.. algorithm::

.. summary::

.. relatedalgorithms::

.. properties::

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


This algorithm applies to the time-of-flight correction which considers the specified elastic peak position. The new X-axis data :math:`t_c` are calculated as

:math:`t_c = t + t_{el} - t_f`

where :math:`t` is the time-of-flight in the input workspace, :math:`t_{el}` is the theoretical elastic time-of-flight, calculated from the neutron wavelength and sample-detector distance, :math:`t_f` is the time-of-flight from sample to detector, corresponding to the elastic peak position specified in the *EPPTable*.

.. note::
   The input *EPPTable* can be produced using the :ref:`algm-FindEPP` algorithm.


If position of the elastic peak is :math:`t_f = 0` for a particular detector, this detector will be masked in the output workspace and warning will be produced. No correction is applied in this case.

Restrictions on the input workspaces
####################################

-  The unit of the X-axis of the input workspace must be **Time-of-flight**.

-  Input workspace must contain *wavelength* and *TOF1* sample logs. *TOF1* is the time of flight of neutron from source to sample (in microseconds).

-  Input workspace must have an instrument set.

-  Number of rows of the table workspace *EPPTable* must match to the number of histograms of the input workspace.

-  Table workspace must have column *PeakCentre*.


Usage
-----

**Example 1: Apply correction to a sample workspace.**

.. testcode:: ExCorrectTOF1

    # create workspace with appropriate sample logs
    ws = CreateSampleWorkspace(Function="User Defined", UserDefinedFunction="name=LinearBackground, \
                               A0=0.3;name=Gaussian, PeakCentre=8123.34, Height=5, Sigma=75", NumBanks=1,
                               BankPixelWidth=1, XMin=6005.25, XMax=9995.75, BinWidth=10.5,
                               BankDistanceFromSample=4.0, OutputWorkspace="ws",SourceDistanceFromSample=1.4)

    lognames = "wavelength,TOF1"
    logvalues = "6.0,2123.34"
    AddSampleLogMultiple(ws, lognames, logvalues)

    # create the EPP table
    table = CreateEmptyTableWorkspace(OutputWorkspace="epptable")
    table.addColumn(type="double", name="PeakCentre")
    table_row = {'PeakCentre': 8128.59}
    for i in range(ws.getNumberHistograms()):
        table.addRow(table_row)

    # apply correction
    wscorr = CorrectTOF(ws, table)

    print("Correction term dt = t_el - t_table =  {:.2f}".format(8190.02 - 8128.59, 2))
    difference = wscorr.readX(0) - ws.readX(0)
    print("Difference between input and corrected workspaces:  {}".format(round(difference[10],2)))

Output:

.. testoutput:: ExCorrectTOF1

    Correction term dt = t_el - t_table =  61.43
    Difference between input and corrected workspaces:  61.43


**Example 2: Apply correction to the TOFTOF data.**

.. testcode:: ExCorrectTOF2

    import numpy

    # load TOFTOF data
    ws_tof = LoadMLZ(Filename='TOFTOFTestdata.nxs')

    # find elastic peak positions
    epptable = FindEPP(ws_tof)

    # apply TOF correction
    ws_tof_corr = CorrectTOF(ws_tof, epptable)

    # apply units conversion to the corrected workspace
    ws_dE = ConvertUnits(ws_tof_corr, Target='DeltaE', EMode='Direct', EFixed=2.27)
    ConvertToDistribution(ws_dE)

    print("5 X values of raw data:  {}".format(numpy.round(ws_tof.readX(200)[580:585],2)))
    print("5 X values corrected data:  {}".format(numpy.round(ws_tof_corr.readX(200)[580:585],2)))
    print("5 X values after units conversion:  {}".format(numpy.round(ws_dE.readX(200)[580:585], 2)))

Output:

.. testoutput:: ExCorrectTOF2

    5 X values of raw data:  [ 8218.59  8229.09  8239.59  8250.09  8260.59]
    5 X values corrected data:  [ 8218.61  8229.11  8239.61  8250.11  8260.61]
    5 X values after units conversion:  [ 0.02  0.03  0.03  0.04  0.05]

.. categories::

.. sourcelink::