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

20.17. ERROR HANDLING IN CIA 267
The CIA routine reduce (see also Section 20.2.4) not only averages the IMAGEs to EXPO-
SUREs, it also creates corresponding RMS images and weight images. Each pixel or element of
the weight image contains the total number of IMAGE pixels that have been averaged to the
EXPOSURE pixel. Each pixel in the RMS image contains the standard deviation of this sample
of IMAGE pixels. To summarize using some CIA pseudo code 2 :
raster_pds.image[i, j, k] = $
average( raster_pds.cube[i, j, raster_pds.from[k]:raster_pds.to[k]] )
raster_pds.npix[i, j, k] = $
total( raster_pds.cube[i, j, raster_pds.from[k]:raster_pds.to[k]] )
raster_pds.rms[i, j, k] = $
stdev( raster_pds.cube[i, j, raster_pds.from[k]:raster_pds.to[k]] )
Similarly, the routine raster scan (see also Section 20.4) not only creates the raster MOSAIC
image, but also a corresponding RMS image and weight image. In this case, each RMS pixel
contains the standard deviation of all IMAGE pixels that sample the same sky pixel as the
MOSAIC pixel. Again in CIA pseudo code:
raster_pds.npixraster[i, j, k] = total( raster_pds.cube[i, j, *] )
raster_pds.rmsraster[i, j, k] = stdev( raster_pds.cube[i, j, *] )
The RMS and weight images that correspond to Figure 3.3 are given in Figures 20.11 and
20.10. These figures were generated with the commands:
CIA> tviso, raster_pds.npixraster
CIA> tviso, raster_pds.rmsraster
To obtain the standard error of the MOSAIC image (a measure of the quality of the calculation
of the mean, in this case, the quality of calculated MOSAIC pixel values) we simply divide
the MOSAIC image by the square root of the MOSAIC weight image.
CIA> raster_std_err = raster_pds.rmsraster / sqrt( raster_pds.npixraster )
2 Note however that this is for purposes of illustration only; unlike reduce we do not take account of unusable
pixels in the MASK.