11.1 MECS event corrections

Some corrections are mandatory (unless one is interested in raw detector events), while other corrections are optional (but require the mandatory ones).

Selection of data in fancy spatial regions (other than a single standard XY box), although strictly not a correction, is implemented as an optional correction.

• Positional dependency of gain

  This correction is mandatory if other corrections are wished. The raw PHA (energy) of an event is corrected dividing it by a factor g(x,y) which is the relative gain w.r.t. the detector centre (mapped in raw detector positions), i.e. the gain is effectively brought back to the detector centre.

• Linearization

  This correction is mandatory if other corrections are wished. The raw detector positions x,y of each event are first converted to mm on the focal plane using a cubic relationship (with the first order term having a slight energy dependency), and then to "new" pixels of user selected size.

• Time dependency of gain

  This optional correction multiplies the positionally-corrected PHA of each event by a correcting factor. This takes into account the gain history described elsewhere (relative gain vs time) and also normalizes to a common energy scale (so-called PI channels).

• Selection using arbitrary spatial regions

  This is not properly a correction, but just the selection or rejection of an event on the basis of its position laying inside or outside a region of interest. For convenience rejected events are assigned an illegal position (so that they are automatically rejected by preexisting software)

• Mandatory corrections

  These are enabled as :

  xasset correction Enabled

  Additionally all primary accumulations programs will currently require three additional arguments following all others. THIS IS PROVISIONAL AND WILL BE CHANGED IN THE FUTURE TO USE ENVIRONMENT VARIABLES. These extra arguments may take the following forms :

  N npix seed

  S pixsize seed

  In the first form one requires npix pixels to cover the original field of view, in the second case one requires the pixel size to be pixsize mm. In either case seed is a seed for a random number generator.
  The current default uses 256 pixels covering the field of view, which gives a pixel size (of approximately 0.17 mm) DIFFERENT for the three MECS units and corresponding to the unlinearized pixel size close to the detector centre.

• Time dependency of gain

  The (optional) correction of the time dependency of gain is enabled as :

  xasset gainhistory gainfile

  where gainfile is a gain history produced by the appropriate command

• Arbitrary spatial region selection

  The (optional) selection of data using spatial regions is enabled in either of the following two ways :

  • xasset selectfile ifile
    xasset selectmethod Map

    In this form selectfile is a XAS image file containing a mask defining the region of interest of arbitrary shape. The image is interpreted as a binary mask, all pixels with a non-zero value define the region of interest.
    No facilities are provided to generate such maps, but it is very easy to do so e.g. in IDL or MIDAS.

  • xasset selectfile rfile
    xasset selectmethod Region

    In this form selectfile is a SAOimage-style region file (recommended filetype is .region) which is interpreted as follows :
    • lines starting with # are comments
    • all positions are in linearized pixels of the current size setting
    • at the beginning it is assumed that all pixels are to be rejected
    • any non-comment line defines a region, which can be of INCLUDE or EXCLUDE type
    • regions are processed strictly in the order in which they appear, flagging the relevant pixels to be accepted or rejected (and eventually changing a previous selection)
    • the following SAOimage-style syntaxes are supported and interpreted as explained

      BOX(xc,yc,xs,ys,angle)

      a rectangular box of full sizes xs,ys with centre in xc,yc, optionally rotated by a given angle (rotation of the X-size w.r.t. the X-axis)

      CIRCLE(xc,yc,radius)

      self-explanatory

      ELLIPSE(xc,yc,xs,ys,angle)

      an ellipse of SEMI-major and minor axes xs and ys, centred in xc,yc, optionally rotated by a given angle (rotation is interpreted as the rotation of the semimajor axis w.r.t. the X-axis ; this semantics is seemingly different from the SAOimage one but appears to give the same results.

    • an EXCLUDE region is defined prefixing with a minus sign, e.g. -BOX, -CIRCLE, -ELLIPSE
    • no other SAOimage syntaxes are supported, in particular polygons are not supported, and expressions are not supported. Annuli can be obtained using in region file a CIRCLE followed by a concentric -CIRCLE
    • the following additional syntax is supported

      SECTOR(xc,yc,radius,theta1,theta2))

      a sector (pie-slice) of the circle of centre xc,yc and given radius, included between the two angles theta1 and theta2. Such angles are in degrees from the X-axis in the range 0-360. It is not allowed to have a sector crossing the +X axis (i.e. from -45 to +45), an equivalent effect can be obtained using two adjacent sectors (0-45 and 315-360)

  xasunsetting the selectfile variable is sufficient to disable the selection.

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