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

284 CHAPTER 21. USING SLICE WITHIN CIA
Figure 21.3: The resulting maps for the first flat-field determination with the DivSky. Note
that the “emission gradient”, produced by the long-term transient is much smoother now, and
pointing imprints are mostly gone.
Another important point is that the ltt action tries to remove ghosts before it estimates
the long-term transient component, this is made using the variable raster (which contains the
flat-fielded cube of raster pointings, see table 21.1). However the action itself does not create
this cube. Therefore make sure it exists (it generally will since it is created and updated by the
make map action) before you start the ltt action 4 .
In fact, the long-term transient determination adds only one new parameter: ltt thresh.
This is similar to the flat thresh parameter, in the sense that it allows to discard pixel readouts
from the long-term transient determination. Typically, this is used to discard pixels observing
strong sources where the short-term transient correction may not have been ideal. On each raster
position, pixels ltt thresh×σ above the mean level of the current sky position are discarded
(note that the exact meaning is opposite to that of flat thresh: a higher value implies that less
pixels are discarded!). In fact, SLICE displays the raster positions with the discarded pixels
masked. A good way to check that your setting is correct is that the location of strong sources
are masked.
In our example, for the LW3 band, and to a lesser extent for the LW2 band as well, the
DivSky method always produces the oscillating artifacts, and thus we are going to use the
Perturbed Single Flat-Field method, with parameters set as in Table 21.3. In this observation
using ltt thresh = 6 is quite satisfactory. In that case, the commands to issue to SLICE
are, for the LW3 case:
CIA> red param=set red param(tdt=’65801627’,flat smooth window=4,nplanes=60,$
CIA> flat thresh=10,ltt thresh=6)
CIA> act = set act(/ltt)
CIA> slice pipe
Figure 21.5 plots the long-term transient corrections that are derived with this setup. On
4 Be aware that putting the two actions in your call to set act willnotworkattheltt action is executed
before the make map one.