Further author information:
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# The Advanced CCD Imaging Spectrometer on the Chandra X-ray Observatory:
twenty-five years of on-orbit operation

Catherine E. Grant  Kavli Institute for Astrophysics and Space Research,
Massachusetts Institute of Technology, Cambridge, MA, USA  Marshall W. Bautz
Kavli Institute for Astrophysics and Space Research, Massachusetts Institute
of Technology, Cambridge, MA, USA  Paul P. Plucinsky  Center for Astrophysics
— Harvard & Smithsonian, Cambridge, MA, USA  Peter G. Ford  Kavli Institute
for Astrophysics and Space Research, Massachusetts Institute of Technology,
Cambridge, MA, USA

###### Abstract

As the Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray
Observatory completes a quarter century of on orbit operations, it continues
to perform well and produce spectacular scientific results. The response of
ACIS has evolved over the lifetime of the observatory due to radiation damage,
molecular contamination, changing particle environment, and aging of the
spacecraft in general. We present highlights from the instrument team’s
monitoring program and our expectations for the future of ACIS. Performance
changes on ACIS continue to be manageable, and do not indicate any limitations
on ACIS lifetime. We examine aspects of the design and operation of ACIS that
have impacted its long lifetime with lessons learned for future instruments.

###### keywords:

Chandra X-ray Observatory, ACIS, X-ray detectors, CCDs

##  1 INTRODUCTION

The Chandra X-ray Observatory, the third of NASA’s great observatories in
space, was launched shortly after midnight on July 23, 1999, aboard the space
shuttle Columbia[1]. After a series of orbital maneuvers, Chandra reached its
operational orbit, with initial 10,000-km perigee altitude, 140,000-km apogee
altitude, and 28.5∘ inclination. The first light observation of the supernova
remnant Cassiopeia A immediately demonstrated the power of the new observatory
by revealing in real-time the compact object at the core of the remnant
(Figure 1).

Figure 1: (left) A Mega-second image of the young supernova remnant Cassiopeia
A as imaged by Chandra and ACIS over a ten year period. The image colors
indicate the location of specific elements (Si, S, Ca, Fe) within the remnant
determined by using the energy resolution of the CCD camera to resolve X-ray
spectral features. The small white point source in the center is the neutron
star which had not been identified previous to Chandra. (right) The Chandra
Operations Control Center as the first light observation of Cas A was
underway. The central compact object was immediately apparent in the first
images. (Image credit: NASA/CXC/SAO)

Chandra’s High-Resolution Mirror Assembly (HRMA) provides exquisite sub-
arcsecond X-ray imaging that was ground-breaking at launch and continues to be
unmatched by any X-ray observatory in operation or in active development. Two
focal plane instruments, the Advanced CCD Imaging Spectrometer (ACIS) and the
High-Resolution Camera (HRC) can be moved in and out of the telescope focus.
In addition, two grating assemblies can be moved in and out of the optical
path: the High-Energy Transmission Gratings (HETG) and the Low-Energy
Transmission Gratings (LETG). ACIS provides moderate spectral resolution
imaging and serves as the readout detector for the HETG, while HRC has
slightly better spatial resolution, minimal spectral resolution, and is the
readout detector for the LETG. For an overview of the mission and more details
about the instrument complement, see the Chandra Proposers’ Observatory
Guide.111https://cxc.cfa.harvard.edu/proposer/POG/ ACIS is the workhorse for
Chandra science with more than 90% of the observation time.

With the exception of rare spacecraft anomalies and radiation shutdowns,
Chandra has been in continuous operation for nearly twenty-five years.
Chandra’s orbit allows for high observing efficiency of approximately 70%,
with the remaining time spent transiting Earth’s radiation belts. Twenty-five
years later, the observing efficiency remains close to the theoretical
maximum. Demand for Chandra observations is high, with steady oversubscription
factors of 4-6. Refereed publications are expected to exceed 10,000 during
this 25th anniversary year. On average, PIs receive their data within two days
of the end of an observation and have access to a suite of specialized
software tools and threads to help analyze their data.

All of this has resulted in spectacular scientific results too numerous to be
listed here. Chandra is particularly well suited to studying crowded fields,
like the Galactic Center and star forming regions, and complicated spatial
structures such as sloshing and feedback in galaxy clusters. A growing number
of studies make use of Chandra’s long lifetime and high spatial resolution to
map tiny motions of supernova remnants and other nearby objects.

