  In the framework of the ESA Athena mission, the X-ray Integral Field Unit
(X-IFU) instrument to be on board the X-ray Athena Observatory is a cryogenic
micro-calorimeter array of Transition Edge Sensor (TES) detectors aimed at
providing spatially resolved high-resolution spectroscopy. As a part of the
on-board Event Processor (EP), the reconstruction software will provide the
energy, spatial location and arrival time of the incoming X-ray photons hitting
the detector and inducing current pulses on it. Being the standard optimal
filtering technique the chosen baseline reconstruction algorithm, different
modifications have been analyzed to process pulses shorter than those
considered of high resolution (those where the full length is not available due
to a close pulse after them) in order to select the best option based on energy
resolution and computing performance results. It can be concluded that the best
approach to optimize the energy resolution for short filters is the 0-padding
filtering technique, benefiting also from a reduction in the computational
resources. However, its high sensitivity to offset fluctuations currently
prevents its use as the baseline treatment for the X-IFU application for lack
of consolidated information on the actual stability it will get in flight.
