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. The last (optional)
  step is the interactive selection of secondary sources
  around confirmed stars; in this case the tolerance box on
  the click location is just 3x3.
  The PSF stars are cleaned from the contaminating sources,
  if specified, background-subtracted, centered with sub-pixel
  accuracy, normalized, stacked and finally combined by a
  median operation. Centering involves interpolation, which
  is allowed only on well-sampled data.
  If present, saturated stars should 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.

  'Fitting box size':
      Size of box to fit the secondary sources to subtract.

  'Normalize PSF stars to':
      The PSF stars can be normalized either to unit total flux
      or to unit maximum. The latter choice is recommended when
      a PSF star lies near the array edge or is contaminated by
      many secondary sources.
 
  'Median superposition':
      The stacked stellar images can be superposed with a
      weighted median operation: each frame is replicated a
      number of times proportional to the square root of the
      original central intensity. This weight is proportional
      to the signal to noise ratio in the photon noise case,
      which applies for bright sources.

  '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.