:Author: Jérôme Kieffer
:Date: 01/12/2016
:Keywords: generic description of the calibration procedure
:Target: General audience

.. _calibration:

Calibration
===========

The determination of the geometry of the experimental setup for the diffraction pattern
of a reference sample is called calibration in pyFAI.
A geometry setup is composed of a detector, the six refined parameters like the distance
and fixed parameters like the wavelength (or the energy of the beam), they are all
saved together into a text files named ".poni" (as a reference to the point of
normal incidence) which is subsequently used for processing the experiment.

The program **pyFAI-calib** is used for calibrating
the experimental setup using a constrained least squares optimization on
the Debye-Scherrer rings of a reference sample (:math:`LaB_6`, :math:`CeO_2`, silver
behenate :math:`AgBh`, …) and saves the results into a *.poni* file.
Alternatively, geometries calibrated using Fit2D can be
imported into pyFAI, including geometric distortions (i.e. optical-fiber
tapers distortion) described as *spline-files*.
For Fit2D compatibility, please refer to the :ref:`tutorial` on basic usage of pyFAI.

By storing all parameters together in a single small file, the risk of mixing two
parameters is highly reduced and we believe this helps performing better
science with fewer mistakes.

While entering the geometry of the experiment in a poni-file is possible, it is
easier to perform a calibration, using the Debye-Sherrer rings of a reference
sample called calibrant.
About 30 calibrants are provided by pyFAI like :math:`LaB_6`, ceria :math:`CeO_2`,
silicon, corrundum or silver behenate.
Among other simple compound, all of the NIST
`Standard Reference Materials <http://www.nist.gov/mml/mmsd/sustainable-materials/diffraction-metrology.cfm>`_
have been are tabulated and are directly available as calibrant.
One can alternatively provide its own calibrant description files which is
a simple text-file containing the largest d-spacing (in Angstrom) for a set of
Miller plans.
A useful reference to generate this file is the American Mineralogist database [AMD]_
or the Crystallographic Open database [COD]_.

The calibration procedure is divided into 4 major steps:

Pre-processing of images:
-------------------------
The typical pre-processing consists of the averaging (or better median filtering) of darks images.
Dark current images are then subtracted from data and corrected for flat.
The pre-processing is best performed using the *pyFAI-average* tool, which documentation
is available in the :ref:`manpage`.

If saturated pixels exists, the are likely to be treated like peaks but their positions
will be wrong.
It is advised to either mask them out or to desaturate them (pyFAI provides an option,
but it is expensive in calculation time).
A Mask drawing tool, called *pyFAI-drawmask*, is installed together with pyFAI and
its documentation available in the :ref:`manpage`.

To start the calibration the *pyFAI-calib* tool will need:

* an image with Debye-Sherrer rings
* the energy or the wavelength
* the calibrant name or the d-spacing file of the calibrant
* the detector description.

Peak-picking
------------

Once started, *pyFAI-calib* will ask you to select rings.
The Peak-picking consists in the identification of peaks and groups of peaks
belonging to same ring.
It can be performed by 4 methods described hereafter.

Massif detection
................

This method consists in making the difference of the original image and a blurred
image. Then look for a large contiguous region of positives values, corresponding
to a single group of peak.
The blurring parameter can be adjusted using the *"-g"* option in pyFAI-calib.

Blob detection
..............

The approach is based on difference of gaussians (DoGs) as described in the
blob_detection_ article of wikipedia.

.. _blob_detection: http://en.wikipedia.org/wiki/Blob_detection

It consists in blurring the image by convolution with a 2D gaussian kernel and making
differences between two successive blurs (called Difference Of Gaussian or DoGs).
In theses DoGs, keypoints are defined as the maxima in the 3D space (y,x,size of
the gaussian). After their localization, keypoints are refined by Savitzky Golay
algorithm or by an interpolation at the second order which is equivalent but uses
less points. At this step, if the estimation of the maximum is too far from the maximum,
the keypoint will be considered as a fake maximum and removed.

Steepest ascent
...............

This is very naive implementation which looks for the nearest local maximum.
Subsequently a sub-pixel optimization is performed based on a second order expansion
using the local gradient and hessian.

Monte-Carlo sampling
....................

Series of peaks can be extracted using the Steepest Ascent on randomly selected seeds.
This method can be biased towards an already known geometry by starting from
points which are supposed to be on the ring.

Refinement of the parameters
----------------------------

After selecting groups of peaks, each of them is assigned to a Debye-Scherrer ring number
(0-based numbering in python)
and associated to a d-spacing value hence a theoretical 2\theta value.
A supervised least-squares refinement, performed on the difference of peak position's
2-theta values versus the expected ones from calibrant provides the 6-geometry parameters
fitted.

The default optimization procedure is the Sequential Least SQuares Programming
implemented in *scipy.optimize.slsqp*.
The cost function is the sum of the square of the difference between the expected and
calculated 2\theta values for the various peaks. This sum is dependent on the number
of control-points.


Validation of the calibration
-----------------------------

Validation by an human being of the geometry is an essential step:
pyFAI will overlaps to the diffraction image, the lines corresponding to the various diffraction
rings expected from the calibrant.
Those lines should be in pretty good agreement with the rings
of the scattering image.
The average error per control point (delta 2\theta error in radian) is printed out
and offers a quantitative measurement of the relative quality of the fit for similar
setups/experiment.
Nevertheless its absolute value has no meaning, except the lower, the better.

Subsequently, pyFAI offers some validation options in to check the quality of the fit.
some of them global, some of them limited to given rings.

