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PSFSTACK

PSFSTACK

Name


  psfstack

Purpose


  Generate an average numerical psf by stacking observed images.

Description


Category


  CCD data processing

Calling Sequence


  psfstack,image,x,y,psf

Inputs


  image - Input image from which psf is collected.
  x - Scalar or vector list of positions of stars (x coordinate).
  y - Scalar or vector list of positions of stars (y coordinate).

Optional Input Parameters


Keyword Input Parameters


  APODIZE - Flag, if set turns on an apodization constraint that forces the
              PSF decrease to zero by the edge. This is implemented by
              looking at the mean radial profile of the final PSF. In
              this profile, the routine looks for a local minimum. From
              that point to the edge (corners, really) the PSF is reduced
              linearly from that radial distance to the corners so that
              the edge goes to zero. It is best to not use this but if
              you have problems with the PSF turning up at the edge, this can
              help make it better.
  DW - half-width of box centered around PSF, final size is 2*DW+1,
            default size is DW=9 pixels. It is really important that
            DW be larger than FWHM. Usually, DW ~ 2*FWHM
  SILENT - Flag, if set suppresses all printed output.
  SKY1 - Inner radius of the sky annulus (in pixels), default=10.
  SKY2 - Outer radius of the sky annulus (in pixels), default=40. The sky
              value is computed for every source to get a local sky value.
  SNRTHRESH - Signal-to-noise ratio threshold. Any source pointed to by x,y
                that has a lower SNR than this will be ignored. The default
                is 25.
  TOPCLIP - Fraction of the brightest pixels that are excluded before doing
              the initial median on the psf stack. This should be a number
              between 0 and 1, though if you set it to 1 everything would be
              excluded. The default is 0.

Outputs


  psf - Final average psf, normalized to unit _volume_

Keyword Output Parameters


  FLUX - Fraction of PSF volume interior to FWHM from center of PSF.
  FWHM - FWHM of final stacked PSF
  XCEN - Centroided X location of center of PSF within the psf array
  YCEN - Centroided Y location of center of PSF within the psf array

Common Blocks


Side Effects


Restrictions


Procedure


Modification History


  97/10/22, Written by Marc W. Buie, Lowell Observatory
  2013/03/30, MWB, added APODIZE keyword
  2013/03/31, MWB, added SKY1, SKY2, TOPCLIP, FWHM, XCEN, YCEN, FLUX
                      also some tweaks to the algorithm that leads to
                      a significantly improved PSF.
  2014/01/17, MWB, changed the procedure that leads to the FWHM calculation.
                      Was using an aperture of radius=DW, now uses DW/2.



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