2.1

Electroformed foils

The electroforming replication technology has been successfully used to develop numerous X-ray optics mirrors for space telescopes (Hudec et al., 2000). This technique has been modified to produce smooth and flat foils from high quality flat masters (Hudec et al., 1991). Although originally developed for the SODART X-ray telescope, this is still a promising alternative technology even for future experiments. The developments have lead to further increase of the thickness uniformity of the foils electroformed from flat masters below 2 % as well as to further improvements of surface quality and other parameters. The common foil material is nickel but alternative materials and alloys are also possible. The additional layers including multilayers can be deposited on the reflecting surface to further improve the reflectivity and energy response.

2.2

Sandwich double-sided foils

Some applications, such as the Schmidt Lobster Eye Optics, require double-sided X-ray reflecting foils. This is a still relatively new approach since all the previous and most of the recent X-ray reflecting foils and flats were one-sided. The innovative techniques to develop such foils have been exploited in the US-Czech Science and Technology Program with promising results (Gorenstein et al., 1996, Inneman et al., 1999).

2.3

Glass foils

The volume density of glass is nearly four times less if compared with electroformed nickel layers. This is why we have carried out an extended study focused on applications of gold-coated glass foils with thickness between 0.03 mm and 1 mm. The glass foils may be used either as flats, or alternatively may be shaped or thermally slumped to achieve the required geometry. In spite of promising inherent properties of glass a lot of work is still waiting to be done in order to mature glass foil technologies to prove their potential and to show successful applications. The glass foils and flats were used e.g. in Lobster Eye X-ray telescope prototypes (with thickness of foils between 0.03 and 0.75 mm and sizes between 3 and 300 mm). The experience learned this way can be preferably applied also for the other types and arrangements of multi-foil X-ray optics.

2.4

Amorphous metals

Although mostly discussed and applied because of their unique magnetic properties, the amorphous (glossy) metals and alloys also exhibit excellent mechanical properties if compared with classical crystalline materials. The results of studies obtained so far (Hudec et al., 2001) indicate that the mechanical stiffness may be nearly 4 times better and hence nearly 4 times improvement may be expected in the weight reduction of the mirror assembly.

On the other hand, the application of this technique in the development of innovative X-ray optics is completely new and needs to be carefully tested and further exploited. From the three available technologies to produce amorphous metal alloys, only one seems to be suitable for X-ray mirror shells (Hudec et al., 2001). We have started study and technology developments in this direction with expected preliminary results in the year 2003.

2.5

Ceramics

The ceramics replication represents a promising alternative to nickel electroforming of X-ray mirror shells and segments (Citterio et al., 2002). The main advantage of the ceramics is a rich variety of suitable materials with volume densities typically between 2.0 and 3.0, and in some cases even less. The method however still needs a substantial effort and improvements (Hudec et al., 2001), especially in the minimizing the internal stresses, in the optimizing the material, and in the application of the reflecting layer. Moreover, there are different alternative methods of ceramics application such as plasma spraying, chemical and physical vapor deposition, which still require careful exploitement to select the optimized technique. The plasma spraying method can be also applied to replicate shells based on various alloys and metals including innovative light materials such as lithium (volume density 0.5).

Figure 2:

The focal plane image from the large (30 x 30 cm) LE 2 D module (0.75 mm thick glass plates, optical light).

2.6

Glossy carbon

We propose completely new innovative materials such as glossy carbon as possible alternative substrate/material for future large X-ray telescopes like the ESA project XEUS where the severe weight constraints exclude the classical recently widely used technologies and approaches. As far as we know, this material has been never discussed and studied before for this type of application.

The preparation methods of glass-like carbons include thermosetting resins, filler, blending, moulding, polymerisation/curing, machining, carbonisation and graphitisation. The preparation procedure for composite glass-like high-density carbons was developed relatively recently. Additions of filler material are quite effective to suppress shrinkage and also to release readily the evolved gases at the polymerisation and curing stages. While the preparation of thick, glass-like carbon products is still difficult, the thin layers and films are much easier to be produced. In the thin film glass carbon techniques, the resin is coated on to a silica-glass plate, cured, peeled, and carbonised, and, if necessary, it can be graphitized. It is obvious that this may be considered as one of promising completely new techniques to be exploited as a possible alternative for future large segmented X-ray mirrors/foil telescopes.

