Stacking Photometry Issue

2013-03-07

After some reading through email threads and looking at Eddie's figures, I constructed a similar test/data set. I chose three i-filter stacks along an RA slice at dec = -27, and compared the CAL_PSF_MAG values calculated by the static sky CMF to the median value of CAL_PSF_MAG for those objects in the warp CMFs. This nicely reproduced the figure that Eddie had made. The three positions I chose were (RA,DEC,stack_id) red=(170,-27,1111371), green=(197,-27,1157219), blue=(200,-27,1238704). Based on Eddie's ucal comp figure (reproduced below), the red and green positions sample the area that is mottled with large offsets, and the blue position is a "good" position where the offsets seem small. Constructing the <m_W> - m_SS plot shows that the blue position is good, with a single population scattered around zero:

Based on this figure, I selected the objects with m_SS < 20 AND (<m_W> - m_SS) < -0.25 to isolate the offset magnitudes, and then plotted them on the images. The small number of points shown in the blue position are generally found near NAN/masked regions, or for stars with NANs in the core (saturated?). However, the other two positions have these objects in regions that can be clearly seen to be the "interchip gaps" of one of the input pointings.

Red:

Green:

Blue:

Pre 2013-03-07