Technical Instrument Report ACS 2013­002
Technical Instrument Report ACS 2013­02
ACS/WFC Amplifier D Read Noise Anomaly:
Sudden Jump from 3.8 to 5.0 Electrons
Dan Coe, Norman Grogin, David Golimowski, Sara Ogaz, Roberto J. Avila
December 31, 2013
ABSTRACT
The read noise in the quadrant of the ACS/WFC detector read out by amplifier D rose suddenly
from 3.8 to 5.0 electrons RMS on January 20, 2013. For the following month until the next CCD
anneal, this read noise fluctuated between 4.5 and 5.0 electrons until settling at 5.0. This new
value is comparable to pre­SM4 read noise levels (March 2002 ­ January 2007) and does not
significantly affect most observations that are sky background limited. We measure no significant
variation in the amplifier gain and verify that photometry has been stable over this time period.
Only the amplifier D quadrant was affected. This anomaly does not appear to be related to the
ASIC installed during SM4. A similar read noise jump occurred in ACS/WFC amplifier A on June
29, 2003. Both events were likely due to cosmic rays impacting the amplifiers on the detector.
1. Introduction
The CCD read noise of the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) has
been relatively low and stable since its installation in 2002. Read noise may limit the sensitivity of
observations obtained in narrowband filters or with short exposure times. However most
broadband filter observations are limited in sensitivity by sky backgrounds, not read noise.
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Technical Instrument Report ACS 2013­002
The ACS/WFC CCD pixel data are read out by four amplifiers, one at each corner of the detector.
Each amplifier imparts a different read noise level onto its respective quadrant of data (see Figure
1). These levels have remained generally stable over the lifetime of ACS, except for four discrete
events:
● On June 29, 2003, the WFC1 amplifier A read noise jumped ~20% (Figure 2)
● After the July 2006 switch to side 2 electronics, the WFC2 amplifier C read noise increased
~4%
● After the Hubble Servicing Mission 4 (SM4) ACS repair, read noise levels dropped ~25%
for all amplifiers due to different signal processing.1
● On January 20, 2013, the WFC2 amplifier D read noise jumped ~30% (Figure 3)
The fourth and most recent event is the subject of this report. We discovered this read noise jump
as part of an investigation into the noise contribution of dark current during read­out. We discuss
those measurements in the next section after providing more background about the detectors here.
ACS/WFC consists of two CCDs (WFC1 and WFC2), each 4144 x 2048 pixels, butted at the long
edge with a physical gap corresponding to roughly 50 pixels (Figure 4). Each of the shorter edges
contains 24 columns of non­sensitive “pre­scan” pixels, resulting in a 4096x4096 pixel detector
area. In full­frame readout mode, four amplifiers simultaneously read out quadrants of the detector
: the WFC1 CCD is read out by amplifiers A and B; WFC2 by amplifiers C and D. Each amplifier
therefore reads 4144/2 = 2072 columns of pixels.
After an ACS exposure, successive rows of pixels are read out in turn by the serial register. Pixel
charges are shifted across the row toward the readout amplifier, taking ~46 ms to read all 2072
columns. Then all the rows are parallel­transferred toward the serial register (taking ~3 ms), and
the next row is read out. During this ~100 second overall read­out time, dark current continues to
accumulate in the pixels, and contributes to the image noise. An ongoing study of the read­out
dark current led to the discovery of the read­noise jump detailed in this report. Subsequent to the
2048 rows of detector read­out, a further 20 row read­outs are performed to create a "virtual
overscan". This virtual overscan contains read­out dark current and cosmic­ray hits, as well as
deferred charge released by lattice defects (responsible for the worsening ACS/WFC charge
transfer efficiency; see Anderson & Bedin 2010).
Fluctuations in read­out noise subsequent to the observed anomalies, as well as the ultimate settling
of read­out noise levels after anomalies, both appear sensitive to ACS/WFC anneals. At
1
The CCD Electronics Box Replacement (CEB­R) processes the pixel data onboard using a dual­slope
integrator (DSI) rather than the clamp­and­sample method used by the original CEB. This lowers the read
noise at the expense of introducing spatial variations in the bias level that must be corrected during ACS
image reduction. For details, see the ACS Data Handbook v7.1, Section 3.4.1.