The response of ACIS has evolved over the lifetime of the observatory due to
radiation damage, contamination accumulation, and degradation of thermal
control. This has resulted in some loss of low-energy effective area and a
general need for careful calibration in the temporal, spatial, and spectral
domains, and as a function of the focal plane temperature. The ACIS instrument
team maintains an ongoing monitoring program, has developed software and
calibration products to improve instrument response, and flight software
patches that further protect the instrument to help ensure a long lifetime.

In this paper, we discuss some aspects of the twenty-five years of Chandra on-
orbit operation from the perspective of the ACIS instrument. We begin with a
more detailed description of the instrument in Section 2, then a brief summary
of current status and a deeper dive into selected topics in Section 3. Section
4 outlines a few lessons learned before a summary in Section 5.

##  2 Detailed Description of the Instrument

The ACIS focal plane consists of two arrays. The first, ACIS-I, is 2 $\times$
2 CCDs tilted to best match the focal surface of the HRMA for highest spatial
resolution. The second, ACIS-S, intended as a readout for the HETG, is a
linear 6 CCD array tilted to match the Rowland circle of the gratings. Eight
of the CCDs are front-illuminated with two back-illuminated CCDs in ACIS-S:
one at the ACIS-S aimpoint and one further along the array where lower energy
X-rays are dispersed. In practice, ACIS-S has also been used for imaging
smaller fields, as the response of the BI CCD is desirably for some
investigations. Photos of the ACIS focal plane are shown in Figure 2 and more
characteristics are listed in Table 1. ACIS was built by MIT and Lockheed
Martin under the direction of PI Gordon Garmire from Penn State University
(now at the Huntingdon Institute for X-ray Astronomy).

Table 1: ACIS characteristics  Detectors | 10 frame-transfer X-ray CCDs
---|---
|  8 front-illuminated
|  2 back-illuminated
CCD format | 1024 $\times$ 1024 pixel image array
Image area pixel size | 24 $\times$ 24 $\mu$m
Depletion depth |  $\sim$70 $\mu$m (FI), $\sim$45 $\mu$m (BI fully depleted)
Output ports | on-chip MOSFET, 4 / CCD
Focal plane arrays: |
ACIS-I | 2 $\times$ 2 CCD array
| tangent to mirror focal surface
| 16.9′$\times$ 16.9′
ACIS-S | 1 $\times$ 6 CCD array
| tangent to grating Rowland circle
| 8.4′$\times$ 51.1′
Focal plane temperature |  $-120$∘C setpoint
|  $-120$∘C to $-105$∘C current allowed range
Optical blocking filter (Al/Polyimide/Al) | 120 nm / 200 nm / 40 nm (ACIS-I)
| 100 nm / 200 nm / 30 nm (ACIS-S)
System noise | 2–3 e- RMS
Nominal frame time | 3.2 sec (full frame)
Allowable frame times | 0.1 to 10.0 sec
Maximum serial rate | 100 kHz (10 $\mu$sec / pixel)
Image-to-framestore parallel rate | 25 kHz (40 $\mu$sec / row)
Instrument output data rate | 24 kbps

Figure 2: (top) A photograph of the ACIS focal plane as it would appear
looking down the optic axis. The $2\times 2$ CCD square Imaging Array and six
CCD linear Spectroscopy Array are visible, as are various structures that
provide support and thermal functions. In flight, the CCDs are covered by an
Optical Blocking Filter. (bottom) A side view of the focal plane shows the
gold coated framestore covers and how the CCDs are tilted to best match the
focal surface.

The ACIS CCDs are CCID-17 frame transfer devices designed and manufactured for
Chandra by MIT Lincoln Lab, out of high-resistivity float-zone Silicon. Each
CCD has 1024 $\times$ 1024 24-$\mu$m pixels in the imaging array and three
phase charge transfer through triple layer polysilicon transfer gates. The FI
CCDs have a depletion depth of $\sim$70 $\mu$m and the BI CCDs are fully
depleted with a thickness of $\sim$45 $\mu$m. The buried channel includes an
additional 2-$\mu$m trough implant to mitigate radiation damage. Figure 3 is a
schematic diagram of an ACIS CCD showing the transfer direction and readout
path. Each CCD has four single-stage on-chip MOSFET readout amplifiers.
Fastest frame time is achieved by utilizing all four, but it is also possible
to clock each half of the framestore out through one end or the other, if
desired. The framestore is covered by 3 mm of gold coated aluminum.

