The Variance Maps have patterns.

We investigated the variance maps for different stages/processing:

 * chip (md)
 * warp (md)
 * stack (md and sas)

There are periodic patterns associated with warp and stack weight maps, which are not seen on the chip weight maps.  

Here is a zoom of one of the unconv.wt.fits for SAS footprint: (stack_Id= 336346). You can see an odd periodic background. The yellow circles are the false detections.
[[Image(skycell.1316.089.336346.sas.wt.png)]]

Here is a zoom of one of the unconv.wt.fit stacks for a recent MD refstack, it not quite as strange, but still, there is an underlying odd, periodic background.
[[Image(md01.wt.png)]]

Chip stage weight maps do not have this background (click to zoom in, this is for chip_id 310753 and XY53)

The full chip:

[[Image(xy53ch310753.png)]]

The weight map of one of the cells in XY53

[[Image(chip.wt.png)]]

The detrended fits of that same cell in XY53

[[Image(chip.png)]]


Warp stage (for the same chip) has a very strange background for the weight map (click to zoom in)

One of the skycells (the weight map, same exposure, same cell, but warp stage)

[[Image(warp.wt.png)]]

A zoom of the weight map for that cell - the background has a periodic pattern

[[Image(warp.wt.zoom.png)]]

And one of the warps (same cell, it looks okay)

[[Image(warp.png)]]

We think something weird is happening with the variance/weight maps in the warp stage - this might cause the funny false detections in the SAS.footprints, and likely causes problems elsewhere...