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

288 CHAPTER 21. USING SLICE WITHIN CIA
from the readouts in bins 1 and 2; if more than two thirds of the readouts fall in bin 2
and 3 (rather normal distribution, around the mean level), the sky value is taken from the
readouts in bins 2 and 3. If none of these combinations represents more than two thirds
of the readouts, the sky value is taken from the readouts in bins 2 and 3 as well.
7. As what we have now is a map of residuals, we add the smoothed map that had been
subtracted to create them and this is the new sky map.
From this description I hope it is rather clear that the parameters to the methods will be
those related to the ghost filtering, the smoothing window for the map and the threshold for the
bad pixels detection. Here they are, listed in more details.
• size filter This is the size of the smoothing window applied to the sky map, both
to determine location of ghosts and to identify bad pixels. Although it is in principle
equivalent to the keyword in the DivSky flat-field method, if the map contains undefined
values (and it generally does), it is already smoothed twice with windows of 5 and 7 pixels,
further smoothing is thus not required and at least small windows should be used.
• bad pix thresh This is the number of σ above and below 0 that we use as a threshold
in the examination of the residual maps. The range of acceptable values depend quite
strongly on the noise used, either global or local (see below).
• /local If set, the current error map will be used to examine the residual maps instead of a
median noise level. In principle this option should be used, although the bad pix thresh
should then be higher than in the median noise option.
• ghost thresh As the name implies, used to flag ghosts.
• peak thresh Also related to ghosts
21.7 Frequently Asked Questions and Problems
In this section, I list the questions that can arise from a use of SLICE and whose answer did
not really fit in the flow of this introduction (or are worth restating).
Q: I have applied the ltt action and subtracted the correction derived by SLICE,
yet after flat-field correction with DivSky, the long-term transient is apparently still
there, what is happening?
A: This is typically a property of DivSky which uses the previously existing map (which
in general was made prior to the long-term transient correction and therefore still shows it) to
estimate the true sky. If the long term transient is strong enough then it can be detected by
SLICE as a flat-field defect (it is in the image but not in the cube itself because it has been
removed) and it will create a new image still affected by what seems to be a long-term transient,
although now it is in the flat-field and not in the cube. The solution is to apply another flat-field
method right after the long-term transient correction to create an estimate of the sky free of the
long-term transient component, and then use DivSky.
Q: I’m trying to run the ltt action on my raster and I encounter an IDL crash
in the routine correct ltt. Apparently it is trying to access a non-existing element
of table si raster. Why is that?