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

Chapter 22
Second order corrections
This chapter describes second order calibration routines. Depending on your data such calibration
maybe entirely unnecessary. All second order correction routines should be used with
care.
22.1 Jitter correction
Spacecraft jitter can cause a slight displacement of a source from IMAGE to IMAGE. This
displacement is limited to about 0.5 ′′ (Kessler et al. 1996). Figure 22.1 shows the distribution
of jitter offsets for one particular observation. Jitter is most noticeable when a point source is
observed. You can check for jitter in your data by using x3d to flick through the IMAGEs in
.CUBE while watching for sub-pixel shifting of a source.
Jitter correction is performed in two steps. Firstly the jitter offsets from IMAGE to IMAGE
are computed and secondly these offsets are applied to the data.
22.1.1 Computing the jitter offsets
In general, the jitter computation is performed as follows:
1. All the IMAGEs corresponding to a single ISO pointing are extracted from the PDS CUBE.
These IMAGEs are averaged and the maximum pixel, [i max avg ,j max avg ] is taken as a first
estimate of the position of the center of the source.
2. A sub-cube is made from the IMAGEs, centered on CUBE[i max avg ,j max avg ,k]andof
radius jitter, wherejitter is the expected maximum jitter amplitude and defaults to 1
pixel. For each frame of this sub-cube, the maximum pixel, [i max ,j max ] k is determined.
3. A new sub-cube, SUBCUBE is again made from the IMAGEs, this time each sub-cube
frame, SUBCUBE[i, j, k], is centered on CUBE[[i max ] k , [j max ] k ,k] and of radius bsize.
4. A fit is applied to each sub-cube frame SUBCUBE[i, j, k] and the center of the fitted
function is taken as the observed source location, [i source ,j source ] k
5. The jitter offsets in the x and y-axis, du and dv are taken as
du k =[i source ] k − median([i source ] k )
dv k =[j source ] k − median([j source ] k )
du and dv are placed in the PDS fields DU and DV.
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