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asmooth (asmooth-2.32) [xmmsas_20211130_0941-20.0.0]

Smoothing of mosaics

Image mosaics assembled from several separate pointings look better if the result is divided by the mosaiced exposure and then smoothed. The withexpimageset facility of asmooth allows you to do this. Users who wish to make mosaics from XMM EPIC data however need to exercise some caution. The largest component of the exposure spatial variation in XMM EPIC images is the mirror vignetting function, which changes typically by a factor of three between the optic axis and the edge of the field of view. The problem is that a significant fraction of the background in such images arises from sources (eg cosmic-ray fluorescence, soft protons, electronic noise) which are subject to little or no vignetting. Dividing an image which comprises a flat part plus a vignetted part by a purely vignetted function is going to leave one with an output image which appears to have a hollow centred on the optic axis - ie, is anti-vignetted. It is therefore advisable to subtract the flat background component first before dividing by the exposure map. NOTE however that this will destroy the Poisson nature of the input image, which introduces a couple of complications: firstly, if you also want the variance of the output, you will need to explicitly supply an invarianceset and not leave it to the task to calculate it; secondly, if you want to adaptively smooth your mosaic (see section 3.3.5) you will have to adopt a more complicated procedure than usual.

The recommended procedure for producing simply-smoothed mosaics is as follows:

  1. Make a mosaic of the raw images.

  2. Make a mosaic of the exposure maps.

  3. Make a mosaic of the nonvignetted background (NVB) maps. (It is hoped that in future there will be a sas task which will be able to decompose XMM EPIC background into its various components; unfortunately, for the present you are on your own.)

  4. Subtract the NVB mosaic from the raw mosaic.

  5. Do

      asmooth inset=<(raw-NVB) mosaic> smoothstyle=simple
      withexpimageset=yes expimageset=<exp img mosaic> remultiply=no
    

    If you also want a map of the output variance, add to this

      readvarianceset=yes invarianceset=<raw mosaic>
      writevarianceset=yes outvarianceset=<pick a name>
    

The recommended procedure for producing adaptively-smoothed mosaics is as follows:

  1. Make a mosaic of the raw images.

  2. Make a mosaic of the exposure maps.

  3. Make a mosaic of the nonvignetted background (NVB) maps. (It is hoped that in future there will be a sas task which will be able to decompose XMM EPIC background into its various components; unfortunately, for the present you are on your own.)

  4. Adaptively smooth the raw mosaic and save the convolvers using parameters writeconvolvers, outconvolversset and outindeximageset. I don't at present have a recommendation for use of withexpimageset or expmapuse at this stage: best to experiment to see which produces the most acceptable eventual output.

  5. Subtract the NVB mosaic from the raw mosaic.

  6. Do

      asmooth inset=<(raw-NVB) mosaic> smoothstyle=withset
      inconvolversarray=<conv set saved from step 4>
      inindeximagearray=<index img saved from step 4>
      readvarianceset=yes invarianceset=<raw mosaic>
      withexpimageset=yes expimageset=<exp img mosaic> remultiply=no
    

XMM-Newton SOC -- 2021-11-30