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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

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).

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.