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March 5, 2012

As discussed in the March 2 status, I've been looking into how the dark model may be introducing the gradients observed in the science data. To see what (if any) functional dependence these gradients had, I selected dark frames used in the construction of the current dark model, and calculated the detproc (overscan corrected, no dark applied) and detresid (overscan and current dark model applied) images. I used only XY67 in this study, as it has clear science image gradients, and was a useful test case. For each variable that could influence the dark, I selected two exposures that spanned the range, while attempting to keep the remaining variables constant. Here are the profile plots for these tests (profile the same as that used in the previous study of XY67: a large 300 pixel box covering the row of cells xy10-xy17):

dateobs

exp_name	dateobs	exp_time	ccd_temp	detproc	detresid
o5605g0022d	2011-02-13	10	-78.455
o5743g0645d	2011-07-01	10	-79.6983

exp_time

exp_name	dateobs	exp_time	ccd_temp	detproc	detresid
o5641g0690d	2011-03-21	0.001	-87.2183
o5638g0035d	2011-03-18	300	-87.0017

ccd_temp

exp_name	dateobs	exp_time	ccd_temp	detproc	detresid
o5630g0494d	2011-03-10	30	-88.4883
o5736g0606d	2011-06-24	30	-77.745

ccd_temp2

exp_name	dateobs	exp_time	ccd_temp	detproc	detresid
o5654g0593d	2011-04-03	10	-86.765
o5612g0382d	2011-02-20	10	-72.1067

30s test

exp_name	dateobs	exp_time	ccd_temp	detproc	detresid
o5666g0013d	2011-04-15	30	-84.43
o5666g0645d	2011-04-15	30	-84.37

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.

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.

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.