The WSPC Handbook of Astronomical Instrumentation

The following sections are included:

• Contents
• Foreword

In this chapter, the main configurations of X-ray astronomical telescopes for space astronomy are introduced. To this end some historical notes on the development of X-ray optics are given and some perspectives on future applications are discussed. X-ray telescopes were introduced at the beginning of the history of X-ray astronomy in order to improve the imaging and flux sensitivity achievable with direct view detectors. Since refractive lenses cannot be used with X-rays (due to the very unfavorable optical constants in this energy band), X-ray telescopes are generally based on grazing-incidence mirrors, working at glancing angles (typically of the order of 1 degree). The classical geometries utilized for making X-ray astronomical mirrors are derived from H. Wolter’s work, initially developed for use in X-ray microscopy applications. These two-reflection mirrors are pseudo-cylindrical shells whose profiles follow conical curves. Other configurations that evolved from Wolter’s design have also become interesting for astronomical applications. Kirkpatrick–Baez and lobster eye optics represent other, completely different, geometries that can be used for X-ray telescopes, but Wolter’s optics configurations remain the most widely used system adopted for X-ray telescopes.

This chapter discusses the production of silicon pore optics, a novel class of lightweight high energy optics made from high quality 12″ silicon wafers. The technology is being developed by the European Space Agency to enable the next generation of X-ray observatories, such as the Athena mission planned for launch in 2033. Silicon Pore Optics will allow building a high-resolution telescope with several square meters of effective area. We will show the production steps from silicon up to final optic, present design parameters for future applications and discuss other applications for this technology.

The utility of an astronomical X-ray telescope, like that of a telescope of any other band, is largely determined by its angular resolution, photon collecting area, field of view, and energy bandwidth. Current and past X-ray telescopes, such as Chandra, XMM-Newton, Suzaku, and NuSTAR, have optimized for one or more of these four qualities at the expense of the others. The future of X-ray astrophysics depends on the realization of these qualities to the largest extent possible in a single telescope at an affordable cost. The next-generation X-ray optics development effort at the NASA Goddard Space Flight Center, which we describe in this chapter, has as its objective to develop a process of making X-ray mirror assemblies that keep Suzaku’s light weight and low cost while attempting to first achieve and then exceed Chandra’s sub-arcsecond angular resolution. We have adopted an implementation method based on precision-polished mono-crystalline silicon mirror segments, which are aligned and integrated into meta-shells as an intermediary step. These meta-shells in turn will be aligned and integrated to become a mirror assembly. This approach simplifies the construction of a large mirror assembly into the building and testing and then aligning and integrating of several meta-shells. The core of our effort is the development and maturation of techniques to build and test meta-shells, including fabrication, coating, alignment, and bonding of lightweight and thin mirror segments. As of 2017, we have made lightweight mirror segments using mono-crystalline silicon that are similar to Chandra’s mirrors in image quality. We have constructed and tested small mirror modules that, under full illumination, produce 2.2 arcsecond X-ray images (halfpower diameter), which is limited by errors in alignment and bonding procedures. We expect that the image quality will continue to improve, reaching and surpassing Chandra’s 0.5 arcseconds in the early 2020s. Through finite element analysis and targeted environmental tests of mechanical mockups, we have also validated the meta-shell approach to building large X-ray mirror assemblies.

Scientific progress in understanding the universe requires data across the electromagnetic spectrum. To address the cutting edge scientific problems of the coming decades, we need X-ray data with sub-arcsecond resolution. To use such precise angular resolution effectively and efficiently, large collecting areas are necessary. These requirements dictate that future X-ray telescopes are made from highly nested, thin shells. To control the figure and alignment of such flimsy optics, we believe on-orbit adjustability is required. Accordingly, the Smithsonian Astrophysical Observatory and The Pennsylvania State University have embarked on a project to control the mirror figure by depositing piezoelectric material on the back (non-reflecting) side of 200mm long pieces of 0.4mm thick glass. Detailed optical metrology on the ground determines the voltages used on-orbit to impart the stresses needed to produce the required X-ray reflecting shape. We have experimentally verified this concept (i.e. established Technology Readiness Level 3), and expect to develop it as a candidate technology for the Lynx mission to be presented to the 2020 Decadal Committee for Astronomy and Astrophysics.

This chapter describes the process of implementing the piezoelectric adjustors, and the concept of a large area, high angular resolution X-ray telescope for an observatory to be used by astronomers worldwide.

We know of three basic geometries that utilize grazing-incidence reflection of X-rays to produce X-ray images. In all three two successive reflections are required to produce a meaningful image largely free of coma. Wolter systems employ surfaces of revolution with axial symmetry, in which the two successive reflections occur in the same plane. Kirkpatrick–Baez systems utilize reflecting surfaces that are almost planar and are set such that the two reflection planes are perpendicular to one another. The third is the lobster eye geometry which comprises an array of small, closely packed tubes or pores that have a square cross-section and imaging is achieved by grazing-incidence reflections from the planar internal surfaces of the pores. Lobster eye optics are peculiar. The two reflections that produce an image occur from adjacent internal walls of each pore, but the combined response of hundreds or thousands of individual pores is required to produce a recognizable image or picture. Lobster eye X-ray optics are difficult to make but they offer two major advantages over Wolter systems. The field of view is not limited by the small grazing angles required to reflect X-rays and, in principle, a single lobster eye optic can produce a continuous image of the entire sky. Providing the width of the pores is large compared with the thickness of the pore walls, lobster eye optics are intrinsically very low mass. X-ray telescopes using lobster eye optics have great potential for X-ray transient imaging and for applications where low-mass is crucial.

