Luisa Rebull &
for sending the data used here to the DIS Calibration Archive.
History: The 60 Hz noise pickup in DIS has been a
long-standing problem, documented as early as September 1995 in a
DIS Status Report by Karen Gloria.
The effect is clearly seen in
blue channel CCD bias frames as a regular banding pattern
but it can also be seen in the red channel.
The column plots, which display the average of 51 columns in blue CCD bias frames, show that the problem was there in the beginning and grew worse until May 1996. The 60 Hz pickup can be seen faintly in the oldest data, 94-NOV-18, with a 5-10 DN peak amplitude. In 95-NOV-27, it's stronger, and by 96-MAY-04 it's up to 20 DN.
Repair: Because the noise was seen only when the instrument was on the telescope, the problem was almost certainly a ground loop. The most likely circuit was through the shielding of the coaxial cable cables carrying the data and clocking signals.
The solution was to replace the cables with fiber optics. This was done in April 1997 by Jon Brinkmann with good results, as seen in the two "after" plots. There is still some low-level 60 Hz noise, but it's weak and unlikely to affect science performance.
The double-peaked histogram: A double-peaked count distribution in the red CCD has been reported recently (by Jon Holtzman, Jim Lauroesch, and Mark Hammergren) but in fact the symptom is present in the earliest data we have (Nov 1994). The double peak is not correlated with the 60 Hz pickup and in fact our noisy "before" picture does not show it while the 60 Hz-free "after" picture does. It's possible that the cause of the double peak increases the readout noise, which was a high 21 electrons as reported by Karen Gloria in the April 11, 1997 night report.