The cyclic reprocessing environment of IDU allows a more sophisticated LSF/PSF
modelling to be performed, relative to the calibration determined in the real-time
system (see Section 2.3.2). In this data release the
approach, however, mirrors that used in FL LODC with a few indirect improvements. For
example, the bias and background models available in IDU are more complete and
have fewer artefacts due to joins between OBMT intervals. The increased
processing power available also enables a more flexible solution to be
determined (extra spline knots and parameter dependencies).
In future data reductions the main difference with respect to the early LSF/PSF
will be the use of a full 2D PSF model, rather than an ALxAC LSF approximation,
which is known to limit the performance, particularly when the PSF is
asymmetric. This full 2D PSF model will be conceptually similar to the LSF model
but it shall use a linear combination of 2D basis functions instead of 1D basis
functions. Knowledge of the scene will also permit the full 2D PSF mapping to be
extended into the ‘far’ wings, well outside the region sampled by the stellar
windows. These diffraction features cause a great number of false detections
on-board and so form a valuable data set, along with data from Virtual Objects.
The ultimate aim for the extended LSF/PSF model is to incorporate a Charge
Distortion Model (CDM) to handle the effects of Charge Transfer Inefficiency (CTI).
The CDM is a very complex problem to solve, and it requires an accurate
knowledge of the input signal, so this remains under development. Fortunately
the level of radiation damage is significantly lower at this stage of the
mission than feared before launch (by a factor of ).