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.