Expected to launch in 2021 Spring, the Imaging X-ray Polarimetry Explorer (IXPE) is a NASA Astrophysics Small Explorer Mission with significant contributions from the Italian space agency (ASI). The IXPE observatory features three identical x-ray telescopes, each comprised of a 4-m-focal length mirror module assembly (MMA, provided by MSFC) that focuses x-rays onto a polarization-sensitive, imaging detector (contributed by ASI-funded institutions). This paper summarizes the MMA’s design, fabrication, alignment and assembly, expected performance, and calibration plans.
The Imaging X-ray Polarimetry Explorer (IXPE) will add polarization to the properties (time, energy, and position) observed in x-ray astronomy. A NASA Astrophysics Small Explorer (SMEX) in partnership with the Italian Space Agency (ASI), IXPE will measure the 2–8-keV polarization of a few dozen sources during the first 2 years following its 2021 launch. The IXPE Observatory includes three identical x-ray telescopes, each comprising a 4-m-focal-length (grazingincidence) mirror module assembly (MMA) and a polarization-sensitive (imaging) detector unit (DU), separated by a deployable optical bench. The Observatory’s Spacecraft provides typical subsystems (mechanical, structural, thermal, power, electrical, telecommunications, etc.), an attitude determination and control subsystem for 3-axis stabilized pointing, and a command and data handling subsystem communicating with the science instrument and the Spacecraft subsystems.
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment demonstrates the technique of focusing hard X-ray (HXR) optics for the study of fundamental questions about the high-energy Sun. Solar HXRs provide one of the most direct diagnostics of accelerated electrons and the impulsive heating of the solar corona. Previous solar missions have been limited in sensitivity and dynamic range by the use of indirect imaging, but technological advances now make direct focusing accessible in the HXR regime, and the FOXSI rocket experiment optimizes HXR focusing telescopes for the unique scientific requirements of the Sun. FOXSI has completed three successful flights between 2012 and 2018. This paper gives a brief overview of the experiment, focusing on the third flight of the instrument on 2018 Sept. 7. We present the telescope upgrades highlighting our work to understand and reduce the effects of singly reflected X-rays and show early science results obtained during FOXSI's third flight.
NASA’s Marshall Space Flight Center (MSFC) maintains an active research program toward the development of high-resolution, lightweight, grazing-incidence x-ray optics to serve the needs of future x-ray astronomy missions such as Lynx. MSFC development efforts include both direct fabrication (diamond turning and deterministic computer-controlled polishing) of mirror shells and replication of mirror shells (from figured, polished mandrels). Both techniques produce full-circumference monolithic (primary + secondary) shells that share the advantages of inherent stability, ease of assembly, and low production cost. However, to achieve high-angular resolution, MSFC is exploring significant technology advances needed to control sources of figure error including fabrication- and coating-induced stresses and mounting-induced distortions.
The Imaging X-ray Polarimetry Explorer (IXPE) will expand the information space for study of cosmic sources, by adding polarization to the properties (time, energy, and position) observed in x-ray astronomy. Selected in 2017 January as a NASA Astrophysics Small Explorer (SMEX) mission, IXPE will be launched into an equatorial orbit in 2021. The IXPE observatory includes three identical x-ray telescopes, each comprising a 4-m-focal-length (grazing-incidence) mirror module assembly (MMA) and a polarization-sensitive (imaging) detector unit (DU). The optical bench separating the MMAs from the DUs is a deployable boom with a tip/tilt/rotation stage for DU-to-MMA (gang) alignment, similar to the configuration used for the NuSTAR observatory. The IXPE mission will provide scientifically meaningful measurements of the x-ray polarization of a few dozen sources in the 2-8 keV band, over the first two years of the mission. For several bright, extended x-ray sources (pulsar wind nebulae, supernova remnants, and an active-galaxy jet), IXPE observations will produce polarization maps indicating the magnetic structure of the synchrotron emitting regions. For many bright pulsating x-ray sources (isolated pulsars, accreting x-ray pulsars, and magnetars), IXPE observations will produce phase-resolved profiles of the polarization degree and position angle.
Si Hybrid CMOS detectors (HCDs) are sensitive to X-rays between approximately 0.2 – 20 keV. HCDs can provide superior performance to traditional CCDs in multiple areas: faster read out time, windowed read out mode, less susceptible to radiation & micrometeoroid damage, and lower power consumption. X-ray detectors designed for use in astronomical observatories must have an optical blocking filter to prevent the detectors from being saturated by optical light. We have previously reported on the successful deposition of an Al optical blocking layer directly onto the surface of HCDs. These blocking filters were deposited with multiple thicknesses from 180 – 1000 Å and successfully block optical light at all thicknesses, with minimal impact expected on quantum efficiency at the energies of interest for these detectors. The thin Al layer is not expected to impact quantum efficiency at the energies of interest for these detectors. We report energy dependent soft X-ray quantum efficiency measurements for multiple HCDs with different optical blocking filter thicknesses.
We report on the characterization of four HAWAII Hybrid Si CMOS detectors (HCD) developed for use as X-ray
detectors as part of a joint program between Penn State University and Teledyne Imaging Sensors (TIS).
Interpixel capacitive crosstalk (IPC) has been measured for standard H1RG detectors as well as a specially
developed H2RG that uses a unique bonding structure. The H2RG shows significant reduction in IPC, as reported
by Griffith et al. 2012. Energy resolution at 1.5 & 5.9 keV was measured as well as read noise for each detector.
