Investigate/fix noisy low-r region of PDF in POLARIS reduction

[RichardWaiteSTFC]

Describe the bug

Comparison of PDF produced by Mantid (LEFT) and Gudrun (RIGHT)

This could be due to the implementation of the fourier filter here

mantid/scripts/Diffraction/isis_powder/polaris_routines/polaris_algs.py

Line 304 in 3a7989f

def fast_fourier_filter(ws, rho0, freq_params=None):

• The output from this filter has regular bin-width (the input doesn’t) taken from smallest bin-width in data
• If no rmax is supplied in the freq_params then it uses hard-coded rmax=200 – I’m not sure how reasonable this is?

It also affects PDF with merge_banks=False

To Reproduce
(1) Ask me for files
(2) Update paths in .yaml files
(3) Run this script

from mantid.simpleapi import *
import numpy as np
from isis_powder import polaris, SampleDetails

config_file_path = r"C:\Users\xhg73778\Documents\POLARIS\PR34144Files\polaris_config_example.yaml"
polaris = polaris.Polaris(config_file=config_file_path, user_name="test")


sample_details = SampleDetails(height=4.0, radius=0.2985, center=[0, 0, 0], shape='cylinder')
sample_details.set_material(chemical_formula='Si', packing_fraction=0.6)
sample_details.set_container(radius=0.3175, chemical_formula='V')
polaris.set_sample_details(sample=sample_details)

# focus
polaris.create_vanadium(first_cycle_run_no="98532", multiple_scattering=True, mayers_mult_scat_events=1)
polaris.focus(run_number="98533", input_mode='Summed', # sample_empty="98532", # "98531"
              mode="PDF", do_absorb_corrections=True, do_van_normalisation=True,
              van_normalisation_method="Absolute",
              empty_can_subtraction_method= "PaalmanPings",
              sample_empty_scale=1.0)

# produce PDF
polaris.create_total_scattering_pdf(run_number="98533", 
                                    q_lims=[[-1, 0.5, 0.5, 0.5, 0.5],[-1, 30, 30, 30, 30]],
                                    delta_q=0.02,
                                    delta_r=0.02,
                                    pdf_type="g(r)",
                                    freq_params=[0.91],
                                    lorch_filter=True,
                                    placzek_order=2,
                                    sample_temp=300,
                                    pdf_output_name="pdf",
                                    merge_banks=True)

Expected behavior
Would produce result similar to gudrun

Screenshots

Platform/Version (please complete the following information):

• OS: [e.g. Windows, RHEL 7, Ubuntu, macOS]
• OS Version:
• Mantid Version [e.g. 6.0.0]

Additional context