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Technical Instrument Report ACS 2013­002
approximately one­month intervals, the CCDs are warmed from ­81° C (their current operating
temperature) to +19° C. This annealing partially remedies the CCD damage responsible for
so­called “warm” and “hot” pixels : those with anomalously large dark current (ACS Instrument
Handbook for Cycle 22, Sections 4.3.4­5; see also ACS ISR 2002­09).
Figure 1: Complete read noise history of the four read­out amplifiers of the ACS Wide
Field Channel from December 2002 to November 2013, including the January 2013
anomaly. Amplifiers A (blue) and B (cyan) read out the WFC1 CCD; amplifiers C (red)
and D (magenta) read out the WFC2 CCD. These values are measured from calibration
bias frames taken regularly throughout the ACS lifetime (see Section 2). We note that all
WFC pre­defined sub­array apertures use the lowest­noise and highly­stable amplifier B.
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Technical Instrument Report ACS 2013­002
Figure 2: On June 29, 2003, the ACS WFC1 amplifier A read noise jumped ~20% from
5.3 to 6.2 e­ RMS. After the next anneal (dashed vertical lines) a month later, the read
noise settled to 5.9 e­. The ACS Team investigation of this anomaly concluded the
initial jump was likely due to radiation damage and occurred after a transit of HST
through the intense radiation environment of the South Atlantic Anomaly (SAA).
Figure 3: On January 20, 2013 (two days after an anneal), the ACS WFC2 amplifier D
read noise jumped 30% from 3.8 to 5.0 e­ RMS. It then fluctuated at levels intermediate
with 4.6 e­, finally stabilizing at 5.0e­ after the next anneal on February 14, 2013.
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Technical Instrument Report ACS 2013­002
2. Read Noise Measurements
We measured read noise using bias frames (zero exposure time) taken as part of the ACS CCD
Daily Monitor calibration programs2. Over the lifetime of ACS, these programs have obtained
both bias and dark frames used to process ACS science observations. The dark frames
(1000­second integrations) are used to measure dark current and track the buildup of hot pixels,
while the bias frames are used to track the bias level and to measure the read noise (primarily, plus
any dark current that accumulates during the read­out). These calibration exposures were
originally obtained daily and are now obtained 3­4 times per week.
Each raw bias image (Figure 4) contains correlated structure that must be removed prior to
read­noise analyses. Such artifacts include cosmic­ray hits, transient bad columns, and in the case
of post­SM4 exposures, static gradients (ACS ISR 2012­02) and low­amplitude 1/f noise between
rows (ACS ISR 2011­05). For our initial analysis, we subtracted "superbias" images (bi­weekly
combinations of 20­30 bias frames; ACS ISR 2004­07) from the raw bias frames. Ultimately we
preferred a method that did not rely on these derived products. We instead subtracted each raw
bias frame from the one obtained immediately prior. This was better at removing virtually all of the
large scale structures, aside from the post­SM4 “bias­striping” 1/f noise, known to have 0.9e­
standard deviation. Assuming consecutive bias images have similar but pixel­uncorrelated read
noise properties, their difference image is noisier by a factor of sqrt(2). In our final analysis, we
divide by this factor to recover the noise of each individual image. We verified that this method
gave similar results to subtracting superbias images from the raw bias frames (noting that the higher
S/N superbias images do not contribute significantly to the noise of the differenced images).
For consistency, we analyzed the subset of bias images commanded to have GAIN = 2. In
practice, each amplifier has a slightly different gain factor, as listed in Table 1.
Table 1: Gain value of each ACS/WFC amplifier when commanded with GAIN=2, before
and after SM4. (From the ACS Instrument Handbook for Cycle 22, Table 4.1.)