Figure 3: A schematic drawing of an ACIS CCD showing the transfer direction
through the image and framestore sections of the CCD to the serial register.
Charge from each half of the CCD can then be clocked out entirely to the node
at one end or the other or can be split in half to use all four amplifier
nodes.

ACIS runs in photon-counting mode, with frametimes of $\sim$3 seconds. On-
board processing identifies potential X-ray event candidates and telemeters
the subset most likely to be X-rays and not particles to the ground. Event
candidates are identified as pixels that are local maxima and have a
pulseheight above an event threshold. On-board filtering removes events with
pulseheights above an upper threshold and event morphologies that are more
likely to be charged particle events. While the removed events themselves are
not telemetered, the instrument does keep an accounting of the numbers of
events removed and why. We use one of these counters, the high energy reject
rate, as a proxy for the particle rate seen by the spacecraft (Section 3.5)
and another of the counters, number of pixels above the event threshold, as
part of the on-board radiation monitor (Section 3.6) .

ACIS offers many selectable observing parameters, including the choice of
CCDs, restrictive energy filters, and reporting additional information from
neighboring pixels in the telemetry stream. Most options include trade-offs
between faster readout, spatial coverage, and telemetry limitations. The
choices and trade-offs are best described in the Chandra Proposers Observatory
Guide.222https://cxc.cfa.harvard.edu/proposer/POG/ To date, 1385 unique ACIS
configurations have been built in response to observer requests and are
available for use.

The focal plane CCDs and the camera body are passively cooled via two large
radiators protected from direct sunlight, then maintained at the desired
temperature with heaters. Since January 2000, the focal plane temperature
setpoint has been $-120\mbox{${}^{\circ}$}$C. As the spacecraft has aged and
the multi-layer insulation (MLI) has degraded, temperatures of all spacecraft
subsystems have increased. The implications for ACIS will be discussed further
in Section 3.3.

More details of the ACIS CCDs and the ACIS focal plane can be found in Ref. 2
and 3, respectively.

###  2.1 External Calibration Source

A key element of ACIS on-orbit operations and calibration has been the
External Calibration Source (ECS), which fully illuminates all ten CCDs while
ACIS is in the stowed position. Since the discovery of the initial radiation
damage (further discussed in Section 3.1), a continuing series of observations
of the ECS have been undertaken–just before and after the instruments are
safed for perigee passages–to monitor the performance of the ACIS CCDs.

The ECS consists of three radioactive 55Fe sources, two of which include
fluoresence targets. The ECS produces strong lines of Mn-K$\alpha$ and
Mn-K$\beta$ at 5.9 keV and 6.5 keV and a much weaker complex of unresolved
Mn-L lines at $\sim$700 eV. Fluorescence targets add lines from Al-K at 1.5
keV and Ti-K$\alpha$ and Ti-K$\beta$ at 4.5 keV and 4.9 keV. The access to
multiple energies has proven invaluable in studying gain changes on short
time-scales and the energy and spatial dependence of charge transfer
inefficiency, as well as the change in low-energy efficiency due to molecular
contamination. Figure 4 shows the spectrum of the ACIS External Calibration
Source and labels the important features.

The frequent cadence of ECS observations, roughly twice every three days, has
allowed for close monitoring of ongoing radiation damage and rapid
investigation of potential instrument anomalies, while still providing a long
baseline for detailed studies of performance changes. In addition to the
regular ECS observations near perigee, much longer exposures are often
undertaken during observatory shutdowns due to solar storms and other
spacecraft anomalies.

Figure 4: An on-orbit spectrum of the ACIS External Calibration Source on an
ACIS-I FI CCD. Only data from the first 256 rows are included to minimize CTI
degradation. Lines used for calibration and monitoring purposes are labelled.
The remaining features are instrumental.

55Fe has a half-life of $\sim$2.7 years. Figure 5 shows the decline in the
count rate of the Mn-K$\alpha$ line throughout the Chandra mission due to the
decay of the calibration source. Clearly over the twenty-five year lifetime of
Chandra, the count rate from the ECS has dropped substantially, requiring more
thoughtful analysis and some reduction in the ability to simultaneously
measure spatial and temporal effects. The weakest line from the low energy
Mn-L complex at $\sim$700 eV was essentially lost in the underlying background
spectrum after 2010. The stronger lines remain well measured and should
continue to provide valuable calibration and monitoring data into the future
with the appropriate spatial and temporal binning.

