1.

INTRODUCTION

The X-ray optics is a crucial part of most X-ray imaging instruments. The X-ray telescopes with optics achieve much better signal/noise ratio than X-ray experiments without optics - this allows e.g. the detection of faint sources. The use of X-ray optics further allows imaging, precice localization, photometry, spectroscopy, variability studies, and estimation of physical parameters of X-ray emitting regions (temperature, electron density…). The space experiments with X-ray optics are also well suited for monitoring of X-ray sky for variable and transient objects including X-ray novae, X-ray transients, X-ray flares on stars and AGNs, galactic bulge sources, X-ray binaries, SGRs (Soft Gamma Ray Repeaters) and X-ray afterglows of GRBs (Gamma Ray Bursts). The X-ray optics represent an important part of numerous past, recent, and future space projects (EXOSAT, ROSAT, Einstein, Fobos, AXAF, XMM, ABRIXAS, BeppoSAX, ASCA…). In the laboratory, there are numerous applications of the X-ray optics e.g. in plasma physics, laser plasma, biology, crystallography etc. Most of the laboratory applications require superior imaging quality.

2.

THE X-RAY OPTICS

The basic division of X-ray optics can be summarized as follows: (1) diffractive optics (Fresnel lenses + pinholes): not useful for space due to small apertures, (2) refractive optics: limited use, not useful for space, (3) reflective optics, based on total reflection (grazing incidence), and (4) reflective optics, based on normal incidence (multilayers). In this paper, we will focus on the reflective optics with the most widely used types listed below.

Wolter optics: The Wolter type optics [Wolt 52] is in wide use in space-borne X-ray astronomical experiments (ROSAT, EXOSAT, Salyut 7, Fobos, XMM, AXAF..). It is based on double reflection on two surfaces (requirement for a superior imaging – the Abbe sine condition). Depending on the combination of surfaces, there are various modifications (Wolter I, II, III types), mostly used is the Wolter I type (paraboloid + hyperboloid).

Fig. 1:

KORONAS X-ray Wolter mirror, 1979

Double-cone approximation of Wolter optics: The Wolter arrangement surfaces can be approximated by less laborious and hence less expensive conical profiles. These mirrors have lower imaging quality but in many cases larger collecting area since the conical shells can be made very thin. These mirrors usually represent high throughput systems, and can be preferably used in foil telescopes (SODART).

Conical, ellipsoidal and paraboloidal optics: These mirrors with only one reflection find most of their applications in the laboratotry either as collimating or focussing and/or imaging elements.

Kirkpatrick-Baez optics: This configuration is used mostly for experiments not requiring large collecting area (solar, laboratory). Recently, large modules of KB mirrors have been suggested also for stellar X-ray experiments [Gore 98].

Normal incidence optics: This optics requires the multilayer deposition on the mirror surface allowing its use under normal incidence. The use of multilayers results in narrow spectral range, and in the past has been mostly used in solar experiments (TEREK Fobos, TEREK KORONAS…) as well as in laboratory applications.

Lobster-eye optics: This optics has been suggested in 70ies for very wide field X-ray imaging but not yet used in space mostly due to severe manufacturing problems. Recently, the first test modules are available for both the Schmidt as well as for the alternative Angel configurations (Inneman et al., 1999).

The development and manufacture of various types of X-ray optics are heavily affected by manufacturing problems, making their production laborious and hence expensive. The most dominating problems can be summarized as follows: (1) the required microroughness is < 3 nm, in many applications however < 1 nm, (2) the X-ray optics is frequently represented by hollow inner surface, and (3) there are very strict requirements on the slope errors and shape deviations.

3.

X-RAY OPTICS IN THE CZECH REPUBLIC

The early stages of the X-ray optics developments in the Czech Republic are closely related to the INTERKOSMOS Space Programme (Soviet and East European equivalent of ESA operated until 1989). All of the X-ray imaging telescopes onboard Soviet spacecrafts were equipped with the Czech X-ray optics (exception: X-ray normal incidence mirrors in the special channel of the TEREK telescopes onboard the Fobos and Koronas spacecrafts). Later on, also the laboratory applications have started. In almost all cases, replicated grazing incidence mirrors of various geometries, types and arrangements have been designed and developed. The replication technology proved to be powerful tool in solving numerous and various different problems and demands.

