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.