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N30 Analysis

Chip N30 exhibited behavior consistent with light damage starting on the afternoon of 2012-11-07. Data obtained that morning appear nominal. A spike in the photodiode data occurred about 9 UT (6am local time), and approximately coincident with some saturated sky flats obtained at the end of the previous night, yet calibration data obtained immediately after the saturated sky flats also appear nominal. Below are images that illustrate this, along with some analysis.

Photodiode data

The photodiode data indicate a spike at approximately 9 UT (6am local time) on 2012-11-07.

Flat comparison

Here are a collection of file numbers that illustrate the change in the behavior of chip N30:

Flats the afternoon before:

  • 147946-147955: 10s Y-band flats.

Several of the last exposures of the night (starting ~9am UT (6am local) on 2012-11-07):

  • 148351: Y-band standard star field. N30 looks normal.
  • 148361-148367: Y, z all with ~60000 ADU/pixel (saturated) and obtained from 9:24 to 9:33 UT. See exp148367n30.png below.
  • 148368-148374: bias frames
  • 148375: u-band flat with ~14000 ADU in N30 and other chips. N30 looks normal.
  • 148416: 10s Y-band dome flat with ~18000 ADU/pixel in N30 and other chips. N30 looks normal. Obtained about 10:30 UT. See exp148146n30.png below.

First exposures (calibrations) the next afternoon: (starting ~8pm on UT 2012-11-07):

  • 148437: 10s Y-band dome flat with ~14000 ADU/pixel in N30, about ~18000 ADU/pixel on others. Obtained about 20:19 UT. See exp148437n3.png below.

One of the saturated sky flats: (centered on N30)

Y-band twilight

Y-band dome flat obtained the same morning shortly after the saturated flat: (centered on N30)

Y-band dome flat obtained the same afternoon: (centered on N30)

10s Y-band flat

The ratio of a Y-band flat obtained the afternoon before the photodiode spike (147950) to one obtained the morning after the spike (148416) indicates that the flats are very consistent with one another at this point. The figure below shows the ratio of the flats for chip N4 (left) and N30 (right).

Ratio of flats obtained before and after the PD spike for N30 (left) and N4 (right).

Photon Transfer Curve

We took photon transfer curve data on 2012-11-09 to evaluate chip N30 and compare it with the other CCDs. Analysis of these data with the script ptc.py indicated that the full well for N30 was approximately 10000 ADU, whereas the values for other chips were on order 40000 ADU. The derived gain for N30 was about 0.05 (ADU/e-), but not well fit. The two images below are the output of ptc.py for the two amplifiers of N30.