The structure of glass-like carbons has been studied over many years. Glass-like carbons are known to be typical, more or less, of isotropic, non-graphitizable carbons. The recent evolutions indicate that the structures of thermosetting resins can be effectively modified to control the structure of resultant glass-like carbon. This way, a glass like carbon with no porosity can be prepared from a chemically hydrophilic thermosetting resin (Marsch et al., 1997).

The glass-like carbons have bulk densities around 1.5 g cm^-3 (although they can be as small as 1.4 g cm^-3 and even 0.6 g cm^-3 if an extended porosity may be accepted) which are almost equal to those of the conventional synthetic graphite and lower than any previous material considered for future large area X-ray mirrors. The glossy carbons with high porosity can even reach bulk densities of 0.6 g cm^-3. The bending strength of glass like carbons amounts to 50-200 MPa, the Young‘s modulus to 20-32 GPa, and the C.T.E. amounts to about 1 x 10^-6 C^-1. Glass like carbons are hard materials as shown by their shore hardness of 100, and of 70-80 after graphitization. However, they have little mechanical shock resistance and belongs to typical fragile materials. This can be, on the other hand, affected by the selection of a suitable filler. They exhibit low self-lubricity and high abrasion resistance reflecting their special structures, compared with conventional graphite.

The applications of glass-like carbons have been rather limited for the past few dozens of years. It is just recently that they have attracted much more interest in terms of industrial applications. Among the parameters, the glass like carbons seems to be favourable because of their low density and low thermal expansion. The large-size composite glass-like carbon thin plates have been already successfully produced for fuel cell separator s (Marsch et al., 1997).

3.1

Foil telescopes

High throughput high collecting area X-ray telescopes can be formed by shaping of thin foils, mostly in double conical Wolter 1 geometry applications. High quality electroformed Nickel foils have been developed for the SODART space telescope (Hudec et al, 1991) confirming that the electroforming on high quality glass masters can results in high quality smooth (micro roughness typically 1 nm) nickel foils with thickness between 10 microns and 1 mm. Alternative less quality but also of less cost foils e.g. plastic foils, aluminium replicated foils or industrial aluminium foils have been also considered an/or used in space experiments with low requirements for the final angular resolution..

3.2

Lobster Eye telescopes

The Lobster Eye Wide Field X-ray telescopes in Schmidt arrangements are based on perpendicular arrays of double-sided X-ray reflecting flats. In the first prototypes developed and tested, double-sided reflecting flats produced by epoxy sandwich technology as well as gold coated glass foils have been used (Inneman et al., 1999, 2000). Recently, micro Schmidt lobster eye arrays with foils thickness as low as 30 microns have been developed and tested in order to confirm the capability of these systems to achieve fine angular resolutions of order of a few arc min. The thin foils are separated by 50 microns in these prototypes. On the other hand, large lobster eye systems with Schmidt geometry have been designed and constructed, achieving dimensions up to 30 x 30 x 60 cm. Their optical tests have confirmed the expected performance according to calculations (computer ray-tracing). The X-ray tests of the large LE modules are planned in collaboration with the Max Planck Institute of Extraterrestrial Physik in Garching, Germany at their X-ray test facility Panter in Neuried later this year. The calculations and the measurement results indicate that the lobster eye telescope based on multiarray of modules with thin and closely spaced glass foils (analogous to those already assembled and tested) can meet the requirements e.g. of the ESA ISS Lobster mission (including the angular resolution) and can hence represent an alternative to the recently suggested MCP technique.

3.3

Large segmented Wolter telescopes

The future large space based X-ray telescopes will require extremely large collecting area i.e. very light X-ray optics shells. The highly nested shells of Wolter 1 geometry must be of very low thickness and of low volume density in order to keep the weight of the whole assembly in still reasonable limits. Recently, we have designed and developed a test module array based on 30 x 30 cm glass foil plates 0.75 mm thick and parabolic shape of the surfaces. This module has been designed and constructed to exploit and to test the possible use of shaped glass foils as an alternative for the future ESA X-ray space mission XEUS. The parabolic shape of the particular shells has been obtained by bending of the foils, the alternative technology of thermal shaping of the glass foils is tested recently. X-ray and optical tests are planed for the 2. half of the year 2002.