The long Chandra lifetime allows us to measure the 55Fe half-life directly
from the ACIS ECS data. A fit to this data yields a half-life of $2.740\pm
0.005$ years, which agrees well with the often quoted value in the literature
of $2.737\pm 0.011$ years[4].

Figure 5: The decline in the count rate of the ACIS ECS due to radioactive
decay of the 55Fe source. The reduction in the number of data points in recent
times is due to a combination of fewer cold ECS data sets and poor fits due to
low counts.

##  3 Instrument Status and On-orbit Performance Evolution

ACIS continues to perform superbly with no limitations on its future operation
for many years. All ten CCDs and forty readout chains are fully functional
with minimal increase in cosmetic defects due to radiation damage. The system
readout noise from all chains remains close to that at launch. All the
instrument electronics and power supplies are nominal, operating on the
original primary units with no failures. Redundant electronics remain
available, should they be needed. The CCD output chains are multiplexed to the
six front-end processing chains, so the loss of one front-end board would only
mean a reduction in the number of available CCDs rather than the loss of a
specific CCD.

There is no evidence of light leaks through the Optical Blocking Filter (OBF)
and no indication of damage due to micrometeoroid impact on the filter or the
CCDs. There has been a continuing gradual deposition of molecular
contamination on the OBF which is further discussed in Section 3.4.

The ACIS flight software has been patched eleven times since launch including
bug fixes, improvements to on-board processing, and enhancements to the flight
software capabilities. Some of these patches will be discussed in Section 3.6.
The ACIS boot code stored in an on-board EEPROM is checked monthly for data
corruption and has remained fault-free.

The only failures in the ACIS instrument to date are to two temperature
sensors, one on the power supply box (in 2016) and one on the cold radiator
(in 2023) which intermittently read open circuits. Neither is required for on-
going thermal modeling and other thermal sensors can be used for health and
safety monitoring. The team has concentrated some effort on identifying
temperature sensors that are necessary for operations and how their function
might be duplicated, were they to fail in the future.

###  3.1 Charge Transfer Inefficiency

Much of the early history of ACIS performance concerns charge transfer
inefficiency (CTI). Chandra’s highly elliptical 2.7-day orbit exposes it to a
wide range of particle environments, and transits the Earth’s radiation belts
at perigee.

Soon after launch it was discovered that the FI CCDs had suffered radiation
damage from exposure to soft protons scattered off the Observatory’s grazing-
incidence optics during passages through the Earth’s radiation belts.[5] The
BI CCDs were unaffected. Since mid-September 1999, ACIS has been protected
from the protons by moving the CCDs to the stowed position during radiation
belt passages and there has been an ongoing effort to prevent further damage
and to develop hardware and software strategies to mitigate the effects of
charge transfer inefficiency on data analysis.[6, 7]

Our primary measure of radiation damage on the CCDs is charge transfer
inefficiency (CTI), the fraction of charge lost during pixel-to-pixel
transfer. The eight front- illuminated CCDs had essentially no CTI before
launch (CTI $<10^{-6}$), but are strongly sensitive to radiation damage from
low energy protons ($\sim$100 keV) which preferentially create traps in the
buried transfer channel. After eight radiation belt transits the FI CCD CTI
had increased to $\sim 10^{-4}$. The framestore covers, 2.5-mm of gold coated
aluminum, are thick enough to stop this radiation, so the initial damage was
limited to the imaging area of the FI CCDs. Radiation damage from low-energy
protons is now minimized by moving the ACIS detector away from the aimpoint of
the observatory during passages through the Earth’s particle belts and during
solar storms. Continuing exposure to both low and high energy particles over
the lifetime of the mission slowly degrades the CTI further.[8, 9] The two
back-illuminated CCDs (ACIS-S1,S3) had measurable CTI before launch in both
the imaging and framestore areas (CTI $\sim 10^{-5}$) and in the serial
transfer array. Owing to their much deeper charge-transfer channel, BI CCDs
are insensitive to damage by the low-energy protons that damage FI devices.

Figure 6 shows the parallel CTI from January 2000 onward, measured using the
5.9 keV line of the ECS after applying corrections for temperature dependence
and sacrificial charge due to the particle background, and grouping the data
into year-long bins. The lack of data in the last few years is due to a
combination of factors: the weakening 55Fe source, operational changes that
reduce the time dedicated to these measurements, and an increase in high focal
plane temperatures.