Measured gain [e­ / DN]
in GAIN=2 mode
WFC1
A
WFC1
B
WFC2
C
WFC2
D
pre­SM4
(Mar 2002 ­ Jan 2007)
2.002
1.945
2.028
1.994
post­SM4
(May 2009 ­ present)
2.020
1.886
2.017
2.011
2
CAL 9647, 9673, 9674, 9675, 10042, 10059, 10060, 10061, 10367, 10389, 10729, 10758, 11041, 11042,
11043, 11879, 11995, 11996, 12384, 12397, 12427, 12729, 12792, 12783, 13152, 13153, 13154, 13589,
13590, 13591 (PIs Riess, Siriani, Golimowski) http://www.stsci.edu/hst/acs/analysis/calib_plan
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Technical Instrument Report ACS 2013­002
We divided each ACS/WFC image into the four quadrants read out by the four amplifiers. Within
each quadrant, we calculated the RMS noise in each row of pixels as follows (summarized in
Figure 5):
1) Remove time­independent correlated structure:
a) take difference of two consecutive bias raw frames
(increases noise RMS by sqrt(2) to be factored out at end)
2) Multiply counts (DN) by gain (e­ / DN) to convert image into units of electrons
3) Measure and subtract bias striping (post­SM4 images only):
a) sigma­clip each row of 2), rejecting >2­sigma outliers and iterating 5 times
b) take mean of each row
c) subtract from 2)
4) Detect and mask cosmic rays:
a) take absolute value of 3)
b) run the source­detection code “SExtractor” (Bertin & Arnouts 1996), yielding a
segmentation map of all detected sources (defined here as 5 or more pixels with
values >1.5x the background RMS level)
5) Measure noise:
a) measure RMS in each row of 3) excluding pixels of detected sources from 4)
b) divide by sqrt(2) : see 1) above
We tested this procedure on simulated bias images with fixed read­noise and found it returned the
correct input RMS values to better than 0.1% accuracy. This assures us, for example, that our
SExtractor clipping of cosmic­ray hits is not overly aggressive.
The first rows of a bias image to be read out contain primarily read noise, with negligible Poisson
noise from accumulated dark current. As the entire array is read out over the next ~100 seconds,
dark current increasingly contributes to the noise measured in each row (Figure 6). This additional
Poisson variance is roughly proportional to distance from the serial register. The maximum
contribution, in the rows furthest from the serial register and at recent times when dark current is
the largest, amounts to an additional half­electron RMS of noise. We will document this in more
detail in a forthcoming Instrument Science Report (ACS ISR; Coe et al. 2014).
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Technical Instrument Report ACS 2013­002
Figure 4: One of the ACS/WFC raw bias frames (zero exposure time) used to measure
read noise in this study (jc5egmbmq_raw.fits obtained July 10, 2013). The
ACS/WFC detector is comprised of two CCDs (WFC1 and WFC2) read out by four
amplifiers (A, B, C, and D) in quadrants, as shown. Each quadrant has been scaled
independently here to suppress the differences in bias pedestals (DC offsets). Note the
static large­scale gradients and the transient bad­columns, row­striping, and cosmic­ray
hits, all of which must be removed before assessing the read noise. Each quadrant
contains 2048 x 2048 "science" pixels that are sensitive to astronomical sources plus
additional regions used for calibrations: 24 "pre­scan" columns and 20 "virtual overscan"
rows. Contrast is enhanced in the yellow zoom­in box to illustrate the lower counts in
the pre­scan columns at left (insensitive to dark current) and to show the deferred­charge
trails, extending away from the serial register, from cosmic ray hits during read out.
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Technical Instrument Report ACS 2013­002
Figure 5: Method to measure noise in raw bias frames. We take the difference between
two consecutive raw bias frames (C = B ­ A), identify bias striping (D) and correct for it
(E), take the absolute value (F), detect cosmic rays with SExtractor (G), and mask them
(H) from the de­striped image (E). For pre­SM4 images, which lack the bias striping,
steps D and E are omitted, and F = abs(C).
Figure 6: Noise measured along each row of the raw bias frames as a function of
distance from the serial register, before (left) and after (right) the January 2013 read
noise jump in amplifier D (magenta). For each amplifier, we plot individual points for
each row and a solid line showing the best linear fit. We take the y­intercepts (closest to
the serial register) as the intrinsic read noise levels. With increasing row number, there
is increasing Poisson noise from dark current during the ~100 second read­out.
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Technical Instrument Report ACS 2013­002
3. Rejecting the Hypothesis of a Gain Drop
Based on other observations, we have been able to reject the alternate hypothesis that the January
2013 amplifier D anomaly indicated gain drop by 30% rather than a read noise increase by 30%.
