XStarFinder_Run help page



  GENERAL DESCRIPTION

  Given a stellar field and an approximation of the Point Spread
  Function (PSF), the XStarFinder_Run application detects the
  stellar sources and estimate their position and flux. The image
  may be contaminated by a smooth background emission.
  The basic analysis procedure consists of 3 phases:
   1) detection of presumed stars above a given threshold in the
      background-removed image
   2) check and analysis of detected objects, sorted by
      decreasing intensity
   3) re-fitting
  The procedure above may be iterated: a new list of objects is
  formed by searching in the image after subtraction of the
  previously detected stars. Then the analysis proceeds on the
  original frame. This iteration is very useful to detect close
  binaries and resolve crowded groups, down to separations
  comparable to the PSF FWHM. At the end of the last iteration
  an optional de-blending strategy may be applied (see below,
  in the PARAMETERS description), allowing one to detect close
  sources at separations somewhat smaller than the PSF FWHM.
  The analysis of each object (step 2) includes the following
  steps:
  - object re-identification, after subtraction of already known
    stars, to reject spurious detections associated to PSF features
    of brighter sources
  - correlation check, to measure the similarity of the object
    with the PSF
  - local fitting, to determine position and flux; fitting takes
    into account the contribution of other stars and the local
    background, approximated with a slanting plane
  - upgrading of a 'stellar field model', which contains a
    replica of the PSF for each detected star; it is basically
    used to take into account the contribution of bright sources
    when analyzing fainter and fainter ones.


 
  PARAMETERS

  'Detection threshold(s)':
      Enter one or more detection thresholds separated by
      commas.
      The number of thresholds specifies also the number of
      iterations of the basic analysis procedure (see steps 1,
      2, 3 in the GENERAL DESCRIPTION).
      A detection threshold represents the minimum central
      intensity of an acceptable star, after removing the local
      background contribution.
      An effective choice is to select two detection levels,
      both equal to (3 * Sigma), where Sigma is an estimate of
      the noise standard deviation.

  'Relative threshold':
      If this option is set, the detection threshold is considered
      as a relative threshold, in units of the noise standard
      deviation. This button is active only if the noise is
      defined on input, i.e. if the noise computation procedure
      has been run before using this application.

  'Correlation threshold':
      Scalar value, representing the minimum correlation to accept
      an object.

  'No. of sub-pixel offsets':
      The correlation of the object with the PSF is affected by the
      relative off-centering between the two patterns; this parameter
      specifies the number of sub-pixel offsets to maximize the
      correlation.

  'Noise':
      Set this option to use the input noise array to perform a
      weighted PSF fitting on the presumed objects. When this
      option is applied, an estimate of the formal errors on
      astrometry and photometry will be computed.

  'Box size for background estimation':
      An accurate background estimate is necessary for accurate
      objects detection and for a more reliable computation of
      the correlation coefficient of each object with the PSF.
      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 set by default to the same value that has
      been used in the PSF extraction procedure (XPsf_Extract).

  'Apply deblender':
      Set this options to apply the deblending strategy, based on
      blends recognition by thresholding. Deblending is applied
      at the end of the last iteration of the basic analysis
      procedure, i.e. when all the 'resolved' objects are supposed
      to have been detected, thus allowing a more careful and
      detailed analysis of crowded groups.
      De-blending requires a very stable PSF, in order to avoid
      local PSF variations to be exchanged for blends.

  'Final re-fitting iterations':
      At the end of each iteration (see GENERAL DESCRIPTION),
      all the currently known stars are re-fitted once.
      At the end of the whole analysis, the re-fitting may be
      further iterated to improve the astrometry and photometry.
      This parameter is to set the number of final re-fitting
      iterations.



  CONTROLS/BUTTONS

  'Processing':
      Analyze stellar field, applying the currently defined options.

  'Save results':
      Save list of stars on ASCII file. The output list is organized
      as a 7 columns table, with the following format:

      X  Y  Flux  Sigma_X  Sigma_Y  Sigma_Flux  Correlation

      where
      - X, Y  are the coordinates of a star in pixels
      - Flux  is the total flux
      - Sigma_X, Sigma_Y, Sigma_F  are the formal error estimates
             on X, Y, Flux respectively
      - Correlation  is the correlation coefficient of the star;
             notice that the correlation coefficient of a star
             found by de-blending (if applied) is set to -1
                   

  'Help':
      Display this help page.

  'Exit':
      Quit XStarFinder_Run.