A systematic way of data reduction for the NIC polarimetry mode has been devised and implemented
to an open software called NICpolpy in the programming language python (tested on version 3.8–3.10 as
of writing). On top of the classical methods, including vertical pattern removal, a new way of diagonal
pattern (Fourier pattern) removal has been implemented. Each image undergoes four reduction steps,
resulting in “level 1” to “level 4” products, as well as nightly calibration frames. A simple tutorial and
in-depth descriptions are provided, as well as the descriptions of algorithms. The dome flat frames (taken
on UT 2020-06-03) were analyzed, and the pixel positions vulnerable to flat error were found. Using the
dark and flat frames, the detector parameters, gain factor (the conversion factor), and readout noise are
also updated. We found gain factor and readout noise are likely constants over pixel or “quadrant”.
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