A 30% drop in the gain (e­ / DN) would have mimicked the read noise jump but also would have
made all astronomical sources appear 30% brighter when observed in that quadrant of the ACS
detector. We find no such variation in repeated observations of stars in 47 Tuc obtained by the
ACS Photometric Stability calibration programs 12734 and 13159 (Figure 7). We analyzed
observations bracketing the read noise jump (Table 2) at four different orientations (HST roll
angles). We confirmed photometric stability over time both within amplifier D and among
different amplifier pairings as derived for the same stars.
A 30% gain drop also would have inflated dark current measurements by 30% as derived from the
Daily Monitor observations. We found no such increase in the amplifier D dark current (Figure 8).
We analyzed six superdark calibration images bracketing the read noise jump. Similar to the
superbias images, superdarks are combinations of 20­30 dark frames obtained over periods of two
weeks, twice per anneal cycle. For all amplifiers, we observe the expected gradual shift in
histogram peaks rightward and downward as more warm / hot pixels (with high dark current)
accumulate in the tail of the distribution. We observe no significant (~30%) horizontal shift in the
amplifier D dark current histogram before/after the January 2013 anomaly. Rather, the amplifier D
histogram peak drops ~10% with respect to the others, which is consistent with a sudden
broadening in the histogram due to increased read noise.
Similarly, a 30% gain drop would have affected the amplifier D bias level. Based on observations
from CAL program 13157, we find the amplifier D bias pedestal has remained constant across the
time period of the January 2013 anomaly. Based on this accumulated evidence, we conclude the
amplifier D gain has remained constant across the January 2013 anomaly, to the 1% percent level
or better.
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Technical Instrument Report ACS 2013­002
Figure 7: ACS photometric stability from CAL programs 12734 and 13159 monitoring
the star cluster 47 Tucanae. Left: Four observations, each 339 seconds in filter F606W,
obtained between March 20, 2012 and June 29, 2013 (bracketing the January 2013
anomaly), all aligned to the same field of view on the sky. The magenta squares mark
the ACS quadrant read out by amplifier D. Right: Photometric differences (Δm = m1 ­ m2)
between stars observed March 20, 2012 (m1) and June 29, 2013 (m2) plotted versus
average instrumental magnitude (m1 + m2) / 2. The 16 boxes group objects by quadrant
pairings. For example, "DC" compares March 20, 2012 quadrant D photometry to June
29, 2013 quadrant C photometry. We find no significant photometric shifts (<1%).
Similar results are obtained for all six pairing combinations among the four images at
left.
Table 2: ACS/WFC observations of star cluster 47 Tucanae obtained by the ACS CCD
Stability Monitor program, analyzed here to verify photometric stability (no gain change).
Date [yyyy­mm­dd]
V3 Position Angle [deg]
Program ID
2012­03­20
353.9
12734
2012­10­31
201.7
13159
2013­03­13
347.0
13159
2013­06­29
95.4
13159
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Technical Instrument Report ACS 2013­002
Figure 8: Histograms of the WFC dark current measured in six calibration superdark
images, three obtained before the Jan 2013 read noise jump and three obtained after. If
the observed event were a 30% gain decrease rather than a read noise jump, the
amplifier D histogram would have shifted 30% to the right. Instead there is a ~10% drop
in the amplifier D histogram peak, consistent with broadening from increased read noise.
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Technical Instrument Report ACS 2013­002
Figure 9: ACS/WFC amplifier D bias level and read noise history in HST Calibration
Cycle 20. Dashed vertical lines are monthly CCD anneals monitored by CAL program
13157. The dotted vertical line marks recovery from the February 2013 HST safing
event, unrelated to the January 2013 ACS/WFC anomaly. Top: Bias levels derived from
CAL program 13157 remain flat despite the read noise jump observed in the bottom two
panels. Middle: Read noise measured in the 24­pixel wide physical pre­scan columns,
which are insensitive to dark current. Bottom: Read noise measured in the first 200
detector rows of the image area (subject to minimal increased noise from dark current).
