2 Background and context

At X-ray energies the present generation of X-ray instrumentation for astronomy depends on grazing angle reflection optics in a nested Wolter-I configuration. In this way the Chandra mission offers angular resolution of 0.5 arcsec (Jerius et al. 2000) and, with thinner, less precise, optics XMM-Newton obtains an effective area of more than 0.4 m² (Stockman et al. 1999). Both of these instruments are limited to photon energies below $\sim$10 keV.

X-ray missions actively under study, in particular Constellation-X (White & Tananbaum 1999) and Xeus (Bavdaz et al. 1999), will make possible considerable improvements in sensitivity and spectral resolution. They will not emphasise angular resolution, which will remain 10³-10⁶ times worse than the diffraction limit. At X-ray energies only interferometry, discussed below, offers the prospect of ultra-high angular resolution.

Moving to hard X-rays and gamma-rays, the sensitivity of the current generation of instruments - Compton-GRO and Integral - is very much poorer than that obtained in the X-ray band. This is largely because it is not currently possible to concentrate the flux as is done at lower energies with mirror optics. The imaging techniques, too, are indirect and have poor angular resolution - of the order 0.1-10$^\circ$.

Grazing incidence optics with multilayer coatings are being actively pursued by a number of groups for focussing energies up to about 100 keV (e.g. Yamashita et al. 1998; Ogasaka et al. 2000; Craig et al. 1998; Hussain et al. 1999; Mao et al. 1999). In principle such techniques can focus radiation of even higher energies, though the number of layers for efficient reflection would become very large and the grazing angles very small, so the area of coated surface for a given effective area would be huge. The angular resolution of multilayer mirrors depends on the precision of the surface of the substrate and of the coating. At present it is little better than one arc minute and there seems little prospect either of achieving large effective areas or of approaching the diffraction limit using this technology.

MAXIM is a mission currently under study that would use X-ray interferometry to allow images to be reconstructed with a resolution corresponding to a baseline of 1 m (pathfinder version) or even 100 m ("event horizon'' version). Using X-rays of ${\sim}1$ to 6 keV respectively this results in resolutions of 100 and 0.1 $\mu''$ (Cash et al. 2000). MAXIM can be regarded as a large modified Wolter III telescope with unfilled aperture in which the sampled parts of the aperture are approximated by individual plane surfaces. To get sub micro arcsecond resolution at keV energies requires an entrance aperture of hundreds of metres and the idea is that the primary mirror segments would be on 32 separate spacecraft and the secondary mirror segments and detector array on two more.

In the gamma-ray band a first step towards attaining flux concentration has recently been made through the use of Laue diffraction in Ge-Si crystals in a telescope for gamma-rays of 170 keV (Laporte et al. 1999) and this technology has been proposed for part of the HXT for the Constellation-X mission (Gorenstein et al. 1996). Concentric rings of carefully oriented crystals diffract the incoming flux towards a common focal point, each ring using a different set of crystal planes. This technique is expected to become important as a way of collecting flux onto a small detector but extremely high angular precision would only be obtained with very carefully aligned, highly perfect, crystals, that have an extremely narrow band-pass. Furthermore the approach does not lend itself to true imaging.

Other approaches to gamma-ray imaging for astronomy and their limitations have been reviewed by Skinner (2001b). None apart from the use of diffraction and/or refraction appear to offer the prospect of extremely high angular resolution and sensitivity. Purely refractive optics could be considered in some circumstances, but Yang (1993) has shown how absorption limits the effective area possible. Systems in which diffraction plays the major role turn out to be preferable and it is on such designs that we will concentrate here.

Diffractive lenses in the form of Fresnel Zone Plates (FZPs) were used in early solar X-ray imaging (Kraemer et al. 1978), but since then have not found application in high energy astronomy, due to their low efficiency and small apertures. Masks with FZP patterns cut into them have been proposed for astronomy both as simple coded masks (Mertz & Young 1961) and in pairs in a modulation collimator configuration in which Moiré fringes are recorded (Desai et al. 1998). Such systems, however, have no flux concentrating capability and are quite distinct from those advocated here. What is discussed here is the possible application to gamma-ray astronomical applications of true diffractive lenses with large effective area.