The Transiting Exoplanet Survey Satellite (TESS) will discover thousands of exoplanets in orbit around the brightest stars in the sky. This first-ever spaceborne all-sky transit survey will identify planets ranging from Earth-sized to gas giants. TESS stars will be far brighter than those surveyed by previous missions; thus, TESS planets will be easier to characterize in follow-up observations. For the first time it will be possible to study the masses, sizes, densities, orbits, and atmospheres of a large cohort of small planets, including a sample of rocky worlds in the habitable zones of their host stars.
During 2014 and 2015, NASA's Neutron star Interior Composition Explorer (NICER) mission proceeded success- fully through Phase C, Design and Development. An X-ray (0.2-12 keV) astrophysics payload destined for the International Space Station, NICER is manifested for launch in early 2017 on the Commercial Resupply Services SpaceX-11 flight. Its scientific objectives are to investigate the internal structure, dynamics, and energetics of neutron stars, the densest objects in the universe. During Phase C, flight components including optics, detectors, the optical bench, pointing actuators, electronics, and others were subjected to environmental testing and integrated to form the flight payload. A custom-built facility was used to co-align and integrate the X-ray "con- centrator" optics and silicon-drift detectors. Ground calibration provided robust performance measures of the optical (at NASA's Goddard Space Flight Center) and detector (at the Massachusetts Institute of Technology) subsystems, while comprehensive functional tests prior to payload-level environmental testing met all instrument performance requirements. We describe here the implementation of NICER's major subsystems, summarize their performance and calibration, and outline the component-level testing that was successfully applied.
The Hitomi (ASTRO-H) mission is the sixth Japanese X-ray astronomy satellite developed by a large international collaboration, including Japan, USA, Canada, and Europe. The mission aimed to provide the highest energy resolution ever achieved at E > 2 keV, using a microcalorimeter instrument, and to cover a wide energy range spanning four decades in energy from soft X-rays to gamma-rays. After a successful launch on 2016 February 17, the spacecraft lost its function on 2016 March 26, but the commissioning phase for about a month provided valuable information on the on-board instruments and the spacecraft system, including astrophysical results obtained from first light observations. The paper describes the Hitomi (ASTRO-H) mission, its capabilities, the initial operation, and the instruments/spacecraft performances confirmed during the commissioning operations for about a month.
We report here the performance of the SXI on ASTRO-H that was started its operation from March,02 2016. The SXI consists of 4 CCDs that cover 38' X 38' sky region. They are P-channel back-illumination type CCD with a depletion layer of 200 μm. Charge injection (CI) method is applied from its beginning of the mission. Two single stage sterling coolers are equipped with the SXI while one of them has enough power to cool the CCD to -110°C. There are two issues in the SXI performance: one is a light-leak and the other is a cosmic-ray echo. The light-leak is due to the fact that the day-Earth irradiates visible lights onto the SXI body through holes in the satellite base plate. It can be avoided by selecting targets not on the anti-day-Earth direction. The cosmic-ray echo is due to the improper parameter values that is fixed by revising them with which the cosmic-ray echo does not affect the image. Using the results of RXJ1856.5-3754, we confirm that the possible contaminants on the CCD is well within our expectation.
The NICER1 mission uses a complicated physical system to collect information from objects that are, by x-ray timing science standards, rather faint. To get the most out of the data we will need a rigorous understanding of all instrumental effects. We are in the process of constructing a very fast, high fidelity simulator that will help us to assess instrument performance, support simulation-based data reduction, and improve our estimates of measurement error. We will combine and extend existing optics, detector, and electronics simulations. We will employ the Compute Unified Device Architecture (CUDA2) to parallelize these calculations. The price of suitable CUDA-compatible multi-giga op cores is about $0.20/core, so this approach will be very cost-effective.
