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add PTC turnoffs
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youtsumi committed Dec 9, 2024
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Persistence optimization
############################################

Leftover signal in the following darks after a blast of illumination has been observed. It is called "Persistence".
Persistence has been observed in an early prototype E2V sensor as early as 2014 ([D2014]_). It was confirmed that the amplitude of the persistence decreased as the parallel swing voltage got smaller. This is consistent with the Residual Surface Image [J2001]_ -- the excessive charges are being stuck at the surface layer. The level of persistence is about 10--20 ADU, and the decaying time constant is about 30sec.
Leftover signal in the following dark after a blast of illumination has been observed. It is called "Persistence".
Persistence has been observed in an early prototype E2V sensor as early as 2014 ([D2014]_). It was confirmed that the amplitude of the persistence decreased as the parallel swing voltage got smaller. This is consistent with the Residual Surface Image [J2001]_ -- the excessive charges are being stuck at the surface layer. The level of persistence is about 10--20 ADU, and the decaying time constant is about 30 sec.

During the EO testing in 2021, we also found the persistence made a streak toward the readout direction from the place where the bright spot located in a previous image. We call this trailling persistecnce.
During the EO testing in 2021, we also found the persistence made a streak toward the readout direction from the place where the bright spot located in a previous image. We call this trailing persistence.

E2V sensors have another major problem so-called "tearing", which is considered as the consequence of the non-uniform distribution of holes. Our primary focus in the opimization was given to mitigate the tearing over years and we have successfully elminate the tearing by bringing the e2v voltage from the unipolar voltage (both parallel rails high and low are posititve) to the bipolar voltage (the parallel high is positive and the low is negative) following the formula [2]_. However, the persistence issue still remained unchanged.
E2V sensors have another major problem, so-called "tearing", which is considered a consequence of the non-uniform distribution of holes. Our primary focus in the optimization was given to mitigate the tearing over years, and we have successfully eliminated the tearing by bringing the E2V voltage from the unipolar voltage (both parallel rails high and low are positive) to the bipolar voltage (the parallel high is positive, and the low is negative) following the formula [Bipolar]_. However, the persistence issue still remained unchanged.

For the persistence issue, if this is the residaul surface image, two approaches could be taken as discussed in [U2024]_. Either 1) establishing the pinning condition where the holes make a thin layer at the front surface so to get excessive charges recombined with the holes. or 2) narrowing the parallel swing so not to get the accumulated charges in the silicon don't get close to the surface state.
For the persistence issue, if this is the residual surface image, two approaches could be taken as discussed in [U2024]_. Either 1) establishing the pinning condition where the holes make a thin layer at the front surface so that the excessive charges recombine with the holes or 2) narrowing the parallel swing so that the accumulated charges in the silicon do not get close to the surface state.

The pinning condition could be established by bringing the parallel low voltage down as low as -7V or lower. The transition voltage needs to be empirically determined. However, E2V pointed out the measured current flow increases as the parallel low voltage goes low, which increases the risk of damaging the sensor by making a breakdown [1]_. Also, The excessive charges could get recombined by the thin layer of the holes, which could disturb the linearity at the high flux end whhere charges start to intaract with the holes.
The pinning condition could be established by bringing the parallel low voltage down as low as -7V or lower. The transition voltage needs to be empirically determined. However, E2V pointed out that the measured current flow increases as the parallel low voltage goes low, which increases the risk of damaging the sensor by making a breakdown [1]_. Also, the excessive charges could get recombined by the thin layer of the holes, which could disturb the linearity at the high flux end where charges start to interact with the holes.

The parallel swing determines the fullwell. Depending on whether the accumulated charges spread over the columns or interact with the surface layer, there are blooming fullwell regime and the surface fullwell regime. The fullwell between these two regime is considered as the optimal fullwell [J2001]_, where we don't have persistence and as high dynamic range as possible. Seeing the persistence, we likely operate the sensor in the surface fullwell condition and we need to go to narrower voltage to get the blooming fullwell or the optimal fullwell. The obvious downside is to narrow the fullwell.
The parallel swing determines the fullwell. Depending on whether the accumulated charges spread over the columns or interact with the surface layer, there are blooming fullwell regimes and the surface fullwell regime. The fullwell between these two regimes is considered as the optimal fullwell [J2001]_, where we don't have persistence and as high dynamic range as possible. Seeing the persistence, we likely operate the sensor in the surface fullwell condition and we need to go to a narrower voltage to get the blooming fullwell or the optimal fullwell. The obvious downside is to narrow the fullwell.

The voltages are defined relative to each other. Changing the parallel swing (for example) also requires changes all other voltages to operate the sensor properly, for example properly reset the amplifier. The initial voltage was given in the original formula [2]_ but to go to the narrow voltage we had to switch to the new formula in order to satisfy constraints [3]_.
The voltages are defined relative to each other. Changing the parallel swing (for example) also requires changes to all other voltages to operate the sensor properly, for example, properly reset the amplifier. The initial voltage was given in the original formula [Bipolar]_ but to go to the narrow voltage we had to switch to the new formula in order to satisfy constraints [PersistenceMitigationVoltage]_.

[S2024]_, set up a single sensor test-stand at UC Davis. They attempted multiple different approaches mentioned above and reported the results [DavisReport]_. The summary is as follows:

[S2024]_, set up the a single sensor test-stand at UC Davis. They attempted multiple different approaches aformetioned above and reported the result [4]_. The sumamry is as follows
- The new voltages following the rule work fine.
- Narrowing the parallel swing eliminates the persistence
- Lowering the parallel low voltage didn't seem to work as we expect, the going further negative voltage is probably needed.
Note that the UCD setup didn't show up the persistence. It might be due to the characteristic of the sensor, might be due to the difference in the electronics (the long cable between CCD and REB, for example). They need to move the parallel rails up
- Narrowing the parallel swing eliminates the persistence.
- Lowering the parallel low voltage didn't seem to work as we expected; the going further negative voltage is probably needed.

Note that the UCD setup didn't show up the persistence. It might be due to the characteristic of the sensor, or might be due to the difference in the electronics (the long cable between CCD and REB, for example). They need to move the parallel rails up.

.. [1] We note that ITL operates at the parallel low voltage of -8V. We have observed the increased current flow. But we have the software protection so that the current flow goes too high.
.. [Bipolar] https://github.com/lsst-camera-dh/mkconfigs/blob/master/newformula.py
.. [PersistenceMitigationVoltage] https://github.com/lsst-camera-dh/e2v_voltages/blob/main/setup_e2v_v4.py
.. [DavisReport] https://docs.google.com/document/d/1V4o9tzKBLnI1nlOlMFImPko8pDkD6qE7jzzk-duE-Qo/edit?tab=t.0#heading=h.frkqtvvyydkr
.. [J2001] https://www.spiedigitallibrary.org/ebooks/PM/Scientific-Charge-Coupled-Devices/eISBN-9780819480392/10.1117/3.374903
.. [D2014] https://ui.adsabs.harvard.edu/abs/2014SPIE.9154E..18D/abstract
.. [U2024] https://ui.adsabs.harvard.edu/abs/2024SPIE13103E..0WU/abstract
.. [S2024] https://ui.adsabs.harvard.edu/abs/2024SPIE13103E..21S/abstract
Persistence optimization
^^^^^^^^^^^^^^^^^^^^^^^^

Based on this test result, we decided to try out the new voltage with the narrower voltage swing on the main Camera focal-plane. Keeping the parallel low voltage at -6V in order to operate the sensor safely (very conservative limit), we change the parallel swing voltage from 9.3V to 8.0V as well as all the other voltages using the new formula. We overexposed CCDs and took 20 darks after.
The image shown below is mean or median of pixel-by-pixel difference between the first and the last dark exposures, as a function of the parallel swing. As the parallel swing is lowered, the residual signal becomes small, it becomes rougly 10 times lower than the orignal 9.3V. Although we sampled mid points between 8.0 and 9.3V, 8.0V appears to work the best and it could be lower with the penalty of loosing the full well.
Based on this test result, we decided to try out the new voltage with the narrower voltage swing on the main camera focal plane. Keeping the parallel low voltage at -6V in order to operate the sensor safely (very conservative limit), we changed the parallel swing voltage from 9.3V to 8.0V as well as all the other voltages using the new formula. We overexposed CCDs and took 20 darks after.
The image shown below is the mean or median of pixel-by-pixel difference between the first and the last dark exposures, as a function of the parallel swing. As the parallel swing is lowered, the residual signal becomes small; it becomes roughly 10 times lower than the original 9.3V. Although we sampled midpoints between 8.0 and 9.3V, 8.0V appears to work the best and could be lower with the penalty of losing the full well.

.. figure:: sections/figures/e2v_transient_dark_vs_dp.png

When we measure the mean and standard deviation, together with the median of the red (e2v) curves above and arrange them by the swing voltage, we can select a more informed swing voltage to operate with an acceptable amount of persistence. The error bars attached to the mean are adopted from the standard deviations of each histogram.
The remaining charges measured in every amplifier but aggregated by mean or median as a function of the parallel clock swing are shown.

The following figures diplay how the persistence is reduced by the voltage change. The images were processed by the standard instrumental signature removal, get assembled in the full focal-plane view. The dark exposure was taken right after the 400ke- -equivalent flat exposure. The figure shows the disticnt pattern of elevated signal associated with the vendor. The inner part of the focal plane is filled by e2v sensors which has the persistence signal.
The following figures display how the persistence is reduced by the voltage change. The images were processed by the standard instrumental signature removal and get assembled in the full focal-plane view. The dark exposure was taken right after the 400ke-equivalent flat exposure. The figure shows the distinct pattern of elevated signal associated with the vendor. The inner part of the focal plane is filled by e2v sensors which have the persistence signal.

.. figure:: sections/figures/E1110dp93.png

The first dark exposure after a 400k flat image under the parallel swing of 9.3V. (Run E1110)

The next figure shows the same dark exposure but taken with the nallow parallel swing voltage of 8.0V. The distict pattern goes away.
The next figure shows the same dark exposure but taken with the narrow parallel swing voltage of 8.0V. The distinct pattern goes away. This demonstrates the persistence in e2v sensors becomes the level of ITL's ones.


.. figure:: sections/figures/E1310dp80.png
Expand All @@ -47,23 +58,16 @@ The next figure shows the same dark exposure but taken with the nallow parallel

Impact on full-well
^^^^^^^^^^^^^^^^^^^^^^^^
Reduction of the full well is expected by narrowing the parallel swing voltage. This subsection explores how much reduction in the PTC turnoff is observed in the dense PTC run. Two runs are acquired with identical setting except for the CCD operating voltage. As the PTC turnoff is defined in ADU, it needs to be multiplied by PTC_GAIN to make the comparison. The figure below compares the PTC turnoff in electrons and their difference in ratio. The median reduction was 22% .

Impact on Brighter-Fatter effect
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Yassine will put his material here.
.. figure:: sections/figures/PtcTurnoffRatio.png

.. [1] We note that ITL operates at the parallel low voltage of -8V. We have observed the increased current flow. But we have the software protection so that the current flow goes too high.
.. [2] https://github.com/lsst-camera-dh/mkconfigs/blob/master/newformula.py
.. [3] https://github.com/lsst-camera-dh/e2v_voltages/blob/main/setup_e2v_v4.py
Histograms of the PTC turn offs (left) and the ratios of differences (right) between E1113 (9.3V) vs E1335 (8.0V). The median of the reduction is 22%.

.. [4] https://docs.google.com/document/d/1V4o9tzKBLnI1nlOlMFImPko8pDkD6qE7jzzk-duE-Qo/edit?tab=t.0#heading=h.frkqtvvyydkr

.. [J2001] https://www.spiedigitallibrary.org/ebooks/PM/Scientific-Charge-Coupled-Devices/eISBN-9780819480392/10.1117/3.374903
Impact on Brighter-Fatter effect
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Yassine will put his material here.

.. [D2014] https://ui.adsabs.harvard.edu/abs/2014SPIE.9154E..18D/abstract

.. [U2024] https://ui.adsabs.harvard.edu/abs/2024SPIE13103E..0WU/abstract

.. [S2024] https://ui.adsabs.harvard.edu/abs/2024SPIE13103E..21S/abstract

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