= psphotStack =

`psphotStack` is the program used to perform object detection, classification, and photometry analysis for the stack images.  The treatment of stacks is slightly different from the treatment of individual exposures in a few respects:

 * a set of skycell images is processed at once in `psphotStack`, nominally all five filters ''grizy''.
 * both raw (unconvolved) and convolved stacks are loaded.  the raw stack is used for detection, while the convolved stack is used for photometric and morphological analysis.  '''update''': as of r33964, we are using the 'raw' images for both detection and morphological analysis (but not radial apertures, discussed below).
 * objects detected in the input frames are joined across the multiple filters to define common objects.  Any object for which a sources is NOT detected in any of the frames is used to define a source on those frames (MATCHED sources).  PSF photometry is performed on the matched sources.
 * for each set of exposures, psphotStack generates a set of PSF-matched images, convolved to a common PSF
 * a set of radial apertures are used to measure photometry of all objects and (un-detected) matched sources.  this is done for the original convolved stacks and the PSF-matched images with full-width half-max values specified by the [source:trunk/ippconfig/recipe/psphot.config psphot.config] key PSPHOT.STACK.TARGET.PSF.FWHM (nominally 6.0 & 8.0 pixels).

For the radial apertures, the following annulii are used (defined by psphot.config keys @RADIAL.ANNULAR.BINS.LOWER and @RADIAL.ANNULAR.BINS.UPPER):

|| annulus || inner (pix) || outer (pix) || inner (arcsec) || outer (arcsec||
||  0 ||   0.00 ||   0.96 ||  0.00 ||  0.24 ||
||  1 ||   0.96 ||   2.72 ||  0.24 ||  0.68 ||
||  2 ||   2.72 ||   4.16 ||  0.68 ||  1.04 ||
||  3 ||   4.16 ||   7.04 ||  1.04 ||  1.76 ||
||  4 ||   7.04 ||  12.00 ||  1.76 ||  3.00 ||
||  5 ||  12.00 ||  18.56 ||  3.00 ||  4.64 ||
||  6 ||  18.56 ||  29.76 ||  4.64 ||  7.44 ||
||  7 ||  29.76 ||  45.68 ||  7.44 || 11.42 ||
||  8 ||  45.68 ||  72.80 || 11.42 || 18.20 ||
||  9 ||  72.80 || 112.80 || 18.20 || 28.20 ||
|| 10 || 112.80 || 176.88 || 28.20 || 44.22 ||

These have been chosen to match the apertures defined by SDSS (see [http://www.sdss.org/dr7/algorithms/photometry.html#photo_profile photo profile]).