Changes between Version 1 and Version 2 of staticsky.20120706_excess_detections

Timestamp:

Feb 4, 2013, 2:20:00 PM (13 years ago)

Author:

bills

Comment:

--

staticsky.20120706_excess_detections

@@ -3 +3 @@
 Bill Sweeney
 
+psphotStack has found a large number of false detections in some of the stacks. This issue was discussed in
+December in the context of y band and changes were made to detect cases where the number of detected
+sources in y band was much higher than r, i, and z and turn off the feature where all forced photometry
+is performed in all bands for sources only detected in y.
 
-== False Detections from y band stacks 2012-12 ==
+Now that we have completed stack photometry for the most of the sky we can see how widespread the problem is.
 
-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.
+The following plot shows the number of detections per skycell as a function of RA and DEC in g and in y band.
 
-This plot shows the number of matched detetctions in i band verses the number of detected sources in y band.
+[[Image(g_detect.jpg)]]
 
-[[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.
-
+[[Image(y_detect.jpg)]]