4. Discussion
On August 9, 2013, NASA convened a tiger team comprising HST engineers from NASA and
STScI, members of the STScI ACS Team, and outside experts on the ACS (including E. Chang,
M. Loose, and A. Waczynski) to review the January 2013 ACS/WFC anomaly. It was noted that
the June 2003 amplifier A read noise jump, similar in many respects to the January 2013 anomaly,
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Technical Instrument Report ACS 2013­002
was attributed to a cosmic­ray strike of the on­detector read­out electronics during an SAA
passage. The January 2013 jump could not clearly be linked to any particular SAA passage,
although HST does appear to have passed near the SAA on days before and after that event (from
the HST ephemeris). The tiger team also reported that spacecraft telemetry (voltages and
temperatures) were nominal around the time of the January 20, 2013 read noise jump. Space
weather, including high­Earth­orbit proton count­rate, was also generally nominal, although there
were elevated count rates of >10 Mev protons observed on January 18, 2013. The ACS Team
made mention of an Earth­directed coronal mass ejection (CME) observed by the Solar Terrestrial
Relations Observatory (STEREO) on 13 January 2013. However there is no clear evidence
linking this event to the ACS anomaly.
The tiger team consensus is that the 2013 read noise jump does not appear to be related to the
ASIC (Application­Specific Integrated Circuit) control electronics included in the CEB­R
installation during SM4. Similarity to the 2003 event, prior to the ASIC installation, suggests a
common (i.e., non­ASIC) origin for both events. Furthermore, both read noise jumps appear to
have been influenced by CCD anneals, which would not have affected the ASIC whatsoever.
Any culpability of the ASIC, which is also being used extensively on the JWST detectors, could
be definitively assessed by on­board ACS/WFC testing. If the ASIC were responsible for the
elevated noise in the ACS/WFC quadrant read out by amplifier D, then swapping and rerouting the
outputs from WFC2 amplifiers C and D through the ASIC would not shift the quadrant with
elevated noise. After discussing this possible test, the NASA tiger team agreed to defer any
on­orbit testing, pending further monitoring of the read noise by the STScI ACS Team. As of
November 2013, all four ACS/WFC quadrants have had stable noise values since the January
2013 anomaly (see Figure 1).
NASA has documented this anomaly and the ensuing tiger team discussion as HST Anomaly
Report (HSTAR) 13771.
5. Summary
In January 2013, the ACS WFC2 amplifier D read noise jumped ~30%, to 5.0e­. This was the
second significant read noise jump in the history of ACS/WFC not coinciding with pre­planned
adjustments to ACS operations, though the first event impacted amplifier A. These two events
were likely the result of heavy ion (cosmic ray) strike causing damage to the silicon lattice in the
region of the on­chip read­out electronics. The 2013 noise anomaly was likely not related to the
ASIC, was definitely not a gain drop, and it appears to have been an isolated incident.
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Technical Instrument Report ACS 2013­002
No ACS science was compromised as a result of this read noise jump. The amplifier D read noise
rose to 5.0 electrons RMS, roughly equal to its performace during the four years of (pre­ASIC)
WFC operation prior to SM4. Most ACS observations are background limited (Poisson variance
from >25 e­ of dark + sky) rather than read noise limited, such that the extra amplifier D variance
has very little effect upon the overall noise budget. We note that ACS/WFC sub­array apertures by
default use amplifier B, which has the lowest read noise.
The ACS calibration team has updated the CALACS image reduction pipeline to reflect the higher
read noise in amplifier D subsequent to January 2013. We have also updated the ACS Instrument
and Data Handbooks with the new read noise value. The ACS Exposure Time Calculator (ETC)
has not been updated, as it does not treat each quadrant of the ACS/WFC detector separately.
No further action is recommended at this time. The STScI ACS Team will continue to closely
monitor the read noise levels of all four amplifiers.
References
ACS Instrument Science Reports (ISRs): http://www.stsci.edu/hst/acs/documents/isrs
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Gonzaga, S. et al. 2013, ACS Data Handbook, Version 7.1 (Baltimore: STScI)
http://www.stsci.edu/hst/acs/documents/handbooks/currentDHB/acs_cover.html
Grogin, N. et al. 2011, Instrument Science Report ACS 2011­05 (Baltimore: STScI) [ADS]
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http://www.stsci.edu/hst/acs/documents/handbooks/current/cover.html
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