Changes between Version 4 and Version 5 of Stack_Sky_Levels

Timestamp:

Aug 28, 2013, 6:04:57 PM (13 years ago)

Author:

watersc1

Comment:

--

Stack_Sky_Levels

@@ -1 +1 @@
 == 2013-08-28 ==
 
+As the statistics and fitsio both seem to be working as expected, I began looking through the ppStack code to attempt to identify an issue there.  It appears that the important sequence in the code is as follows:
+ 1. Input images are matched against the target PSF, and convovled to match that target.  
+ 2. After convolution, a single ROBUST_MEDIAN background level is subtracted from the convolved images.
+ 3. The convolved images are written to a temporary disk file.
+ 4. Initial and final convolved stacks are constructed, using the disk files to ease the threading of these steps.
+ 5. For the final unconvolved stacks, the list of input original files is passed to the convolution step.  These input files have an unknown (to ppStack), possibly non-zero background level.  
 
+I believe this explains the deviations in the background behavior between the convolved and unconvolved stacks.  As the warps retain a small non-zero background level, failing to remove this results in a stack background level that scales as the number of exposures/exposure time/input proxy.  The solution is to retain a set of background level values for the unconvolved inputs, and optionally apply them during the stacking stage.  This will require a bit of development work to implement.
+
+For an illustration of the input dependence, I've plotted up the measured background level for a set of SAS_22 stacks, separated by filter.
+
+Convolved:
+[[Image(ninput_convolved_check.png,400px)]]
+
+Unconvolved:
+[[Image(ninput_unconvolved_check.png,400px)]]
 == 2013-08-27 ==