  XPsf_Extract help page



  GENERAL DESCRIPTION

  The XPsf_Extract widget extracts a PSF estimate from a
  stellar field image, as a superposition of a set of stars.
  The user is prompted to select the candidate sources by
  clicking with the mouse: each click is allowed to range
  in a 5x5 box centered on the maximum of the object. After
  selection, a blow-up of each source is shown, to allow
  rejection of undesired candidates. Stars too close to the 
  image edge (e.g. closer than half the size of the output 
  PSF) should be avoided.
  The PSF stars are cleaned from the contaminating sources. 
  This task is accomplished in two ways. During the first 
  PSF estimation on a given image, the user may optionally 
  select the secondary sources around the candidate PSF stars, 
  by clicking with the mouse; when the PSF extraction procedure 
  is called after having already analyzed the image, the known
  contaminating sources around the PSF stars are automatically 
  subtracted, using the information collected so far by the 
  program. Then the PSF stars are background-subtracted, 
  centered with sub-pixel accuracy, normalized, stacked and 
  finally combined by a pixel-by-pixel average with median, 
  minimum or mean operation. Centering involves interpolation, 
  which is reliable only on well-sampled data.
  If present, saturated stars might be included as PSF
  stars, because they provide useful information on the PSF
  halo. To ensure correct normalization, the core of these
  stars is approximately repaired, by replacing it with a scaled
  replica of the PSF: accurate positioning is achieved by
  correlation maximization, whereas the scaling factor is
  determined by fitting the wings of the saturated source.
  Nevertheless the repaired region is masked when forming
  the PSF. Saturated stars, if not repaired, would be rejected
  by the detection algorithm, compromising the analysis of
  fainter sources around.
  The XPsf_Extract widget returns the estimated PSF and, as
  a by-product, the input image with repaired saturated stars.
  Notice that saturated stars, if present, are repaired only
  if selected for PSF extraction.
 

 
  PARAMETERS

  'Size of output PSF':
      Set the size (in pixels units) of the output PSF array.

  'Box size for background estimation':
      The background is estimated by interpolating an array of
      local measurements, relative to a set of image sub-regions
      arranged in a regular grid. This parameter specifies the
      size (in units of PSF FWHM) of each sub-region.
      This parameter is only available in the first iteration 
      of the PSF extraction for a given image: in later iterations, 
      the procedure adopts the background estimate provided by 
      the last call to XStarFinder_Run.

  'Fitting box size (FWHM units)':
      Size of box to fit the secondary sources to subtract.
      This parameter is only available in the first iteration 
      of the PSF extraction for a given image: in the following 
      iterations, secondary sources around the PSF stars are 
      automatically subtracted on the basis of the previous 
      astrometric and photometric analysis.

  'Normalization radius':
      The PSF stars are normalized in a way that the integrated 
      flux within a circular region centered on the stellar peak is 
      unit. This parameter specifies the radius of this normalization
      region. Of course, the final PSF estimate is again normalized 
      to a total flux of 1.
 
  'Average type':
      Select the method to combine the PSF stars into a single image. 

  'Source selection':
      It is possible to load an input list of stars. The list must be in an  
      ASCII file with two columns: X position, Y position.
      The positions are in pixels. There is no need to re-load the list 
      for a new iteration. 

  'Saturation threshold':
      Estimate of the saturation level in the input image. This
      field is set by default to a very high value (larger than
      the image maximum), indicating no saturation. A saturation
      threshold smaller than the image maximum indicates the
      presence of saturated stars. If one or more selected
      stars have their maximum above this level, they are
      considered saturated and repair is attempted.
      If the brightest star in the image was saturated and
      has been repaired, it is recommended to display the image
      again and change the display options (see 'Display Image'
      and 'Display Options' buttons below): the upper intensity
      level of the display should be increased.

  'Search box to optimize correlation':
      Accurate positioning of the PSF onto the core of
      saturated stars is performed by correlation maximization.
      The 'search box' specifies the region inside which the
      PSF is positioned to maximize the correlation. The center
      of the 'search box' is represented by the pixel clicked
      on by the user when selecting the star. Notice that, when
      clicking on a saturated star, no tolerance is fixed on
      the position of the click and no search for the nearest
      maximum is performed: the presumed center of the
      saturated star coincides with the click itself.

  'Repair box':
      Size of the region to use when fitting the wings of a
      saturated star, in order to determine the proper scaling
      factor. This box size is expressed in units of the
      saturated core diameter.

  'Sub-pixel positioning accuracy':
      When saturated stars are only slightly corrupted, it is
      possible to achieve sub-pixel accuracy in positioning the
      PSF onto the saturated core. This parameter indicates the
      number of sub-pixel offsets for correlation maximization.



  CONTROLS/BUTTONS

  'Processing...':
      Extract a PSF estimate applying the currently defined options.

  'Display Image':
      Display the image array.

  'Display PSF':
      Display the PSF array.

  'Display Options':
      Call the widget application XDisplay_Opt to modify the display
      options of the currently displayed array (either Image or PSF).

  'Help':
      Display this help page.

  'Exit':
      Quit XPsf_Extract. Before quitting, the user is prompted to
      create an ASCII file with the positions of the PSF stars.