Dark current as a function of temperature is reported from 150 – 210 K and dark current figures of merit are
estimated for each detector. We also discuss upcoming projects including testing of a new HCD called the
Speedster-EXD. This prototype detector will have a low noise, high gain CTIA to reduce IPC and read noise as
well as in-pixel CDS and event flagging. In the coming year PSU and TIS will begin work on a project to
incorporate CTIA and CDS circuitry into the ROIC of a HAWAII HCD like detector to satisfy the small pixel and
high rate needs of future X-ray observatories.
Future space-based X-ray telescope missions are likely to have significantly increased demands on detector read out
rates due to increased collection area, and there will be a desire to minimize radiation damage in the interests of
maintaining spectral resolution. While CCDs have met the requirements of past missions, active pixel sensors are likely
to be a standard choice for some future missions due to their inherent radiation hardness and fast, flexible read-out
architecture. One form of active pixel sensor is the hybrid CMOS sensor. In a joint program of Penn State University
and Teledyne Imaging Sensors, hybrid CMOS sensors have been developed for use as X-ray detectors. Results of this
development effort and tests of fabricated detectors will be presented, along with potential applications for future
We present the results of x-ray measurements on a hybrid CMOS detector that uses a H2RG ROIC and a unique
bonding structure. The silicon absorber array has a 36μm pixel size, and the readout array has a pitch of 18μm;
but only one readout circuit line is bonded to each 36x36μm absorber pixel. This unique bonding structure gives
the readout an effective pitch of 36μm. We find the increased pitch between readout bonds significantly reduces
the interpixel capacitance of the CMOS detector reported by Bongiorno et al. 20101 and Kenter et al. 2005.2
The recent development of active pixel sensors as X-Ray focal plane arrays will place them in contention with
CCDs on future satellite missions. Penn State University (PSU) is working with Teledyne Imaging Sensors
(TIS) to develop X-Ray Hybrid CMOS devices (HCDs), a type of active pixel sensor with fast frame rates,
adaptable readout timing and geometry, low power consumption, and inherent radiation hardness. CCDs have
been used with great success on the current generation of X-Ray telescopes (e.g. Chandra, XMM, Suzaku, and
Swift). However, their bucket-brigade readout architecture, which transfers charge across the chip with discrete
component readout electronics, results in clockrate limited readout speeds that cause pileup (saturation) of bright
sources and an inherent susceptibility to radiation induced displacement damage that limits mission lifetime. In
contrast, HCDs read pixels through the detector substrate with low power, on-chip readout integrated circuits.
Faster frame rates, achieved with adaptable readout timing and geometry, will allow the next generation's larger
effective area telescopes to observe brighter sources free of pileup. In HCDs, radiation damaged lattice sites
affect a single pixel instead of an entire row. The PSU X-ray group is currently testing 4 Teledyne HCDs,
with low cross-talk CTIA devices in development. We will report laboratory measurements of HCD readnoise,
interpixel-capacitance and its impact on event selection, linearity, and energy resolution as a function of energy.
The development of Hybrid CMOS Detectors (HCDs) for X-Ray telescope focal planes will place them in contention
with CCDs on future satellite missions due to their faster frame rates, flexible readout scenarios, lower
power consumption, and inherent radiation hardness. CCDs have been used with great success on the current
generation of X-Ray telescopes (e.g. Chandra, XMM, Suzaku, and Swift). However their bucket-brigade readout
architecture, which transfers charge across the chip with discrete component readout electronics, results in
clockrate limited readout speeds that cause pileup (saturation) of bright sources and an inherent susceptibility
to radiation induced displacement damage that limits mission lifetime. In contrast, HCDs read pixels with low
power, on-chip multiplexer electronics in a random access fashion. Faster frame rates achieved with multi-output
readout design will allow the next generation's larger effective area telescopes to observe bright sources free of
pileup. Radiation damaged lattice sites effect a single pixel instead of an entire row. Random access, multi-output
readout will allow for novel readout modes such as simultaneous bright-source-fast/whole-chip-slow readout. In
order for HCDs to be useful as X-Ray detectors, they must show noise and energy resolution performance similar
to CCDs while retaining advantages inherent to HCDs. We will report on readnoise, conversion gain, and energy
resolution measurements of an X-Ray enhanced Teledyne HAWAII-1RG (H1RG) HCD and describe techniques
of H1RG data reduction.
In a joint program of Penn State University and Teledyne Imaging Sensors, hybrid CMOS sensors have been developed
for use as X-ray detectors. This detector technology can provide major improvements in performance relative to CCDs,
which are the current standard technology used in the focal planes of X-ray telescopes (e.g. Chandra, XMM, Suzaku, and
Swift). Future X-ray telescope missions are all likely to have significantly increased collection area. If standard CCDs
are used, the effects of saturation (pile-up) will have a major impact, while radiation damage will impact the quality and
lifetime of the detectors. By reading out the hybrid CMOS detector in a pixel-by-pixel fashion at high speeds, with an
energy resolution similar to CCDs, CMOS sensors could increase the range of pile-up free operation by several orders of
magnitude. They are also several orders of magnitude more radiation hard than typical CCDs since they transfer charge
through the thickness of the device, rather than across the length of its surface. Furthermore, hybrid CMOS detectors
can be programmed to read out any variety of windowed regions, which leads to versatility and speed. All of this can be
achieved, in principle, while maintaining the same quantum efficiencies achievable in CCDs. Results of this
development effort and preliminary tests of fabricated detectors will be presented, along with potential applications for
future missions such as EDGE and Constellation-X.