| | 1 | == March 1-2, 2012 == |
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| | 3 | Similar to Nigel's work, I've been looking at the gradients, however starting from the individual frames. Choosing one of the input exposures that contributes the bulk of the data to skycell.1405.022, I've placed a 300 pixel wide profile along one row of cells (a vertical column as seen in the stack image below). For reference, my row of cells is the 6th from the left side in the stack image below. The first set of profiles shows the data for the raw image (no continuity correction or background models), the raw image corrected by the continuity correction only, and finally the continuity and background corrected image, which is equivalent to what would have gone into the stack. |
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| | 5 | || Raw || Continuity Corrected || Final || |
| | 6 | || [[Image(gradient_raw.png,300px)]] || [[Image(gradient_continuity.png,300px)]] || [[Image(gradient_final.png,300px)]] || |
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| | 8 | Based on these profiles, it's clear that the initial image has some gradient included, the continuity correction does a reasonable job of aligning these gradients into a large slope, and then the background correction removes this large slope, with a few DN peak-to-peak residual gradient remaining. I checked that this gradient was not due entirely to the increased row-to-row variations evident on the high edge of the chip, and that does not appear to be the case. The best description is that there is an underlying gradient, and as the gradient increases, the row-to-row variations produce higher spikes that trigger the false detections. |
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| | 10 | As one aspect of these gradients is to determine how the dark influences the pattern, I looked at the profile for a dark detresid image (a dark frame processed using the current dark model; note that this detresid was taken ~250 days after the footprint data, and therefore is not a direct comparison) to see what residual patterns are visible. |
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| | 12 | [[Image(gradient_detresid.png,400px)]] |
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| | 14 | This detresid image shows a similar saw-tooth pattern visible along that same row of cells on the same OTA. Checking the detproc image (a dark frame that has not had the dark model applied) shows a different profile shape: |
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| | 16 | [[Image(gradient_detproc.png,400px)]] |
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| | 18 | This suggests that the current dark model assumes a given detector response along the cell pixel columns that possibly no longer matches the real detector response. I am now checking a large sample of dark images to determine how this has changed over time. |
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