Changes between Version 3 and Version 4 of Background_Dark_Model

Timestamp:

Mar 6, 2012, 6:22:51 PM (14 years ago)

Author:

watersc1

Comment:

--

Background_Dark_Model

@@ -37 +37 @@
 === Results ===
 
-Looking at the residual plots for the dateobs and exp_time scans shows that there is some difference between the endpoints of these ranges, but that both have a strong gradient along the x pixel direction on each cell.  Surprisingly, the direction of this gradient reverses in the ccd_temp scan, even though the temperatures studied in the previous scans are only a degree or two different.  Repeating this study with a different exposure time shows that the 30s darks have a different gradient trend, a result confirmed by choosing two 30s darks from a single date with near identical ccd temperatures.
+The above profiles suggested that 30s darks were somehow different than other exposure times.  This seems to be an unfortunate coincidence of the darks selected.  Processing a set of evening darks (of all times) from 2011-04-15 all show the downward slopes, and all from 2011-04-03 show the upward slopes.  Given this behavior, I selected a set of darks from various dates, processed them, and extracted the slope in the first cell.  The following plots show the results of these slopes.  The exposure time was chosen to be the same for all of these exposures, and their position in the night was selected to be the same as well (these are the second 30s evening dark taken on each date between the beginning and end of the current dark model inputs).
 
-Given that the gradient remaining after the dark is applied slopes upward for the 0.001,10,and 300s darks, but downward in the 30s darks, it seems reasonable to assume that some aspect of the exposure time is not being fully modeled by the dark analysis.  The current assumed functional form of the dark model is:
-{{{
- D = c_0 + (c_1 + c_2 ccd_temp + c_3 ccd_temp^2) * exp_time
-}}}
-Previous studies over the summer of 2011 suggested that c_3 is roughly equal to zero for the temperature range GPC1 currently operates at, and so this quadratic term in ccd_temp was dropped, leaving a model that is linear in exp_time and ccd_temp.  If there is some non-linearity in the dark response as a function of exposure time, then it is easy to see how we could arrive at the observed residual structure by fitting a linear model to this response.
-
+[[Image(ccdtemp.png,400px)]]
+[[Image(pontime.png,400px)]]
+[[Image(dateobs.png,400px)]]
 I've requested that a sequence of darks be taken to more finely cover the range of exposure times, and will use this data to develop a more complete dark model that will hopefully not introduce any residual gradients.