An instrument called Neutron Star Interior Composition ExploreR (NICER) will be placed on-board the Inter- national Space Station in 2017. It is designed to detect soft X-ray emission from compact sources and to provide both spectral and high resolution timing information about the incoming ux. The focal plane is populated with 56 customized Silicon Drift Detectors. The paper describes the detector system architecture, the electronics and presents the results of the laboratory testing of both ight and engineering units, as well as some of the calibration results obtained with synchrotron radiation in the laboratory of PTB at BESSY II.
The Soft X-ray Imager (SXI) is an X-ray CCD camera onboard the ASTRO-H X-ray observatory. The CCD chip used is a P-channel back-illuminated type, and has a 200-µm thick depletion layer, with which the SXI covers the energy range between 0.4 keV and 12 keV. Its imaging area has a size of 31 mm x 31 mm. We arrange four of the CCD chips in a 2 by 2 grid so that we can cover a large field-of-view of 38’ x 38’. We cool the CCDs to -120 °C with a single-stage Stirling cooler. As was done for the CCD camera of the Suzaku satellite, XIS, artificial charges are injected to selected rows in order to recover charge transfer inefficiency due to radiation damage caused by in-orbit cosmic rays. We completed fabrication of flight models of the SXI and installed them into the satellite. We verified the performance of the SXI in a series of satellite tests. On-ground calibrations were also carried out and detailed studies are ongoing.
The Transiting Exoplanet Survey Satellite (TESS) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its 2-year mission, TESS will employ four wide-field optical charge-coupled device cameras to monitor at least 200,000 main-sequence dwarf stars with IC≈4−13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from 1 month to 1 year, depending mainly on the star’s ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10 to 100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every 4 months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.
The Transiting Exoplanet Survey Satellite (TESS ) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its two-year mission, TESS will employ four wide-field optical CCD cameras to monitor at least 200,000 main-sequence dwarf stars with IC (approximately less than) 13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from one month to one year, depending mainly on the star's ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10-100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every four months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.
Over a 10-month period during 2013 and early 2014, development of the Neutron star Interior Composition Explorer (NICER) mission  proceeded through Phase B, Mission Definition. An external attached payload on the International Space Station (ISS), NICER is scheduled to launch in 2016 for an 18-month baseline mission. Its prime scientific focus is an in-depth investigation of neutron stars—objects that compress up to two Solar masses into a volume the size of a city—accomplished through observations in 0.2–12 keV X-rays, the electromagnetic band into which the stars radiate significant fractions of their thermal, magnetic, and rotational energy stores. Additionally, NICER enables the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) demonstration of spacecraft navigation using pulsars as beacons. During Phase B, substantive refinements were made to the mission-level requirements, concept of operations, and payload and instrument design. Fabrication and testing of engineering-model components improved the fidelity of the anticipated scientific performance of NICER’s X-ray Timing Instrument (XTI), as well as of the payload’s pointing system, which enables tracking of science targets from the ISS platform. We briefly summarize advances in the mission’s formulation that, together with strong programmatic performance in project management, culminated in NICER’s confirmation by NASA into Phase C, Design and Development, in March 2014.
Soft X-ray Imager (SXI) is a CCD camera onboard the ASTRO-H satellite which is scheduled to be launched in 2015. The SXI camera contains four CCD chips, each with an imaging area of 31mm x 31 mm, arrayed in mosaic, covering the whole FOV area of 38′ x 38′. The CCDs are a P-channel back-illuminated (BI) type with a depletion layer thickness of 200 _m. High QE of 77% at 10 keV expected for this device is an advantage to cover an overlapping energy band with the Hard X-ray Imager (HXI) onboard ASTRO-H. Most of the flight components of the SXI system are completed until the end of 2013 and assembled, and an end-to-end test is performed. Basic performance is verified to meet the requirements. Similar performance is confirmed in the first integration test of the satellite performed in March to June 2014, in which the energy resolution at 5.9 keV of 160 eV is obtained. In parallel to these activities, calibrations using engineering model CCDs are performed, including QE, transmission of a filter, linearity, and response profiles.
