Getting Even

Following the fiscal year-end helium drought, we are now cold again in the bench dewar with the science array.  To recap two posts from August:

-  Data taken with a constant voltage on the ØRowOn line showed a ~25% difference in slope (gain) and a substantial offset that can be zeroed out by adjustment of the VrstOff level.

-  Per guidelines from Raytheon, the ØRowOn pulse was to be clocked opposite the ØRstOn pulse to mitigate/eliminate this effect.

-  The MIRI folks at JPL have published two documents that seem to offer conflicting solutions to this problem.  In an internal doc they suggest clocking ØRowOn identically on destructive and non-destructive frames.  In an SPIE paper the remedy is to never clock ØRowOn.  This discrepancy may just be a typo, but taken literally it's a different approach.

-  Our data up to now has not clocked ØRowOn at all, similar to the MIRI SPIE recommendation...we still see odd-even slope differences of about 25%

In the lab this week we've collected a series of non-destructive frames using the old clock table (the one used at the IRTF) with varying values for the VRstOff voltage, which seems to adjust the offset between odd and even rows, but not the gain.  The array was staring at a 40K blackbody with a scan time of 3s.  Here is a plot for of mean pixel values for even and odd rows at three different VrstOff values:

Turns out that both even and odd rows maintain their slopes as VRstOff varies.  The even rows (which are connected to the output circuit during reset) don't shift much, but the odd rows shift vertically as VRstOff changes.  The slope of the even rows differs as before.

Using our new clock table, which nests application of the reset pulse within a de-assertion of the ØRowOn line so that neither row of a given pair is connected to the output circuit during reset, we see more symmetric behavior.  There is more detail in the lab book, but the basic message is captured in this plot:

Here we have slopes that differ by ~3% and only a small offset between odd and even rows.  VRstOff still works as an offset between even/odd rows, and can be adjusted to suit.  I believe that with the new clock table the odd/even effect is no longer a significant issue.