Changes between Version 12 and Version 13 of staticsky.20120706_excess_detections

Timestamp:

Mar 1, 2013, 5:35:50 PM (13 years ago)

Author:

watersc1

Comment:

--

staticsky.20120706_excess_detections

@@ -3 +3 @@
 == 2013-03-01 ==
 
-Manually running this example stack has clarified what the issue with the variance is.  Bill was correct, it is a result of the matching chi2 and the additional variance term.  
+Manually running this example stack has clarified what the issue with the variance is.  Bill was correct, it is a result of the matching chi2 and the additional variance term.  Here are the fwhm, chi2, additional variance values for each unrejected input, along with the input pixel values of both the warp and variance for two positions: (972,3213) is in the "good region," where the measured stack image variance matches the value in the variance image; (884, 3265) is in the "bad region," where the observed variance is large.
 
 || Input || FWHM    ||chi^2 || Additional variance || (972,3213) || V(972,3213) || (884,3265) || V(884,3265) ||
@@ -26 +26 @@
 || 10    ||-1217.08|| 7237640.00|| -4041.17|| 6502667.00||
 || 11    ||2353.00|| 4089177.00 ||  || ||
+
+From this data, it's clear that the additional variance terms being applied to all inputs other than 5 have the effect of down-weighting those inputs.  The size of this difference in variance (up to 2 orders of magnitude) means that for the regions where input 5 has data, all other inputs are effectively rejected, resulting in a scaling of only input 5.  Where input 5 has no data, all inputs have approximately the same variance, and so the rejection functions normally and the result has an enhanced s/n compared to a single input.
+
+The PSF matching can be definitively blamed for this effect by constructing the stack using "-Db CONVOLVE F", which disables the PSF matching.  In this case, no additional variance is added to any input, and the output looks like what we would expect.  The image below shows this, with the unconvolved stack from a standard ppStack run on the left, and the output with convolution completely removed on the right.  The scales and colorbars are identical for both frames.
 
 [[Image(excess_detection_testA.jpg)]]