The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions developed by the Institute of Space and Astronautical Science (ISAS), with a planned launch in 2015. The ASTRO-H mission is equipped with a suite of sensitive instruments with the highest energy resolution ever achieved at E > 3 keV and a wide energy range spanning four decades in energy from soft X-rays to gamma-rays. The simultaneous broad band pass, coupled with the high spectral resolution of ΔE ≤ 7 eV of the micro-calorimeter, will enable a wide variety of important science themes to be pursued. ASTRO-H is expected to provide breakthrough results in scientific areas as diverse as the large-scale structure of the Universe and its evolution, the behavior of matter in the gravitational strong field regime, the physical conditions in sites of cosmic-ray acceleration, and the distribution of dark matter in galaxy clusters at different redshifts.
We report on the development status of the readout ASIC for an onboard X-ray CCD camera. The quick low- noise readout is essential for the pile-up free imaging spectroscopy with the future highly sensitive telescope. The dedicated ASIC for ASTRO-H/SXI has sufficient noise performance only at the slow pixel rate of 68 kHz. Then we have been developing the upgraded ASIC with the fourth-order ΔΣ modulators. Upgrading the order of the modulator enables us to oversample the CCD signals less times so that we. The digitized pulse height is a serial bit stream that is decrypted with a decimation filter. The weighting coefficient of the filter is optimized to maximize the signal-to-noise ratio by a simulation. We present the performances such as the input equivalent noise (IEN), gain, effective signal range. The digitized pulse height data are successfully obtained in the first functional test up to 625 kHz. IEN is almost the same as that obtained with the chip for ASTRO-H/SXI. The residuals from the gain function is about 0.1%, which is better than that of the conventional ASIC by a factor of two. Assuming that the gain of the CCD is the same as that for ASTRO-H, the effective range is 30 keV in the case of the maximum gain. By changing the gain it can manage the signal charges of 100 ke-. These results will be fed back to the optimization of the pulse height decrypting filter.
A precise measurement of the Cosmic X-ray Background (CXB) is crucial for constraining models of the evolution and composition of the universe. While several large, expensive satellites have measured the CXB as a secondary mission, there is still disagreement about normalization of its spectrum. The Cosmic X-ray Background NanoSat (CXBN) is a small, low-cost satellite whose primary goal is to measure the CXB over its two-year lifetime. Benefiting from a low instrument-induced background due to its small mass and size, CXBN will use a novel, pixelated Cadmium Zinc Telluride (CZT) detector with energy resolution < 1 keV over the range 1-60 keV to measure the CXBN with unprecedented accuracy. This paper describes CXBN and its science payload, including the GEANT4 model that has been used to predict overall performance and the backgrounds from secondary particles in Low Earth Orbit. It also addresses the strategy for scanning the sky and calibrating the data, and presents the expected results over the two-year mission lifetime.
We have studied timing properties of the Amptek Silcon Drift Detectors (SDD) using pulsed X-ray source
designed at NASA Goddard Space Flight Center. The proposed Neutron Star Interior Composition Explorer
(NICER) mission will use 56 of these detectors as X-ray sensors in an attached payload to the International
Space Station to study time variability of millisecond X-ray pulsars. Using a rastered pinhole we have measured
the delay times for single X-ray photons as a function of the impact position on the detector, as well as signal
rise time as a function of impact position. We find that the interdependence of these parameters allows us to
determine photon position on the detector by measuring the signal rise time, and, improve the accuracy of the
photon arrival time measurement.
Soft X-ray Imager (SXI) is a CCD camera onboard the ASTRO-H satellite which is scheduled to be launched
in 2014. The SXI camera contains four CCD chips, each with an imaging area of 31mm×
31 mm, arrayed in
mosaic, which cover the whole FOV area of 38' ×
38'. The SXI CCDs are a P-channel back-illuminated (BI) type
with a depletion layer thickness of 200 μm. High QE of 77% at 10 keV expected for this device is an advantage
to cover an overlapping energy band with the Hard X-ray Imager (HXI) onboard ASTRO-H. Verification with
engineering model of the SXI has been performed since 2011. Flight model design was fixed and its fabrication
has started in 2012.