Total number of X-ray mirrors produced: more than 50

Total number of mirrors flown in space: 8

Total spacecrafts with Czech X-ray optics: 4

Total number of space experiments with Czech X-ray optics onboard: 8

Fig. 2:

Mirror shells of the Salyut 7 X-ray mirror assembly, 1981

Fig. 3:

Mandrels of the double-nested X-ray Wolter 1 mirror, Salyut 7, aperture 240 mm

Fig. 4:

Fobos 1 X-ray mirror, 1988

Fig. 5:

TEREK X-ray telescope and coronagraph, Fobos 1 spacecraft, 1988

3.2

Replication

The idea of replica technology is to create a perfect copy of a negative shaped master and of smooth surfaces. This is very suitable if the negative shape is easier to be produced. The two main advantages of the replication in X-ray optics design and construction are as follows: (1) the negative shapes are usually easier to be produced, and (2) the produced shells may be much thinner than is possible with classical technologies (important for light-weight high throughput nested arrays). The replication by electroforming has been used in the Czech Republic for development and production of X-ray mirrors since 1967. Since then, the technology has been further developed and modified. A number of modifications exist, meeting different demands. The replication is perfectly suitable for the production of X-ray grazing incidence optics since external surfaces can be much easily grinded and polished than inner cavities (especially in the case of small apertures). The masters are of high quality optical glass, glass ceramics or metals. It should be noted that the replication technology in X-ray optics is applied by different groups in various different modifications.

Fig. 6:

Replicated grazing incidence X-ray optics

4.

RECENT AND FUTURE PROJECTS

The widely used Wolter objectives have a very limited FOV (typically of order of 1 degree or less) so they are suitable for pointed observations but not for monitoring and surveys. The wide-field X-ray optics has been suggested in 70ies by Schmidt (orthogonal stacks of reflectors) [Schm 75] and by Angel (array of square cells) [Ange 79] but has not been constructed yet. With this type of reflective X-ray optics, up to 180 deg FOV may be achieved. The difficult production of LE lenses has caused the significant delay between their theoretical description and their construction and real use. The possible solution is offered recently by the replication technology. Recently, the first Lobster-eye X-ray telescope prototypes have been successfully finished. The prototype of Schmidt geometry represents one module and consists of two perpendicular arrays of double-sided X-ray reflecting flats (36 and 42 double-sided flats 100 x 80 mm each). The flats are 0.3 mm thick and gold-coated. The focal distance is 400 mm from the midplane. The FOV of one module is about 6.5 degrees. More such modules may create an array with substantially larger FOV. Advanced Schmidt modules based on 0.1 mm glass gold-coated plates separated by only 0.3 mm have been also developed and tested.

For the Angel geometry, numerous square cells of very small size (about 1x1 mm or less at lengths of order of tens of mm) are to be produced. This is much more difficult than the Schmidt arrangement mentioned above. Nevertheless, the recent results indicate that this demand can be also solved by modified innovative replication technology [Inne 99]. Various test Angel LE modules have been produced so far such as a linear module with 47 cells of 2.5 x 2.5 mm 120 mm long, f = 1.3 m, and a L-shaped module with 2 x 18 cells, dimensions and f as above, surface microroughness 0.8 nm. An advanced LE Angel test module is in development as a 2D module with 96 x 96 = 9 216 cells 2.5 x 2.5 mm, 120 mm long, f = 1.3 m.

The LE X-ray telescopes are extremely important since the discovery of X-ray afterglows of Gamma Ray Burst (GRBs) sources in 1997. The expected rate of GRBs is 1 per day, however the theoretical prediction assumes larger beaming angle in X-rays if compared with gamma rays, hence the actual rate of X-ray afterglows is expected to be substantially larger (nearly 10 x or even more) than the rate of GRBs, hence about 10 X-ray afterglows are expected daily. The sensitivity of LE telescopes is sufficient enough to detect the recently discovered X-ray GRB afterglows. It should be also noted that the localization accuracy of the LE telescopes is of order of 1 arcmin, substantially exceeding the recent localization accuracy of most gamma ray instruments (2 deg and more). It is hence obvious that the LE telescopes are expected to provide a substantial contribution to the science and statistics of GRBs. The additional science of LE X-ray telescopes includes supernova explosions, high energy binary sources, AGNs, blazars, X-ray novae, X-ray transients etc.

The further activities involve the participation on the ESA XEUS project where first test mirror shell has been produced based on replication technology. Within this project, various innovative technologies will be studied and developed such as light-weight ceramics replication and the use of amorfous and glossy metals.

Fig. 7:

Lobster-Eye telescope prototype, advanced Schmidt arrangement (23 x 23 mm plates with 0.3 mm spacing).

5.

DISCUSSION

The (replicated) grazing incidence X-ray optics shows some advantages if compared with other types of X-ray imaging and focussing elements which can be summarized as follows [Hude 99]: (1) wide energy range (no narrow window), (2) large collecting area, (3) resistant to heat and other environment influences, (4) high reflectivity, over 90% depending on the grazing angle, and (5) wide field systems are possible. The grazing incidence X-ray optics is in use within space experiments already for 30 years. Despite of this, some innovative types of the X-ray mirrors are still in the development phase. Moreover, the requirements for future missions are much higher than for those in the past so new and innovative technologies are necessary. The future of astronomical grazing incidence X-ray optics can be briefly summarized as follows:

For laboratory applications, improved imaging quality and superior angular resolution (X-ray microscopes) are required.

Fig. 8:

Tapping AFM images of the sample of replicated surface taken in the test facility of the Astronomical Institute in Brera, Italy – the resulting microroughness RMS is 0.3 nm.

These needs require innovative technologies for the future: