SITCOMTN-148

LSST Camera Electro-Optical Test Results

This is a draft.

By:

• Aaron Roodman
• Yousuke Utsumi

Abstract

This note collects results from the LSST Camera electro-optical testing prior to installation on the TMA. We describe the CCD and Focal Plane optimization and the resulting default settings. Results from eo_pipe are shown for standard runs such as B-protocols, Dense and SuperDense PTCs, gain stability, OpSim runs of Darks, and Darks with variable delays. We also describe features such as e2v Persistence, ITL phosphorescence in coffee stains, remnant charge near Serial register following saturated images, vampire pixels, ITL dips, and others.

Electro-optical setup

Run 7 Optical modifications

hello world.

This section describes run 7 optical changes to the CCOB, projector, etc.

• refresh of setup with items the same as IR2 (CCOB, no narrow-beam)

• diffuser install

• projector

Projector spots

hello world.

This section describes the spots and rectangles tested with the 4k projector

• Projector background

• Spots on many amps

• Spots on one amp

• Optical setup

FCS development

hello world.

This section describes FCS operation and intervention.

• Activity with the FCS

• Fault at end of september & documentation

• autochanger known light leak

• Running FCS in emulator mode (OCS)

Characterization

Dark current and light leaks

This section describes dark current and light leaks in Run 7 testing.

One of the first tests we attempted with the camera was measuring dark current and sources of light leaks in the camera body.

Light leak mitigation with shrouding the camera body

Sources of light leak with the autochanger

After completing the shroud of the camera, we proceeded with several long dark exposures using different filter and shutter conditions to establish our baseline dark condition for testing.

• We acquired 900s darks with different shutter conditions and the empty frame filter in place.

• We acquired 900s darks in different filters with the shutter open

Shutter condition impact on darks

Filter condition impact on darks

Final measurements of dark current

Baseline characterization

hello world.

This section describes baseline characterization and reverification

• first B protocols and PTCS

• comparison to IR2 metrics

• new features in any baseline runs?

Guider operation

hello world.

This section describes guider operation.

• initial guider operation

• power cycling the guiders to get to proper mode

• synchronization

• guider roi characterization

Tree rings

hello world.

This section describes tree rings.

• Tree rings without diffuser

• Tree rings with diffuser

Camera Optimization

Sequencer Optimization

hello world.

This section describes sequencer optimization.

• No-pocket conclusions

• Overlap conclusions

• Serial flush conclusions

Persistence optimization

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 [dmtn-276].

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 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 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 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 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 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:

• 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 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 does not go too high.

[dmtn-276]

https://dmtn-276.lsst.io

[Bipolar] (1,2)

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] (1,2)

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 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.

Fig. 1 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 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.

Fig. 2 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 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.

Fig. 3 The first dark exposure after a 400k flat image under the parallel swing of 8.0V (Run E1310). The figure shows no distinct patterns from persistence in e2v sensors anymore.

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 (E1113 for 9.3V and E1335 for 8.0V). As the PTC turnoff is defined in ADU, it needs to be multiplied by PTC_GAIN to make a comparison. The figure below compares the PTC turnoff in electrons and their difference in ratio. The median reduction was 22% .

Fig. 4 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%.

Impact on Brighter-Fatter effect

Reducing the parallel swing is expected to enhance the brighter-fatter effect (BFE), possibly in an anisotropic way. The BFE can be characterized via the evolution of the variance and covariances of flatfield exposures as a function of flux. In order to evaluate the impact of reducing the parallel voltage swing on e2v sensors, we acquired two series of flatfield exposures with the respective voltage setups and extracted the “area” coefficients the “area” coefficients (Equation (1) in [A2023]) from these two data sets. The area coefficients describe by how much a unit charge stored in a pixel wil alter the area of some other pixel (or itself). We find that reducing the parallele swing from 9.3V to 8V typically increases the area coefficients by 10% (between 5 and 19% depending on distance), and the increase is almost isotropic (along serial and parallel directions). From these measurements, we anticipate that the increase of star sizes with flux will not become more isotropic at 8V than it was at 9.3V, and hence does not introduce new threats on the measurement of the PSF ellipticity

Fig. 5 Scatter plots of area coefficients (one entry per amplifier) measured at 8V and 9.3V. The 9 subfigures correspond to separations between the source of the area distortion and its victim, with the self interaction at the bottom left. The first neighbors increase respectively by 19% in the parallel direction by 14% in the serial direction. So the BFE is slightly larger at 8V but not significantly more anistropic.

[A2023]

https://arxiv.org/pdf/2301.03274

Summary

E2V sensors had persistence. We confirmed changing the E2V CCD operating voltage greatly reduced persistence. As penalties, we observed 22% of full well reduction, and a ~10% increase of the brightter-fatter effect, essentially in an isotropic way.

Thermal Optimization

hello world.

This section describes thermal optimization.

• Background

• Idle flush off & it’s stability

• impact on other parameters

Camera stability

Defect stability

hello world.

This section describes defect stability.

• Bright defects

• Dark defects with picture frame

Bias stability

hello world.

This section describes bias stability.

• Typical bias stability runs

• dark delay

• dark with bias delays

Gain stability

hello world.

This section describes gain stability.