Figure 6: Parallel CTI measured at 5.9 keV and -120∘C for a front-illuminated
and back-illuminated ACIS CCD. The FI CCDs have higher CTI, due to the
radiation belt transits at the start of the mission. Both types of CCDs show a
gradual increase in CTI with time.

CTI increases smoothly and roughly linearly with time. The FI CCDs have much
higher CTI than the BI CCDs, due to the initial radiation damage from low
energy protons. The CTI increase of the FI CCDs ($\delta\mathrm{CTI}\sim
2\times 10^{-6}$/yr) is higher than that of the BI CCD
($\delta\mathrm{CTI}\sim 1\times 10^{-6}$/yr) which indicates that the
accumulated radiation damage must include a mixture of particle energies,
including the low energy protons which preferentially damage the FI CCDs but
not the BI CCDs.

###  3.2 Read Noise

Before launch, the ACIS CCDs had read-out noise less than 2 e- RMS, with a
total system noise for the entire signal chain of 2-3 e- RMS. Since launch
there has been very little change in the system noise even after twenty-five
years of exposure to a space environment. Figure 7 shows the total system read
noise on the front-illuminated CCD at the I-array aimpoint and the back-
illuminated CCD at the S-array aimpoint since January 2000. This was measured
by fitting a Gaussian to the corner pixels of the 3$\times$3 event islands.
Since the corner pixels go through the same process on-board as the X-ray
events, the measured width of the corner pixel Gaussian is representative of
the noise seem by X-ray events including contributions from the entire readout
and processing chain. The back-illuminated device was known to have higher
noise before launch. Both devices show a negligible increase of $\sim$ 0.3 e-
over twenty-five years that is not a significant degradation of instrument
performance.

Figure 7: The total system read noise of the front-illuminated I-array
aimpoint CCD and the back-illuminated S-array aimpoint CCD as a function of
time. There has been very little change in the total system noise since
launch. Very small step changes are due to known instrument anomalies and
operational changes.

###  3.3 Temperature Control

The ACIS focal plane is passively cooled via a radiator and then maintained at
a set temperature via a heater. In January 2000, that set point was
$-120\mbox{${}^{\circ}$}$C. In subsequent years, the multi-layer insulation
(MLI) of the spacecraft slowly degraded, and temperatures throughout the
spacecraft began to rise. ACIS focal plane temperatures are sensitive to the
spacecraft pitch and roll, and sometimes receive extra heating around perigee
due to earthshine on the radiator.

Figure 8 shows the time dependence of focal plane temperature. Each data point
is the mean temperature of a single observing run. It is clear that the
frequency and magnitude of high focal plane temperatures have both increased
with time. In the past few years very little time is spent at the nominal
setpoint value. The radiator itself does not appear to have degraded, as tests
have shown that given the proper spacecraft pitch angle and enough time, the
temperature can drop well below $-120\mbox{${}^{\circ}$}$C. The issue is
rather that the thermal limits of other spacecraft subsystems require pitch
angles that are less favorable for cold ACIS focal plane temperatures.

The radiation induced CTI on ACIS, especially for the FI CCDs, is temperature
dependent, which means that gain and spectral resolution both degrade with
increasing temperature.[10] Best performance for the FI CCDs requires a cold
focal plane. The calibration team has set limits on the maximum temperatures
for good calibration. In addition, the ACIS CTI correction software has a
temperature component to help mitigate the dependence.

Figure 8: Time dependence of focal plane temperature since January 2000. Each
data point is a single science observation. The nominal set point of the ACIS
focal plane is $-120\mbox{${}^{\circ}$}$C, but increasingly drifts to higher
temperatures.

###  3.4 Contamination

Within a few years of launch it was clear that there was excess absorption
from molecular contamination somewhere in the system. The spatial distribution
of the contaminant matched the temperature profile of the OBF, thicker/colder
at the edges, thinner/warmer in the center, so the contaminant seems likely to
be on the OBF, rather than on the CCDs themselves. Regular calibration
observations monitor the optical depth, composition, and spatial distribution
as it has evolved over the lifetime of Chandra.[11, 12] Figure 9 demonstrates
the changing effective area due to contaminant deposition.