The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated
by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the highenergy
universe via a suite of four instruments, covering a very wide energy range, from 0.3 keV to 600 keV.
These instruments include a high-resolution, high-throughput spectrometer sensitive over 0.3–12 keV with
high spectral resolution of ΔE ≦ 7 eV, enabled by a micro-calorimeter array located in the focal plane of
thin-foil X-ray optics; hard X-ray imaging spectrometers covering 5–80 keV, located in the focal plane of
multilayer-coated, focusing hard X-ray mirrors; a wide-field imaging spectrometer sensitive over 0.4–12 keV,
with an X-ray CCD camera in the focal plane of a soft X-ray telescope; and a non-focusing Compton-camera
type soft gamma-ray detector, sensitive in the 40–600 keV band. The simultaneous broad bandpass, coupled
with high spectral resolution, will enable the pursuit of a wide variety of important science themes.
Soft X-ray Imager (SXI) is a CCD camera onboard the ASTRO-H satellite which is scheduled to be launched
in 2014. The SXI camera contains four CCD chips, each with an imaing aread of 31mmx31 mm, arrayed in
mosaic, which cover the whole FOV area of 38'x38'. The SXI CCD of which model name is HPK Pch-NeXT4
is a P-channel type, back-illuminated, fully depleted device with a thickness of 200μm. We have developed an
engineering model of the SXI camera body with coolers, and analog electronics for them. Combined with the
bread board digital electronics, we succeeded in operation the whole the SXI system. The CCDs are cooled down
to -120°C with this system, and X-rays from 55Fe sources are detected. Although optimization of the system is in
progress, the energy resolution of typical 200 eV and best 156 eV (FWHM) at 5.9 keV are obtained. The readout
noise is 10 e- to 15 e-, and to be improved its goal value of 5 e-. On-going function tests and environment tests
reveal some issues to be solved until the producntion of the SXI flight model in 2012.
We report on the performance of an analog application-specified integrated circuit (ASIC) developed for the front-end electronics of the X-ray CCD camera system (SXI: Soft X-ray Imager) onboard the ASTRO-H satellite. The ASIC consists of four identical channels and they simultaneously process the CCD signals at the pixel rate of 68kHz. Delta-Sigma modulator is adopted to achieve effective noise shaping and obtain a high resolution decimal values with relatively simple circuits. We will implement 16 ASIC chips in total in the focal plane assembly. The results of the unit test shows that it works properly with moderately low input noise of <30μV at the pixel rate of 80kHz. Power consumption is sufficiently low of 150mW. Dynamic range of input signals is +-20mV that covers effective energy range of the CCD chips of SXI (0.2-20keV). The integrated non-linearity of 0.2% satisfies the same performance as the conventional CCD detectors in orbit. The radiation tolerance against total ionizing dose (TID) effect and single event latch-up (SEL) has also been investigated. The irradiation test using 60Co gamma-rays and proton beam showed that the ASIC has sufficient tolerance against TID up to 200 and 167krad respectively, which thoroughly exceeds the expected operating duration in the planned low-inclination low-earth orbit. The irradiation of the Fe ion beam also showed no latch-up nor malfunctions up to the fluence of 4.7x10^7ions. The threshold against SEL is larger than 1.68MeVcm^2/mg, which is sufficiently high enough that SEL events should not be a major cause of instrument downtime.
We have developed application specific integrated
circuits(ASICs) for multi-readout X-ray CCDs in order to improve their
time resolution. ASICs with the size of 3mm × 3mm were fabricated by employing a Taiwan
Semiconductor Manufacturing Company(TSMC) 0.35 μm CMOS technology.
The number of channels is 4 and the each channel consists of a
preamplifier, 5-bit DAC and delta-sigma analog-to-digital converters
(ADCs). The measured equivalent input noise at the
pixel rate of 19.5 kHz and 625 kHz are 36 μV and 51 μV,
respectively. The power consumption is about 110 mW/chip at 625 kHz pixel rate,
which is about 10 times lower than that of our existing system.
