During the calibration of the Si(111) monochromator, the position of the DCM pitch motor refined from the rocking_curve() scan was recorded:

7110.62 3.915
7708.59 3.918
8330.97 3.911
8979.97 3.911
9659.87 3.907
11561.40 3.900
11918.10 3.895
13032.8 3.894
18981.0 3.868
19999.6 3.858

This was fit with a line (this is a gnuplot script):

set xlabel '{/*1.2 energy (eV)}'
set ylabel '{/*1.2 DCM pitch}'
set title '{/*1.6 DCM pitch v. energy for Si(111)}'

a(x) = m*x + b
plot 'pitch.dat' using 1:2 title 'refined from rocking curve'
fit a(x) 'pitch.dat' using 1:2 via m, b
replot a(x) title 'linear fit'


Resulting in

RTENOTITLE

The fitted values of slope and offset (m = -4.42156e-06 +/- 1.769e-07, b=3.94956 +/- 0.002209) are then used in the approximate_pitch() function in 76-edges.py to move to the approximately correct value before doing the rocking curve scan.