Figure 9: Observations of a galaxy cluster from 2000 to 2023 showing the
decrease in low energy effective area due to contamination
accumulation.(Source: Chandra Observatory Proposers Guide)

Substantial effort has been put towards identifying the exact composition of
the contaminant and its volatility. That, along with detailed thermal models
of the ACIS instrument, have been used to predict whether a bakeout of the
instrument would be effective.[13] Unfortunately, the unknown contaminant
volatility does not allow us to predict the results of a bakeout, how much
contaminant would be removed, if any, or how fast it might reaccumulate
afterwards. A bakeout also carries some risk of damaging the Optical Blocking
Filter, so an on-orbit bakeout has not been attempted. The effectiveness and
risks of a bakeout of the focal plane to remove contamination are uncertain,
and the consensus is that the risks are not justified by the potential
benefits.

###  3.5 Particle Background

Chandra’s highly elliptical orbit samples a wide range of near-Earth particle
environments, from the radiation belts at closest approach through the
magnetosphere and magnetopause and past the bow shock into the solar wind.
Science observations take place outside the Earth’s radiation belts, where the
quiescent background is dominated by galactic cosmic rays (GCR), primarily
protons $>$ 10 MeV, plus any secondary particles produced through interaction
with the spacecraft and instrument.

GCR protons are the primary source of quiescent background, modulated by the
solar cycle and solar activity. The twenty-five year baseline provided by
Chandra data covers more than two complete solar cycles, allowing a study of
conditions at solar maximum and minimum and for variances between solar
cycles. Figure 10 shows the time dependence of the particle background on ACIS
using the high energy reject rate as a proxy. Variability is present on
multiple timescales. At the largest scales, the two solar cycles can be
compared. Each one is distinct; rates from one cycle could not be used to
accurately predict those in the next. During solar maximum, there is an
additional $\sim 10\%$ variation on timescales of weeks to months from
individual solar storms, but even at solar minimum, there remains variation at
the 5% level.

It is expected that the unfocused particle background experienced by future
missions in L1 or L2 should be very similar to that seen by Chandra. Ref. 14
compares the variability of ACIS to that measured by the precision particle
experiment, Alpha Magnetic Spectrometer on the ISS, and to an equivalent
reject rate on XMM-Newton EPIC-pn, to improve our knowledge of the background
for future missions.

Figure 10: Time dependence of the particle background as measured by the rate
of high energy events on ACIS-S3. Each data point is a single ECS observation.
(top) The full mission history of high energy rates on S3 includes three solar
maxima and two minima. The particle background is anti-correlated with the
solar cycle and varies by a factor of $\sim$2 over 11 years. The cycles are
distinct; rates from one cycle could not be used to accurately predict the
rates in the next cycle. (bottom left) A segment of the ACIS-S3 background
during solar maximum. The sawtooth pattern is typical of solar activity and
individual solar storms. The background variation is $\sim$10% over weeks to
months. (bottom right) A segment of the ACIS-S3 background during solar
minimum, when there was no significant solar activity. Background variations
are smaller, of order 5% over a matter of days.

###  3.6 Flight Software Patches

The ACIS flight software is flexible and has been patched eleven times since
launch. The patches have included bug fixes, improvements to on-board
processing, and enhancements to the flight software capabilities. The most
recent patchload was uplinked in September 2023 with four patches: a) to
correct a bug that could lead to an infinite loop during the bias map
computation in alternating exposure mode, b) to accurately report the state of
the software in all cases after a safing action, c) to report more diagnostic
information in the case of an unexpected condition, and d) to force a restart
and reloading of the software if a front-end processor is powered off
unexpectedly. Updates such as these make ACIS more operationally robust and
help minimize any downtime for some anomalous conditions.

An example of an ACIS flight software patch that has had great impact on the
instrument lifetime is TXings, the ACIS radiation monitor patch. The Chandra
team has implemented procedures to protect ACIS during high-radiation events
including autonomous protection triggered by an on-board radiation monitor.
When an elevated particle environment is reported to the On-Board Computer
(OBC), science observations are stopped, the ACIS CCDs are powered down, and
ACIS is translated to the stowed position. Due to elevated temperatures, the
on-board radiation monitor, EPHIN, became increasingly unreliable and in 2014
was removed from the autonomous radiation protection monitor. The anti-
coincidence shield of the HRC was added to the on-board radiation monitor in
2005, but due to operational changes after HRC anomalies in 2020 and 2022, is
no longer providing that function. Concern about the effectiveness of the on-
board particle monitor motivated exploring other on-board measures of the
radiation environment such as ACIS itself.[15]

