Twenty Rows

As part of our next steps in detector characterization, we need to address the '20-row' issue we observed in our IRTF data.  A basic statement of the problem (please chime in if this isn't accurate!):

-  A quasi-fixed pattern exists in nod-pair results, with bright/dark horizontal banding on ~20-row centers.
-  This pattern does not subtract out, and swamps all but the brightest signals for astronomical sources.
-  Because it does not subtract out it must vary over time, or possibly with illumination (as the odd/even effect seems to).
-  This effect has (so far) only been noticed with the science array (serial 115).

First things first....here's a picture demonstrating the effect:

Difference of two scans looking at the dark flag

of the cal wheel @IRTF, 1s integration time.

In the cut it appears that the '20-row' effect in fact has a spatial period of about 16 rows, and an amplitude of about 30DN p-p (~6DN rms).  With a 1s scan time, the 16 row period is about 16ms, which has vary much the fragrance of 60Hz about it.  1DN is roughly 126µV, or 30 electrons, so this amplitude is about 900 electrons (180 rms).  In a single scan, the odd/even effect is similar in amplitude (at least in non-illuminated regions).

Since the period is so close to 60Hz, it worth wondering whether the '20-row' effect is just a pickup artifact...if so it should be asynchronous and should roll through the frame over time.  The file collected just after the image above shows the same amplitude and period, but is shifted in phase.  Here is an image showing the average these two difference frames:

Difference of the image above and the subsequent frame. dark flag

of the cal wheel @IRTF, 1s integration time

Here the phase difference is about pi, so the pickup about cancels and we can even see the faint cross-dispersed spectrum in the average frame.  In this averaged result the odd-even remains, but would probably disappear if the frames were differenced.

Not airtight, but I'm suspecting our '20-row' effect is just 60Hz pickup....