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omsrclistcomb (omsrclistcomb-2.23) [xmmsas_20211130_0941-20.0.0]

Astrometric alignment of the input source coordinates

If the parameter alignaxes is set to true (default) then the program will try to shift the RA/DEC origins of each source-list file to a common origin. This facility is included to try to remove any slight misalignment between the input source coordinates of each input source-list files due to small OM pointing errors. Omsrclistcomb uses the astrometrically uncorrected celestial coordinates to produce the merged source-list, and this routine can reduce mis-matching and improve the astrometric accuracy.

The routine works in the following way:

  1. Get a list of the number of unique “windows” in the input source-list files (a window has a unique set of WINDOWX0, WINDOWDX, WINDOWY0, WINDOWDY and BINBPE parameters.
  2. For each window do the following:

    1. Find the source-list that has the most sources in that window.
    2. For each other source-list in the window, compute the RA/DEC offsets that are needed to be added to their RAs/Decs to align with the reference source-list coordinates.
    3. Compute the mean offsets.
    4. Subtract the mean offsets from the RA/Dec coordinates of the reference source-list.
    5. Subtract the mean offsets from the computed offsets.
    6. Add these offsets to the RAs/Decs of the other source-lists.

  3. having aligned the source-lists of each window with a reference source-list, the algorithm then tries to align the windows using the following algorithm:

    1. Loop from i=1 to i=the number of windows
    2. Loop from j=i+1 to the number of windows
    3. If all the windows have been aligned exit the routine.
    4. If both windows have been aligned move on to the next j
    5. Otherwise, try to align window j with window i, by matching the source-list in window i having the maximum number of sources with the source-list in window j having the maximum number pf sources. If successful,
      • if neither window has been aligned, or window j has been aligned, add the offsets to the RA/Decs to all the sources in the source-list files of window j. Record that windows i and j have been aligned and move on to the next j.
      • If window j has been aligned, but not window i, subtract the offsets from the RA/Decs to all the sources in the source-list files of window i. Record that windows i has been aligned and move on to the next j.

XMM-Newton SOC -- 2021-11-30