (degrees)


Step 4 - apply the correction to the data

There are two ways to do this, either to apply the correction to an existing file without regenerating (good for when you have a complete scan). Or you can calculate what the goniometer offsets gl, gs, dpsi are, and then use these when you regenerate the sqw file (good for situations when you are still accumulating data, such as on the beamline during an experiment).

Option 1 : apply the correction to an existing sqw file

There is a simple routine to apply the changes to an existing file, without the need to regenerate;

change_crystal_horace(sqw_file, rlu_corr)

where rlu_corr was determined in the steps described above


Option 2 : calculate goniometer offsets for regeneration of sqw file(s)

In this case there is a single routine to calculate the new goniometer offsets, that can then be used in future sqw file generation.

[alatt, angdeg, dpsi_deg, gl_deg, gs_deg] = crystal_pars_correct (u, v, alatt0, angdeg0, omega0_deg, dpsi0_deg, gl0_deg, gs0_deg, rlu_corr, <keyword options>)

The inputs are:

u, v - The notional scattering plane (used when the sqw file was initially generated, before any alignment corrections were performed)

alatt0, angdeg0 - The initial lattice parameters used in the first sqw file generation, before refinement

omega0_deg, dpsi0_deg, gl0_deg, gs0_deg - The initial goniometer offsets used in the first sqw file generation, before refinement (all in degrees)

rlu_corr - The correction matrix determined above.

The following optional keywords can be provided:

u_new, v_new - Replacement vectors u, v that define the scattering plane. Normally these would not be given, and the input u and v will be used. The extent to which u_new and v_new do not correctly give the true scattering plane will be accommodated in the output misorientation angles dpsi, gl and gs below. (Default: input arguments u and v)

omega_new - Replacement value for the orientation of the virtual goniometer arcs with reference to which dpsi, gl, gs will be calculated. (Default: input argument omega) (deg)


The outputs are:

alatt, angdeg - The true lattice parameters: [a_true,b_true,c_true], [alpha_true,beta_true,gamma_true] (in Ang and deg)

dpsi, gl, gs - Misorientation angles of the vectors u_new and v_new (deg)


Option 2a (for use with e.g. Mslice): calculate the true u and v for your misaligned crystal

Following option 2 above, you can recalculate the true u and v vectors using the following method.

[u_true, v_true, rlu_corr] = uv_correct (u, v, alatt0, angdeg0, omega_deg, dpsi_deg, gl_deg, gs_deg, alatt_true, angdeg_true)

The inputs are:

u and v - the notional orientation of a correctly aligned crystal.

alatt and angdeg - the notional lattice parameters of the aligned crystal. These are the same as in crystal_pars_correct above..

omega_deg, dpsi_deg, gl_deg, gs_deg - the calculated misorientation angles, i.e. the output of crystal_pars_correct.

alatt_true, angdeg_true - similarly, the calculated correct lattice parameters


The outputs are:

u_true, v_true - the corrected u and v vectors required for e.g. Mslice.

rlu_corr - the orientation correction matrix to go from the notional to the real crystal (see above)



List of alignment correction routines

Below we provide a brief summary of the routines available for different aspects of alignment corrections. For further information type

help <function name>

in the Matlab command window.

bragg_positions

[rlu0,width,wcut,wpeak]=bragg_positions(w, rlu, radial_cut_length, radial_bin_width, radial_thickness,...
                                                            trans_cut_length, trans_bin_width, trans_thickness)

Get actual Bragg peak positions, given initial estimates of their positions, from an sqw object or file

bragg_positions_view

bragg_positions_view(wcut,wpeak)

View the output of fitting to Bragg peaks performed by bragg_positions

calc_proj_matrix

[spec_to_u, u_to_rlu, spec_to_rlu] = calc_proj_matrix (alatt, angdeg, u, v, psi, omega, dpsi, gl, gs)

Calculate matrix that convert momentum from coordinates in spectrometer frame to projection axes defined by u1 || a*, u2 in plane of a* and b* i.e. crystal Cartesian axes. Allows for correction scattering plane (omega, dpsi, gl, gs) - see Tobyfit for conventions

crystal_pars_correct

[alatt, angdeg, dpsi_deg, gl_deg, gs_deg] = crystal_pars_correct (u, v, alatt0, angdeg0, omega0_deg, dpsi0_deg, gl0_deg, gs0_deg, rlu_corr)

Return correct lattice parameters and crystal orientation for gen_sqw from a matrix that corrects the r.l.u.

refine_crystal

[rlu_corr,alatt,angdeg,rotmat,distance,rotangle] = refine_crystal(rlu0,alatt0,angdeg0)

Refine crystal orientation and lattice parameters

rlu_corr_to_lattice

[alatt,angdeg,rotmat,ok,mess]=rlu_corr_to_lattice(rlu_corr,alatt0,angdeg0)

Extract lattice parameters and orientation matrix from rlu correction matrix and reference lattice parameters

ubmatrix

[ub, mess, umat] = ubmatrix (u, v, b)

Calculate UB matrix that transforms components of a vector given in r.l.u. into the components in an orthonormal frame defined by the two vectors u and v (each given in r.l.u)

uv_correct

[u_true, v_true, rlu_corr] = uv_correct (u, v, alatt0, angdeg0, omega_deg, dpsi_deg, gl_deg, gs_deg, alatt_true, angdeg_true)

Calculate the correct u and v vectors for a misaligned crystal, for use e.g. with Mslice.