We now expect to employ an ASIC as the readout system of X-ray CCD camera onboard the next Japanese X-ray astronomy satellite. We tested the
readout of the prototype X-ray CCDs by using ASICs and the total-dose effects of ASICs. We describe the overview of our ASICs and test results.
We report on the development of high-speed and low-noise readout system of X-ray CCD camera with ASIC and the Camera Link standard.
The ASIC is characterized by AD-conversion capability and it processes CCD output signals with a high pixel rate of 600 kHz, which is ten times quicker than conventional frame transfer type X-ray CCD cameras in orbit.
There are four identical circuits inside the chip and all of them process CCD signals simultaneously. ΔΣ modulator is adopted to achieve effective noise shaping and obtain a high resolution decimal values with relatively simple circuits.
The results of the unit test shows that it works properly with moderately low input noise of ~70 μV at pixel rate of 625 kHz, and ~40 μV @ 40 kHz.
Power consumption is sufficiently low of <120 μuV @ 1.25 MHz. We have also developed the rest of readout and driving circuits. As a data acquisition scheme we adopt the Camera Link standard in order to support the high readout rate of the ASIC.
In the initial test of the CCD camera system, we used the P-channel CCD developed for Soft X-ray Imager onboard next Japanese X-ray astronomical satellite. The thickness of its depletion layer reaches up to 220 μm and therefore we can detect the X-rays from 109Cd with high sensitivity rather than N-channel CCDs. The energy resolution by our system is 379 (±7)eV (FWHM) @ 22.1 keV, that is, ΔE/E=1.8% was achieved with a readout rate of 44 kHz.
X-ray CCDs have superior spatial resolution of ~20μm and moderate energy resolution of ~130 eV(FWHM)
at 5.9 keV. On the other hand, the number of pixels assigned to each readout node is generally so large that it
takes several seconds to process a frame data of the entire chip. Relatively low pixel readout rate in order to
keep readout noise low also limits the timing resolution of X-ray CCDs. Although a large number of readout
nodes is essential to improve the timing resolution, size and power consumption of conventional readout circuits
prevent us from being implemented in X-ray CCD camera systems onboard satellites. We are developing an
application specific integrated circuits (ASIC) for multi readout of X-ray CCD signals. The ASIC with the size
of 3mm×3mm has four channels of readout electronics that employs the delta-sigma (ΔΣ) digitization. The
fabrication process is a 0.35μm complimentary metal-oxide semiconductor (CMOS) process provided by Taiwan
Semiconductor Manufacturing Company (TSMC). The equivalent input noise was about 33μV and the power
consumption was about 70mW per chip at the pixel rate of 44 kHz. When we used the X-ray CCD whose
sensitivity was 3 μV/e-, the equivalent noise charge was 10.8e- and the energy resolution was 168 eV(FWHM)
at 5.9 keV. The noise level of our ASIC is comparable to that of the conventional readout systems.
Delta Sigma digitizers generally have excellent linearity, precision and noise rejection. They are especially well
suited for implementation as integrated circuits. However, they are rarely used for time bounded signals like
CCD pixels. We are developing a CCD video digitizer chip incorporating a novel variant of the Delta Sigma
architecture that is especially well suited for this application. This architecture allows us to incorporate video
filtering and correlated double sampling into the digitizer itself, eliminating the complex analog video processing
usually needed before digitization.
We will present details of a multichannel ASIC design that will achieve spectroscopic precision and linearity
while using much less energy than previous CCD digitizers for technical applications such as imaging X-ray
spectroscopy. The low conversion energy requirement together with the ability to integrate many channels will
enable us to construct fast CCD systems that require no cooling and can handle a much wider range of X-ray
intensity than existing X-ray CCD systems.
