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Introduction

CCDs are widely employed in ground-based astronomical applications, and are key components of several recently launched space missions. However it is only now that a number of important X-ray astronomical applications of CCDs have reached critical development stages, so that this is an appropriate time to review the application of space-borne CCD instruments to cosmic X-ray astronomy, and highlight the particular concerns which are being addressed for X-ray instruments, as distinct from optical applications.

CCDs were rapidly developed after their invention and were being considered as candidate detectors for the Large Space Telescope as early as 1972 [Sequin]. Ground-based astronomers benefitted enormously from the developments funded through this program and from the manufacturers' own in-house programs to develop CCDs as solid state replacements for broadcast standard TV tubes. Fortuitously, some of these devices were able to operate in standard commercial applications and also at the low light levels and cryogenic temperatures required for astronomy.

For over a decade CCDs have been performing useful scientific observations at ground-based observatories, where their high dynamic range, linearity and sensitivity to low surface brightness features have made them the sensor of choice for a wide range of observations. In space-borne applications, they have been used on the Giotto mission to observe Halley's comet [Keller et al], the Galileo planetary mission [Belton et al], and of course the Hubble Space Telescope [Westphal et al].

The first sounding rocket payloads for X-ray astronomy which utilized CCDs have recently been launched [Burrows et al]. Meanwhile, a number of major X-ray astronomy missions have been proposed, or are under development, for which CCDs have been chosen as the focal plane detector. The first of these was the Japanese-US ASCA mission, which was the first cosmic X-ray astronomy satellite to fly CCD detectors. ASCA was launched in early 1993, and two of its four X-ray telescopes were equipped with CCD focal planes. The scientific objectives of ASCA required relatively high spectral resolution (E/ E 10 - 50) over a broad energy range (0.3 - 10 keV), with good throughput at the iron features near 7 keV. The CCD detectors were optimized to maximize spectral resolution and high-energy quantum efficiency. The angular resolution of this mission was limited by its conical foil mirrors to about 2 arcmin (half-power diameter).

The JET-X (Joint European X-ray Telescope) instrument is expected to be launched in 1997, and will utilize CCDs at the focal plane of two co-aligned telescopes [Wells and Lumb]. JET-X will offer higher angular resolution (10 - 20 arcsecs), but with a lower collecting area compared with ASCA. The scientific goals emphasize the source detectability via the improved imaging capability. In the case of ASCA, the high spatial resolution of the CCDs were not exploited to the full, but their excellent energy resolution was a key parameter. Conversely, for JET-X, the improved angular resolution is obtained at the expense of lower throughput, and the performance features which affect imaging properties have received greater attention in the instrument design.

Somewhat farther in the future, the two world-class observatories, NASA's Advanced X-ray Astrophysics Facility (AXAF) and ESA's X-ray Multi-Mirror Mission (XMM), will exhibit a similar complementarity. AXAF will have a diverse range of instruments, including imagers exploiting the very high angular resolution properties of the AXAF mirrors, whilst the XMM observatory has been designed with an emphasis on spectroscopic observations. The AXAF CCD Imaging Spectrometer (ACIS) experiment [Garmire et al] will combine high angular resolution imaging with good energy resolution, and provide a dedicated array of CCDs in the same assembly for readout of transmission grating spectra. XMM will deploy 3 EPIC (European Photon Imaging Camera) CCD focal plane cameras behind its three mirror systems for moderate angular resolution, non-dispersive spectroscopy [Wells and Lumb]. XMM also has reflection grating spectrometers fixed behind 2 mirrors, and these will utilise dedicated CCD instruments as their readout element [Brinkmann et al].

It can be seen that different instruments place different demands on the CCD attributes. We will demonstrate that these requirements may conflict in some cases. Therefore detailed trade-off studies must be made in each case. This work explores some of these issues, and reviews the development status of CCD research for X-ray astronomy applications.

In Section 2 we examine the the mode of operation of CCDs, contrasting the requirements of X-ray and optical applications. Some of the key developments of in the attempts to optimize CCDs for X-ray astronomy are discussed in Section 3, where we also note recent examples of performance data obtained.