Changes between Version 9 and Version 10 of PS1_GPC1_Magic_Status_20090628

Timestamp:

Jun 28, 2009, 10:13:54 PM (17 years ago)

Author:

eugene

Comment:

--

PS1_GPC1_Magic_Status_20090628

@@ -39 +39 @@
 We have also had concerns that some of the Magic false positives are caused by fractional errors in the variance.  We have found that the GPC1 data processed to the difference level has a higher pixel-to-pixel standard deviation in the background than expected from the noise propagation.  In the case of the exposure above, the effect ranges from about 15% to 25% (ie, stdev is 15% to 25% higher than expected).  We have shown that this effect does not appear in our simulated data analysis, which seems to suggest it is not caused by the software specifically.  The last four images illustrate the impact of an error in the variance level of this scale.  
 
-|| [[Image(http://kiawe.ifa.hawaii.edu/eugene/software/magic/72897.raw.clusters.png)]] 
-|| [[Image(http://kiawe.ifa.hawaii.edu/eugene/software/magic/72897.raw.streaks.png)]] 
-|| 
-|| [[Image(http://kiawe.ifa.hawaii.edu/eugene/software/magic/72897.sig29.clusters.png)]] 
-|| [[Image(http://kiawe.ifa.hawaii.edu/eugene/software/magic/72897.sig29.streaks.png)]] 
+|| [[Image(http://kiawe.ifa.hawaii.edu/eugene/software/magic/72897.raw.clusters.png,550)]] 
+|| [[Image(http://kiawe.ifa.hawaii.edu/eugene/software/magic/72897.raw.streaks.png,550)]] 
 || 
 
-In conclusion, there are a number of camera ar
+|| [[Image(http://kiawe.ifa.hawaii.edu/eugene/software/magic/72897.sig29.clusters.png,550)]] 
+|| [[Image(http://kiawe.ifa.hawaii.edu/eugene/software/magic/72897.sig29.streaks.png,550)]] 
+|| 
+
+The top pair of images illustrate the magic analysis using the standard threshold of 2.3 sigma.  The bottom pair illustrate the effect of changing the threshold to 2.75 (roughly 1.2 times the standard value, thus canceling the error in the variance).  The left set of images shows the pixel cluster that were detected in significant peaks in the hough transform; the right set show the streaks which resulted from those detected clusters.  
+
+It is clear that increasing the threshold (or adjusting the variance to the expected value) results in many fewer clusters.  However, the change in the number of actual streaks is not large.  In addition, while the total number is lower, the streaks detected are largely the same structures in both cases.  It is clear that the variance error is having an effect, but the effect is second order.  
+
+We do not understand the source of this 20% variance error.  The main guesses include the impact of the convolution kernel on low-level defects, the influence of CTE, real variability in the background, scintillation, etc.  Michael Wood-Vasey has reported seeing a similar effect (20% error in the variance in the same direction) in Essence data processed with that pipeline.  
+
+Although we do not understand the source of the elevated variance, we can correct for it by measuring it during the difference analysis.  The robust statistic can be used to measure the standard deviation of the pixels in the signal-to-noise image, and adjust the variance of each skycell as needed.  If we implement this, we need to take care to raise an exception if the predicted variance change is too large -- that could be evidence of something else going wrong.
+
+In conclusion, we have a few areas where we can further attempt to address the causes of magic false positives.  There are still a number of camera artifacts that can be masked; we can avoid applying tiltystreak to certain problematic chips; we can check if burntool was run correctly; we can empirically adjust the variance to match the observed pixel standard deviation.  However, there are definitely features which will be difficult to address.  The biggest concern in my mind is the variable nature of the features and structures we have seen.