XMM-Newton SAS Home Page
XMM-Newton Science Analysis System


ommosaic (ommosaic-2.11) [xmmsas_20211130_0941-20.0.0]

Stacking of sky-images after internal image alignment

Before stacking images it is desirable to make sure that they are properly aligned first- by this it is meant that the absolute coordinate values (RA/DEC for sky-images) at the reference pixels are accurate. Any such errors will degrade the mosaiced image and possibly lead to source-detection and source-parameterization problems. To illustrate this, In Figures 4 and 5 we compare two mosaiced images from the stacking of the same two sky-image files, testimage1.fits and testimage2.fits. The latter file is a copy of testimage1.fits, but the reference pixels CRPIX1 and CRPIX2 have both had two pixels added. The image in Figure 4 has been created simply by stacking them, without alignment, using the command - ommosaic imagesets=”testimage1.fits testimage2.fits" mosaicedset=mosaicedimage1.fits mincorr=0”. and the image in Figure 5 created using the command ommosaic imagesets=”testimage1.fits testimage2.fits" mosaicedset=mosaicedimage2.fits mincorr=0.5 correlset=correl.fits”.

Figure 4: Mosaiced sky-image resulting from the stacking of two identical images with small reference pixel offsets without image alignment. The detected source regions are shown.
\begin{figure}\centering
\epsfysize =15cm
\epsfig{file=mosaicedimage1.ps=, height=15cm}\end{figure}

Figure 5: Mosaiced sky-image resulting from the stacking of two identical images with small reference pixel offsets with image alignment. The detected source regions are again shown.
\begin{figure}\centering
\epsfysize =15cm
\epsfig{file=mosaicedimage2.ps=, height=15cm}\end{figure}

For the latter mosaiced image, ommosaic correctly computed the reference pixel offsets (2 pixels) and corrected them internally before stacking the two images.

A comparison of the two figures shows that

  1. The first image is obviously blurred compared to the second one.
  2. Many of the sources classified as extended (cyan region) in the first image are classified as point-like (green regions) in the second one.

Figure 6 shows the correlation image produced by ommosaic and stored in the file correl.fits. The image is 21x21 pixels and each pixel is ten times smaller than a pixel in testimage1.fits. As can be seen from the image, there is a well defined maximum correlation near the centre of the image, and this is a good indication that the image-alignment algorithim worked well.

Figure 6: Correlation image produced by ommosaic
\begin{figure}\centering
\epsfysize =15cm
\epsfig{file=correl.ps=, height=15cm}\end{figure}

XMM-Newton SOC -- 2021-11-30