de Vaucouleurs Model Parameters - Stack Photometry versus Full Force Photometery

Back in February we presented the first results of the new IPP programs psphotFullForce ​http://svn.pan-starrs.ifa.hawaii.edu/trac/ipp/wiki/fullforce_SAStest1.

There we presented plots that send to indicate that the full force results provided a much better match to SDSS model fits than the psphotStack results for de Vaucouleurs models (dev model). There was some scepticism that the difference was so great. In particular the validity of the ​plot was doubted.

It turns out that this scepticism was justified. The ddevrad values for the stack photometry were incorrect. The difference in units between the sdss value (arcseconds) and the IPP values (0.25 arcsecond pixels) was not accounted for. Upon further review we also discovered that the wrong stacks (label SAS.20130620) were used for that investigation rather than the SAS 32 stacks (label SAS.20130703).

In order to examine the dev model results in more detail we re-ran the full force pipeline on a subset of the SAS skycells. To improve statistics we changed the system to select only dev models for forced photometry rather than the usual method of choosing the best of the 3 model fits. The skycells processed were near the center of the SAS field: 332 < ra < 336, -3 < dec < 3.

Here are the differences between the SDSS model radius and the psphotStack and psphotFullForce model paramters for dev galaxies (devFrac_r == 1.0). Here bright galaxies are SDSS r magnitude < 20.

ddevrad = devRad_r*0.25 - EXT_WIDTH_MAJ_dev

Here are both sets of histograms on the same plot:

We find that the lowest chi squared fit is shifted slightly to the left indicating that we get a larger radius. Interestingly this effect is larger for the bright objects.

Here is the plot of the magnitude differences.

Apparently the larger radii are causing more flux to be included in the model magnitude.