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Common mode correction

Most of pixel array detectors produce imaging data which can not be directly used in analysis and need in corrections. Most popular corrections are

• dark rate (pedestal) subtraction,
• bad pixel masking,
• common mode correction,
• gain correction,
• etc.

In this note we discuss common mode correction algorithms. Common mode is a tiny hardware effect of collective pixel intensity variation due to synchronous variation of potentials on sensor chip or ADC at readout process. This effect can be corrected at low sensor illumination, where the number of pixels with energy deposition from photons is small. The spectrum of pixel intensities without photon should be grouped in the peak with small offset of the average from zero (after dark rate correction is applied) due to the common mode effect. This offset can be evaluated and subtracted from all pixel intensities in the group of synchronously fluctuating pixels. This correction is detector-dependent, because detectors may have many groups of pixels. Algorithms are also detector-dependent in order to better account for sensor behaviour. 

Implementation

Below we describe common mode correction algorithms implemented in psana module ImgAlgos::NDArrCalib. This correction along with others can be applied to raw ndarray under control of the psana configuration file for specified detector, for example for  Epix100a:

[ImgAlgos.NDArrCalib]
source  = DetInfo(:Epix100a)
key_in  = ndarray-raw
key_out = ndarray-clb
do_peds = yes
do_cmod = yes
do_stat = yes
...

where keywords do_peds and do_cmod should have value yes or true in order to turn on pedestal subtraction and common mode correction algorithms. Each algorithm may load file with parameters from calib directory, which by default is

/reg/d/psdm/<INS>/<experiment>/calib/<calib-version>/<data-source>/<calibration-type>/<run-range>.data
    for example:
/reg/d/psdm/XPP/xppi0614/calib/Epix100a::CalibV1/NoDetector.0:Epix100a.0/pedestals/0-end.data
/reg/d/psdm/XPP/xppi0614/calib/Epix100a::CalibV1/NoDetector.0:Epix100a.0/common_mode/0-end.data

Content of this file depends on calibration type and algorithm, as shown below.

Algorithms

We use algorithms earlier developed for CSPADs from ImgAlgos::GlobalMethods(#1) and from psalg package (#2 and #3).

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