• No temp variation, fixed flux

• no temp variation, variation in flux

• Temp variation, fixed flux

Sensor features

ITL Dips

hello world.

This section describes ITL Dips.

Vampire pixels

First observations

Vampire pixels were first observed in ComCam observations [need more info to properly give context] - Andy’s study on Oct. 8 - Agnes masking effort

LSSTCam vampire pixel features

The vampire pixels have distinct features, both on the individual defect level, and across the focal plane

Individual vampire features

• General size

• Radial kernel

• uniformity

Vampire features across the focal plane

• sensor type

• static or dynamic

• higher concentrations? Particularly bad sensors?

Current masking conditions

• Bright pixels

• Dark pixels

• Jim’s task

Analysis of flats

• LED effect

• Intensity effect

Analysis of darks

• Previous LED effect

• Intensity of LED effect

• dark cadence and exposure times

Current models of vampires

• Tony & Craig model

• Others?

Serial remnants

hello world.

This section describes incomplete serial flush.

• Background

• Mitigation with sequencers

• discussion of different clears

Phosphorescence

hello world.

This section describes phosphorescence.

• phosphorescence background

• phosphorescence on flat fields

• phosphorescence on spot projections

Conclusions

Run 7 final operating parameters

This section describes the conclusions of run 7 optimization and the operating conditions of the camera. Decisions regarding these parameters were decided based upon the results of the voltage optimization, sequencer optimization, and thermal optimization.

Voltage conditions

Table 1 Voltage conditions

Parameter	dp80 (new voltage)	dp93 (Run 5)
pclkHigh	2.0	3.3
pclkLow	-6.0	-6.0
dpclk	8.0	9.3
sclkHigh	3.55	3.9
sclkLow	-5.75	-5.4
rgHigh	5.01	6.1
rgLow	-4.99	-4.0
rd	10.5	11.6
od	22.3	23.4
og	-3.75	-3.4
gd	26.0	26.0

Sequencer conditions

Table 2 Sequencer conditions

Detector type	File name
E2V	FP_E2V_2s_l3cp_v30.seq
ITL	FP_ITL_2s_l3cp_v30.seq

• v30 sequencers are identical to the FP_ITL_2s_l3cp_v29_Noppp.seq and FP_E2V_2s_l3cp_v29_NopSf.seq. All sequencer files can be found in the github repository.

Other camera conditions

• Idle flush disabled

Record runs

This section describes run 7 record runs.

All runs use our camera operating configuration, unless otherwise noted.

Table 3 Record runs

Run Type	Run ID	Links	Notes
B protocol	E1880
	E2233		Identical to E1880. Acquired after CCS subsystem reboot
PTCs	E1886		Red LED dense. Dark interleaving between flat pairs
	E1881		Red LED dense. No dark interleaving between flat pairs
	E748		nm960 dense
	E2237		Red LED dense. Acquired after CCS subsystem reboot.
	E2016		Super dense red LED. HV Bias off for R13/Reb2. jGroups meltdown interrupted acquisitions, restarted
Long dark acquisitions	E1117
	E1116
	E1115
	E1114
	E1075
Projector acquisitions	E1558		Flat pairs, fine scan in flux from 1-100s in 1s intervals. E2V:v29_NoP, ITL:v29_NoPP
	E1553		Flat pairs, coarse scan in flux from 5-120s in 5s interval.E2V:v29_NoP, ITL:v29_NoPP
	E1586		One 100s flat exposure, spots moved to selected phosphorescent regions.E2V:v29_NoP, ITL:v29_NoPP
	E2181		Flat pairs from 2-60s in 2s intervals. Two 15s darks interleaved after flat acquisition. Rectangle on C10 amplifier.E2V:v29_NoP, ITL:v29_NoPP
	E2184		10 30s dark images to capture background pattern
OpSim runs	E1717		Long dark sequence, no filter changes
	E2330		Short dark sequence, filter changes in headers through OCS
	E1414		30 minutes OpSim run with shutter control, filter change, and realistic survey cadence
	E2328		Flats with shutter-controlled exposure
	E1657		10 hour OpSim dark run, ~50% of darks were acquired properly
Phosphorescence datasets	E2015		10 flats at 10ke- followed by 10x15s darks
	E2014		1 flat at 10ke- followed by 10x15s darks
	E2011		20 flats at 10ke- followed by 10x15s darks
	E2012		10 flats at 1ke- followed by 10x15 s darks
	E2013		10 flats at 10ke- followed by 10x15s darks. Interleaved biases with the darks
Tree ring flats	E1050
	E1052
	E1053
	E1055
	E1056
	E1021
	E1023
	E1024
	E1025
	E1026
Gain stability runs	E1955
	E2008
	E1968
	E1367
	E1362
	E756
	E1496
Persistence datasets	E1503
	E1504
	E1505
	E1506
	E2286
	E1502
	E1501
	E1500
	E1499
	E1498
	E1494
	E1493
	E1492
	E1490
	E1491
	E1489
	E1488
	E1487
	E1486
	E1485
	E1478
	E1477
	E1479
	E1483
	E1484
Guider ROI acquisitions	E1510
	E1518
	E1519
	E1508
	E1509
	E1520
	E1511
	E1521
	E1512
	E1513
	E1514
	E1517