The ACIS team has developed an algorithm which uses data from the CCDs
themselves to detect periods of high radiation and a flight software patch to
apply this algorithm is currently active on-board the instrument.[16, 17] This
patch was installed on ACIS in November 2011; it examines threshold crossing
rates in the ACIS CCDs and signals the OBC if the rates are above pre-set
triggers levels and monotonically increasing. The OBC can then take action to
protect the ACIS CCDs by moving ACIS out of the focal plane and turning off
the radiation-sensitive video boards. The patch has operated as expected with
no impact on regular science operations and has correctly detected an enhanced
radiation environment on multiple occasions and triggered autonomous radiation
protection of the spacecraft. To better match the changing quiescent particle
background which is anti-correlated with the solar cycle, the parameters of
the patch are re-evaluated monthly and updated as necessary.

The loss of HRC data to on-board radiation monitoring in recent years leaves
the ACIS algorithm as the sole input to the autonomous radiation safing system
on Chandra. ACIS and the TXings patch have limitations compared to a dedicated
particle instrument, like EPHIN, the original Chandra particle detector, or
the HRC anti-coincidence shield. The algorithm runs parasitically while ACIS
is taking event data, not during biases or slews, and includes contributions
from both astrophysical X-rays as well as particle radiation. Given these
known limitations, it was decided after the second HRC anomaly to impose
additional radiation conservatism. An update to the TXings patch was uplinked
in 2022, which makes it more sensitive to radiation and does not solely rely
on detecting a monotonically increasing signal.

##  4 Twenty-Five Years of Lessons Learned

After twenty-five years of successful on-orbit operation, there are many
lessons learned that could be useful for future missions. An early example is
the radiation damage suffered by the FI CCDs after radiation belt transits
(Section 3.1). Once the cause was understood, the ACIS experience was
communicated to the XMM-Newton team, which launched CCD instruments into a
similar orbit five months later. So warned, XMM operations included closing
the filter wheel during radiation belt transits, preventing similar
damage.[18] The upcoming SMILE SXI has similar plans.[19]

The contamination accumulation on ACIS (Section 3.4) and on the Suzaku XIS[20]
has motivated the design of warm contamination blocking filters planned for a
number of upcoming missions including the recently launched XRISM Xtend.[21]
In both cases it is believed that the contamination was condensing on the
filters rather than the CCDs. The CCDs are generally the coldest surfaces, but
are separate from the main body of the spacecraft, where more sources of
contamination exist. Actively warming the filter should help keep it clean.

The ACIS External Calibration Source (Section 2.1) has been key to the
scientific success of ACIS. Fully illuminating all ten CCDs with multiple line
energies has been crucial for monitoring and calibrating the spatial,
spectral, and temporal aspects of radiation damage and contamination
deposition.

The versatility of ACIS in reacting to anomalies and changing requirements is
in part due to the ability to patch its flight software, and this in turn is
greatly facilitated by the object-oriented design of its back-end processor
software. Patch testing is improved by a pair of ground simulators: one in
hardware built from spare flight boards, the other in software using virtual
QEMU processes.

Finally, the success of ACIS operation over twenty five years is due to a
large team from MIT, SAO, Penn State, Northrop Grumman, Lockheed Martin, and
Marshall Space Flight Center. In particular, the MIT instrument team and SAO
science operations team work as a blended collaboration which started well
before launch at the sub-assembly calibration stage. These well established
relationships were crucial for the effective team response to instrument
anomalies.

##  5 Summary

The ACIS instrument on the Chandra X-ray Observatory continues to perform well
after twenty five years on-orbit with no limitations on its future operation
for many years. The hardware and software are operating as designed.
Increasing radiation damage, molecular contamination accumulation, and
increasing temperature variation are calibration challenges but are manageable
with mitigations in place. Given the lack of any replacement for Chandra’s
angular resolution capability, we look forward to many more years of ACIS
observations.

###### Acknowledgements.

We are exceedingly grateful to all the scientists and engineers involved in
designing, building, and supporting the continued operation of the Chandra
X-ray Observatory. In particular, we would like to thank the Chandra Science
Operations Team (SOT) and Flight Operations Team (FOT) at SAO, MIT, and
Northrop-Grumman (NG), and the Chandra Project Science team (MSFC) for many
years of fruitful collaboration and constant vigilance. This work was
supported by NASA contracts NAS 8-37716 and NAS 8-38252.

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