cen giving nan
A sad old story - here is a new testcase
OPTICS [1]: a2scan(cpm18, 6.25, 6.45, cpm18t, 0.07, 0.07, 50, 0.1, sleep_time=0.5)
Out [1]: Scan(number=21, name=a2scan, path=/data/id11/inhouse2/blc12169/id11/rz1edges/rz1edges_flat/rz1edges_flat.h5)
OPTICS [2]: cen()
Out [2]: {cpm18: nan, cpm18t: nan}