Changes between Initial Version and Version 1 of GPC_Dark_Trends

Timestamp:

Feb 24, 2009, 4:23:55 PM (17 years ago)

Author:

trac

Comment:

--

GPC_Dark_Trends

@@ +1 @@
+=== GPC Dark Trends ===
+
+I analyzed a set of 140 dark exposures taken during run 1c in the period from 02/08/08-02/11/08. The exposuretimes range from 12 to 100s, the detector temperatures range from -90 to -60. The figure below shows expodure time vs. temperature of the dataset: 
+
+[[Image(htdocs:/images/time_vs_temp.jpg)]]
+
+==== ccd36/cell2 analysis ====
+
+I selected a region x=461..510,y=81..130 in chip36/extension2 with a relatively prominent dark feature:
+
+[[Image(htdocs:/images/selected.jpg)]]
+
+In the first step I took the average of the region and made a second order polynomial fit to the exposuretime (DARKTIME keyword). I found that the first two exposures of each of the 10 dark series have significantly lager counts that the other ones. I plot the residuals after subtracting the best fit as a function of exposure number withing series (different series color-coded):
+
+[[Image(htdocs:/images/dark_number3.jpg)]]
+
+The first blue point s not displayed and has a residual value of 200. From now on I dropped the first two exposures of each series. The two figures below shows the linear fit in exposure time of the remaining 120 dark images and the residuals as a function of temperature (DETTEM keyword):
+
+[[Image(htdocs:/images/timefit.jpg)]][[Image(htdocs:/images/tempres.jpg)]]
+
+There seems to be no clear 2nd order trend like you would expect it(monotonically rising with temperature), so I decided to look at the 30s exposures only and fitted a linear relation to the dark current as a function of temperature:
+
+[[Image(htdocs:/images/temp_30s.jpg)]]
+
+The next step is to do the analysis pixel-by-pixel. I used again the 120 images with different exposure times and fitted a linear relation in time and linear relation in temperature. The coefficients for each pixel are stored in a fits file:
+
+[[Image(htdocs:/images/a1_1.jpg)]][[Image(htdocs:/images/a1_2.jpg)]]
+[[Image(htdocs:/images/a2_1b.jpg)]][[Image(htdocs:/images/a2_2.jpg)]]
+
+Using this basis a dark of any arbitrary exposure time and temperature can be created: dark = a1_1 * time[s] + a1_2 + a2_2 * temperature[C] + a2_2.
+Of course, a1_2 and a2_2 could be combined. Below there are some example residual images:
+
+[[Image(htdocs:/images/res1.jpg)]][[Image(htdocs:/images/res2.jpg)]]
+
+Also interseting to see are the nchi-images which show the quality of both fits, nchi = sqrt( SUM[ dark_residual^2 ] / N ):
+
+[[Image(htdocs:/images/nchi1.jpg)]][[Image(htdocs:/images/nchi2.jpg)]]
+
+Cosmics are clearly visible and should be removed to identify the pixels that are really bad. To do this I included an iterative kappa-sigma-clipping to the fit. This is the resulting nchi-image of the time-fit:
+
+[[Image(htdocs:/images/nchi_better.jpg)]]
+
+Ok, this particular cell (chip36/extension2) seems to be quite 'friendly', meaning correctable. But what about the others?
+
+==== full detector improved analysis ====
+
+In the following I show the results of a full-detector analysis. I dropped one more raw image, because it turned out to have zero values only is some of the chips. Also I changed from doing a linear fit in time and temperature to a 2-dimentional fit: dark = a0 + a1 * time + a2 * temperature.
+
+ * [wiki:Global_results global results]
+ * [wiki:Ccd01_details ccd01 details]
+ * [wiki:Ccd02_details ccd02 details]
+ * [wiki:Ccd03_details ccd03 details]
+ * [wiki:Ccd04_details ccd04 details]
+ * [wiki:Ccd05_details ccd05 details]
+ * [wiki:Ccd06_details ccd06 details]
+ * [wiki:Ccd10_details ccd10 details]
+ * [wiki:Ccd11_details ccd11 details]
+ * [wiki:Ccd12_details ccd12 details]
+ * [wiki:Ccd13_details ccd13 details]
+ * [wiki:Ccd14_details ccd14 details]
+ * [wiki:Ccd15_details ccd15 details]
+ * [wiki:Ccd16_details ccd16 details]
+ * [wiki:Ccd17_details ccd17 details]
+ * [wiki:Ccd20_details ccd20 details]
+ * [wiki:Ccd21_details ccd21 details]
+ * [wiki:Ccd22_details ccd22 details]
+ * [wiki:Ccd23_details ccd23 details]
+ * [wiki:Ccd24_details ccd24 details]
+ * [wiki:Ccd25_details ccd25 details]
+ * [wiki:Ccd26_details ccd26 details]
+ * [wiki:Ccd27_details ccd27 details]
+ * [wiki:Ccd30_details ccd30 details]
+ * [wiki:Ccd31_details ccd31 details]
+ * [wiki:Ccd32_details ccd32 details]
+ * [wiki:Ccd33_details ccd33 details]
+ * [wiki:Ccd34_details ccd34 details]
+ * [wiki:Ccd35_details ccd35 details]
+ * [wiki:Ccd36_details ccd36 details]
+ * [wiki:Ccd37_details ccd37 details]
+ * [wiki:Ccd40_details ccd40 details]
+ * [wiki:Ccd41_details ccd41 details]
+ * [wiki:Ccd42_details ccd42 details]
+ * [wiki:Ccd43_details ccd43 details]
+ * [wiki:Ccd44_details ccd44 details]
+ * [wiki:Ccd45_details ccd45 details]
+ * [wiki:Ccd46_details ccd46 details]
+ * [wiki:Ccd47_details ccd47 details]
+ * [wiki:Ccd50_details ccd50 details]
+ * [wiki:Ccd51_details ccd51 details]
+ * [wiki:Ccd52_details ccd52 details]
+ * [wiki:Ccd53_details ccd53 details]
+ * [wiki:Ccd54_details ccd54 details]
+ * [wiki:Ccd55_details ccd55 details]
+ * [wiki:Ccd56_details ccd56 details]
+ * [wiki:Ccd57_details ccd57 details]
+ * [wiki:Ccd60_details ccd60 details]
+ * [wiki:Ccd61_details ccd61 details]
+ * [wiki:Ccd62_details ccd62 details]
+ * [wiki:Ccd63_details ccd63 details]
+ * [wiki:Ccd64_details ccd64 details]
+ * [wiki:Ccd65_details ccd65 details]
+ * [wiki:Ccd66_details ccd66 details]
+ * [wiki:Ccd67_details ccd67 details]
+ * [wiki:Ccd71_details ccd71 details]
+ * [wiki:Ccd72_details ccd72 details]
+ * [wiki:Ccd73_details ccd73 details]
+ * [wiki:Ccd74_details ccd74 details]
+ * [wiki:Ccd75_details ccd75 details]
+ * [wiki:Ccd76_details ccd76 details]
+
+==== comments ====