Mimir - Dark Current Plots

At the present time (2005/10/04), Mimir shows a time-dependent dark current with a well-fit functional form. The data for the following plots were obtained during the May/June 2005 nights with the array as operated with the 20050529 set of voltages, clocks, and biases.

The first plot shows the run of mean dark counts (in ADUs) for CDS images versus the exposure time.

The log fit indicates we have a "prompt" contribution plus a ln growth with exposure time. The fit equation should be used to predict dark counts in images, but should not be used to correct science images (users should obtain proper dark frames instead).

The second plot shows how the dark current varies with exposure time. These currents were computed from the differences in dark counts between successively longer exposure times, and the X-values are the averages of the starting and stopping exposure times used for the different calculation. The roughly 1/time dark current behavior is consistent with the ln dark count (the integral of the dark current) behavior.

Clearly, we would strongly prefer a lower dark current and one which does not vary with exposure time. During the November 2005 detector servicing run, we will try to understand the origin of these dark count and dark current relationships and cure them.