Changes between Initial Version and Version 1 of staticsky.20120706_excess_detections

Timestamp:

Feb 4, 2013, 2:10:31 PM (14 years ago)

Author:

bills

Comment:

--

staticsky.20120706_excess_detections

@@ +1 @@
+= This page discusses stacks with an excess in false detections in LAP.!ThreePi.20120706 =
+
+Bill Sweeney
+
+
+== False Detections from y band stacks 2012-12 ==
+
+We have noticed that some skycells are getting large numbers of matched (forced photometry) detections. 
+These are caused by false detections in the y band. Recall that forced photometry is performed for all y band sources 
+that are not matched with a detection in another band while detections for the other filters must match 2 other bands
+before forced photometry is done.
+
+This plot shows the number of matched detetctions in i band verses the number of detected sources in y band.
+
+[[BR]]
+[[Image(matched_i.jpg)]]
+
+If we plot the ratio of the number of matched detections to the number of detected sources for each band we can see these outliers.
+The following plots shows the ratio for the r band versus that in the ratio in i band for each stack. The plot on the left shows the full range
+including the outliers wile the plot on the right zooms in on the values near zero.
+[[Image(mdr.jpg)]] [[Image(mdrzoom.jpg)]]
+[[BR]]
+
+The plots for g and z show the same shape while the y band plot does not show the tail.
+
+[[Image(mdy.jpg)]]
+[[BR]]
+
+We propose re-running psphotStack for these skycells changing the rule that matched detections are created for all unmatched y band
+detections if some criteria is met. We can get rid of the worst of the outliers with the following cut. 
+[[BR]]
+
+{{{
+             Nmatched_i / NDetected_i > 0.4 && Nmatched_r /  NDetected_r > 0.6
+}}}
+[[BR]]
+This effect is seen in 3% of the skycells north of 30 degrees declination.
+[[Image(mdr_cut.jpg)]]
+
+
+After testing a more conservative cut was chosen. It is based on the ratio of the number of sources in y
+to the number of sources detected in r, i, and z.
+
+Automatic forced photometry for sources detected in y band but not any other is disabled for skycells where 
+
+{{{
+   (NDetected_y / NDetected_r) > 1.7 && (NDetected_y / NDetected_i > 1.6) &&  && (NDetectected_z > 1.65)
+}}}
+
+About 1 percent of skycells is affected by this feature.
+