Table 1:

LE Schmidt modules developed so far

3.4

K-B X-ray telescopes

The large modules of Kirkpatrick-Baez X-ray optics based on multiple and large flats in K-B geometry have been also suggested for future space missions (Gorenstein, 1998). The K-B modules are based on orthogonal stacks of thin reflectors, each reflector represents a parabola in one dimension.

The modular concepts of Schmidt LE modules, of the large segmented Wolter telescopes (such as XEUS), and of large segmented K-B telescopes are similar: all are based on either planparalel or curved flats and foils. This means that the development of high quality X-ray reflecting foils and flats with high mechanical stiffness and low volume density is extremely important for most of the future X-ray astronomy large aperture projects. The segmented K-B telescopes have the advantage of being highly modular on several levels. All segments are rectangular boxes with the same outer dimensions (Gorenstein, 1998).

Figure 3:

The micro LE Schmidt module (3 x 3 mm, 0.03 mm thick glass foils) in the holder.

4.1

General description

To be able to make some estimates on the LE performance, image quality, and scanning observation problems, we have developed a ray-tracing code matching our requirements.

The first version of our software, currently being operational, works with the set of ideally planar surfaces creating a LE optics. The surfaces has a reflectivity depending on the material and its surface quality. Fully 3-D ray-tracing Monte-Carlo simulations are then performed on such a system.

To verify that even such a simple (only planar surfaces) approach can provide us with the valuable results, we have compared the experiment and the simulation of the experiment for the 8 keV photons at point-to-point focusing X-ray LE-like optics. The model fits the experiment quite well (at least from general point of view).

There is more sophisticated version of the ray-tracing software under development now, which will be able to deal with the curved and distorted surfaces, as well as with the photon scattering on the micro roughness (angle spread function).

4.2

Simulated optics

We have performed a number of various LE designs (varying generally in the foil dimensions and the spacing between the foils). One of the promising designs has 78 mm x 23 mm x 0.1 mm foils and spacing between them 0.3 mm. The focal length is f = 375 mm. We have simulated the Schmidt LE consisting of these foils coated with the gold with the micro roughness σ = 1 nm.

Figure 4:

The experiment (left) and the simulation (right) of the 8 keV point-to-point focusing system. The gain defined as a ratio of the maximum value of the pixel and the value of the directly illuminated pixels is ~570for the experiment and ~600for the simulation. The FWHM of both peaks is also close to each other. The focal length of the system is f = 1.2 m, the pixel size is 24 μm.(Inneman et al. 2000)

The gain, defined as the ratio of the photons gathered inside the FWHM area with and without optics, ranges from ~ 2500 for softer photons (below 2.5 keV) to just several hundreds for harder photons (2.5 – 10 keV). The gain depends on the FWHM, which will be larger in the reality due to the foil distortions, but gain above ~1000 below 2.5 keV can be expected.

The FWHM of a central peak ranges from 2.5 to 4.5 arc min (270 – 480 μm). For most cases the FWHM was below slightly above 3.0 arc min. For real LE the FWHM will be larger, but comparing the model with the experiment show that the blurring will not be immense.

Figure 5:

Atot and Aeff for the described optics, please note that the front area of the optics is ~60 cm2.

Figure 6:

The gain defined as the ratio of the photons gathered inside the FWHM area with and without optic. Simulation of the LE with 78 mm x 23 mm x 0.1 mm foils and 0.3 mm spacing.

We adopt two definitions of the effective areas (Priedhorsky et al., 1996). First, A_tot corresponds with the amount of photons gathered from the single source overall the detector. Second, A_eff corresponds with the amount gathered inside the FWHM area. The goal is to minimize A_tot while maximizing A_eff, e. g. maximize the signal while minimizing the collateral impact on the whole image. A_eff is 2-3 cm² below 2 keV and ~0.3 cm² below 7.5 keV. Atot is ~ 3 times larger.