Full-shell grazing-incidence optics have been used extensively in X-ray astronomy and have produced the best angular resolutions to date, albeit at the expense of mass and cost. Lightweight full-shell optics trade angular resolution for ease of fabrication, lower mass, and lower costs per mirror shell. Current programs in lightweight full-shell grazing-incidence optics for X-ray astronomy and other applications are described, together with research to improve the native resolution of thin-shell optics or to effect post-fabrication improvements. Also covered are techniques for mirror-shell alignment and assembly, intended to reduce image degradation during production of mirror assemblies.

This chapter describes the use and development of reflective multilayer coatings for hard X-ray astronomy. These coatings are designed to have broad spectral response at X-ray energies above ∼10 keV, and are deposited onto mirror substrates used to construct grazing-incidence X-ray telescopes. The principles of operation, methods for the design, fabrication, and characterization of these coatings, and future research directions are described.

Charge-coupled devices (CCDs) have been the detector of choice for all X-ray astronomy missions of the past 20 years. I discuss the operational principles of CCDs used in the X-ray energy band and review CCD instruments on a variety of astrophysics and heliophysics missions.

Invented in the mid-eighties, DEPFET detector amplifier structures have been foreseen as radiation detectors for particle tracking, as X-ray imagers and for position-resolved spectroscopy in basic and applied science. ESA’s BepiColombo and Athena missions, to be launched in 2018 and 2028, respectively, will be equipped with focal plane detectors based on the DEPFET principle. The DEPFET technology allows for very low noise signal amplification at high speed due to its minimal input capacitance. As the DEPFET is made on high resistivity double-sided structured silicon, it exhibits high quantum efficiency and back illumination through an ultrathin radiation entrance window. X-ray detection in single photon counting mode with spectroscopic performance has been demonstrated using X-rays from 277 eV to 10 keV. An electronic shutter has been integrated as well as a non-destructive repetitive readout scheme to obtain noise figures as low as 0.25 electrons (rms). We discuss the basic principles as well as astrophysical applications and perspectives for the future.

X-ray hybrid CMOS detectors have multiple strengths for astronomical telescope applications, with some of the most notable among those strengths being: (1) high quantum efficiency across the 0.2–20 keV bandpass, (2) rapid readout capabilities, (3) low power, (4) radiation hardness, and (5) adaptable readout schemes making use of their ability to read out individual active pixels. While longer wavelength hybrid CMOS detectors (HCDs) have been mature for decades and used for space-flight applications on multiple occasions, the X-ray versions of these detectors have been in the development stages for the past dozen years, with a recent rocket flight having brought one version of these detectors to TRL 9 in April 2018. Ubiquitous industrial use of HCDs for commercial applications (e.g. cell phone cameras) makes it possible to achieve affordable and relatively fast developments for these devices, and the next generation of X-ray HCDs are already showing remarkable features that will enhance X-ray astronomy. The latest devices have already demonstrated event-driven readout with very rapid effective frame rates, in-pixel correlated double sampling, and analog-to-digital signal conversion on-chip. Future X-ray astronomy missions will benefit from their use of X-ray HCDs.

Adiabatic demagnetization refrigerators (ADRs) which provide a temperature of about 100mK for detectors are described, with emphasis on space missions. ADRs use the Carnot cycle with a magnetic field and a paramagnetic salt. A high Carnot efficiency is generally achieved. In order to reduce total mass, a multi-stage system is considered for space missions where the maximum magnet current is limited to a few Amperes and the thermal sink temperature is typically 2–4K. An ADR consists of three key components: a refrigerant pill, a heat switch, and a superconducting magnet. Magnetic shielding of the superconducting magnets is also important. We summarize the materials and designs of these components. We show two examples of ADRs in space missions: the 3-stage ADR for the X-ray microcalorimeter instrument, SXS, onboard the Hitomi (ASTRO-H) satellite, and the ADR designed for the far-infrared instrument, SAFARI, on the SPICA mission. We then describe recent ADR developments aimed at high reliability and/or continuous operations. We finally mention an alternative method to obtain ∼100mK in orbit, a single-shot dilution refrigerator used for the Planck mission, and describe the development status of a closed-cycle dilution refrigerator.

High resolution soft X-ray spectroscopy is a necessary method for many astrophysical investigations. The Critical Angle Transmission grating is a new technology that can provide very efficient spectrometers in the soft X-ray band with high resolving power. Here we discuss the astrophysical motivation, the operating principles, give overviews of fabrication and performance, and finally present a mission concept, including comparisons to existing astrophysical X-ray grating spectometers.

X-ray reflection gratings placed in the off-plane mount are capable of achieving high diffraction efficiency and high spectral resolving power, the two chief assets of a grating spectrometer. Developments in this technology have been accelerated in recent years and new fabrication methodologies are being employed to create a new generation of reflection gratings. Here, we discuss the theory of diffraction, and how it pertains to off-plane gratings. This is followed by a discussion of considerations that are relevant when designing an off-plane grating spectrometer. Empirical results from the measurement of diffraction efficiency and spectral resolving power are also presented to motivate a discussion of the incorporation of these gratings into future space-based X-ray spectrometers.

We review the basic principles of X-ray polarimetry and current detector technologies based on the photoelectric effect, Bragg reflection, and Compton scattering. Recent technological advances in high-spatial-resolution gas-filled X-ray detectors have enabled efficient polarimeters exploiting the photoelectric effect that hold great scientific promise for X-ray polarimetry in the 2–10 keV band. Advances in the fabrication of multilayer optics have made feasible the construction of broadband soft X-ray polarimeters based on Bragg reflection. Developments in scintillator and solid-state hard X-ray detectors facilitate construction of both modular, large area Compton scattering polarimeters and compact devices suitable for use with focusing X-ray telescopes.

The following sections are included:

• Index