ISOCAM Interactive Analysis User's Manual Version 5.0 - ISO - ESA

278 CHAPTER 21. USING SLICE WITHIN CIA
Table 21.2: Observing setup for the NGC 2366 data
Filter N stab N exp N M ∆N ∆M T int PFOV
′′ ′′
s ′′ .pix −1
LW3 28 24 5 5 75.0 75.0 2.1 6
LW2 14 12 5 5 75.0 75.0 5.04 6
Figure 21.1: The raster maps using a standard CIA procedure (see text for details). Left panel
shows the LW3 data, while the right panel shows the LW2 data. Both data sets are affected by
periodic patterns due to bad flat-field determination, as well as long term transients.
21.5.1 The data set
The dataset we are using here is a 5×5 Y-axis raster on the irregular galaxy NGC 2366. It is
performed in LW3 followed by LW2. Table 21.2 provides more details on the observing setup.
These are raster with non-integer step-sizes (12.5 pixels in all direction). This is not a problem
for SLICE which does exact projection back and forth from sky to detector. The important
point is that the raster steps are less than half the array which ensures high redundancy. In
principle, SLICE does not need a very high level of redundancy, but you will be better off with
high numbers (typically when the median overlap factor is higher than 3-4)
Figure 21.1 shows the raster maps obtained with a standard CIA procedure: dark correction
using the dark model, deglitching with the multiresolution method, transient correction with
the Fouks-Schubert model, and flat-field correction using the “auto” method, i.e. the flat-field
is derived from the data.
As can be seen clearly on the figure, we are suffering from periodic patterns which are due to
the use of a single flat for the whole raster, and also from long term transients, both in LW3 and
LW2. On can also see that due to the strong flux decrease between the LW3 and LW2 raster,
we have a falling then increasing long term transient in the LW2 raster.
At that stage, we are ready to use SLICE. An important point to understand is that the
variable flat-field and the long-term transient are not independent effects: one must have no
flat-field residuals in order to accurately determine the long-term transient, and one would need
ideally a long-term transient corrected data set to pin-point variations due to the flat-field. Since