11.1 MECS event corrections
In the case of the MECS a number of corrections need to be applied to raw events in order
to normalize their position and energy information. In principle these corrections could
be applied post-facto to a raw
before using it as input for
however so far a different, faster, way has been implemented, which is to handle all corrections
on request, transparently during the
primary accumulation of images, spectra, time profiles or
Some corrections are mandatory (unless one is interested in raw detector events), while
other corrections are optional (but require the mandatory ones).
Selection of data in fancy spatial regions (other than a single standard XY box), although
strictly not a correction, is implemented as an optional correction.
These are the kind of corrections and a very brief description of their nature.
Positional dependency of gain
This correction is mandatory if other corrections are wished. The raw PHA
(energy) of an event is corrected dividing it by a factor g(x,y) which is
the relative gain w.r.t. the detector centre (mapped in raw detector positions),
i.e. the gain is effectively brought back to the detector centre.
This correction is mandatory if other corrections are wished.
The raw detector positions x,y of each event are first converted to mm on the
focal plane using a cubic relationship (with the first order term having a slight
energy dependency), and then to "new" pixels of user selected size.
Time dependency of gain
This optional correction multiplies the positionally-corrected PHA of each event
by a correcting factor. This takes into account the
described elsewhere (relative gain vs time) and also normalizes to a common
energy scale (so-called PI channels).
Selection using arbitrary spatial regions
This is not properly a correction, but just the selection or rejection of an event
on the basis of its position laying inside or outside a region of interest. For
convenience rejected events are assigned an illegal position (so that they are
automatically rejected by preexisting software)
In order for corrections to be applied during
they must currently be explicitly enabled. Otherwise one obtains uncorrected files using
raw detector X,Y and PHA.