The quantitative study of the changes in the behavior of structures with scale is one of the oldest areas of physics: it was one of Galileo's "Two New Sciences". Nevertheless, it does not have the appreciation it deserves among high energy astrophysicists. While most understand the importance of collecting area and resolution, the connection between them is less well known. This matters: to make a good instrument one must exploit the applied physics, not fight against it.
I will discuss counter-intuitive consequences of some well known scaling laws. I will show that for imaging instruments detector linear resolution is an under-appreciated performance driver. I will discuss the tradeoffs between modularization and integration.
Attention to scaling issues has the potential to enable world class science from small instruments, increase the productivity of larger instruments, and transform extremely large instruments from impractical fantasies to practical realities.
We report on design updates for the XIS (X-ray Imaging Spectrometer)
on-board the Astro-E2 satellite. Astro-E2 is a recovery mission of Astro-E, which was lost during launch in 2000. Astro-E2 carries a total of 5 X-ray telescopes, 4 of which have XIS sensors as their focal plane detectors. Each XIS CCD camera covers a field of view of 19×19 arcmin in the energy range of 0.4-12 keV. The design of the Astro-E2 XIS is basically the same as that for Astro-E, but some improvements will be implemented. These are (1) CCD charge injection capability, (2) a revised heat-sink assembly, and (3) addition of a 55Fe radio-isotope on the door. Charge injection may be used to compensate for and calibrate radiation-induced degradation of the CCD charge transfer efficiency. This degradation is expected to become significant after a few year's operation in space. The new heat-sink assembly is expected to increase the mechanical reliability and cooling capability of the XIS sensor. The new radio-isotope on the door will provide better calibration data. We present details of these improvements and summarize the overall design of the XIS.
The optical chain of the spectroscopic x-ray telescopes aboard the Constellation-X spacecraft employs a reflective grating spectrometer to provide high resolution spectra for multiple spectra as a slitless spectrometer in the spectral feature rich, soft x-ray band. As a part of the spectroscopic readout array, we provide a zero-order camera that images the sky in the soft band inaccessible to the microcalorimeters. Technological enhancements required for producing the RGS instruments are described, along with prototype development progress, fabrication and testing results.
The ASTRO-E X-ray Imaging Spectrometers (XISs) consists of four sets of X-ray CCD camera for the ASTRO-E mission. The XISs have been calibrated at Osaka University, Kyoto University, ISAS and MIT. The calibration experiment at Osaka focuses on the soft x-ray response of the XIS. The calibration of the XIS flight model has been performed since August 1998. We measured the signal-pulse height, the energy resolution and the quantum efficiency of the XIS as a function of energy, all of which are essential to construct the response function of the XIS. The detailed shape of the pulse-height-distribution are also investigated. We also constructed a numerical simulator of the XIS, which tracks the physical process in the CCD so as to reproduce the measured data. With a help of this simulator, we propose a model of the pulse-height-distribution of the XIS for single energy incident x-rays. The model consists of four components; two Gaussians, a constant, plus a triangle-shape component.
The x-ray imaging spectrometers (XIS) are x-ray CCD cameras on-board the Astro-E satellite launched in 2000. The XIS consists of 4 cameras, each of them will be installed on a focal plane of the Astro-E X-ray Telescope (XRT). The XIS not only have a higher sensitivity, which comes from a larger effective area of the XRT and thicker depletion layers of the XIS CCDs, than ASCA SIS. But also have several features that will overcome the radiation damage effects anticipated in the orbit. The calibration experiment at Osaka focuses on the soft x-ray response of the XIS. The calibration system employs a grating spectrometer which irradiates the CCD with dispersed x-rays. We have obtained preliminary results on the XIS proto model, including the energy-pulse-height relation, the energy-resolution relation, and the quantum efficiency at the energy range of 0.25-2.2 keV.
The recent restructuring of the AXAF program has necessitated a review of the design of the ACIS instrument. In this paper we report on the current status of these design activities. We concentrate on changes to the baseline CCD and its impact on aspects such as the operating modes. Also we review changes to the mechanical design with respect to the passive cooling scheme facilitated by the change to a highly eccentric deep earth orbit.