Wide-field X-ray Optics 
  René Hudec, Ladislav Pína, Adolf Inneman, Ladislav Pína 

The recent imaging X-ray telescopes have quite limited field of view of order of 1 degree or so. The development of wide-field X-ray telescopes with large field of view, reasonable angular resolution and high sensitivity can play an important role in future of X-ray astronomy and astrophysics. An important alternative is the Lobster eye X-ray optics theoretically described in the past but not yet constructed and used in a real experiment. We review the wide-field X-ray optics arrangements and discuss their preferences and drawbacks. We report on the design, development, manufacture and tests of first test X-ray objectives based on both Angel and Schmidt lobster eye geometry. We also suggest strategy for further developments in this area and discuss the scientific importance of space experiments based on lobster eye optics.


The major scientific achievements of the X-ray astronomy in the past are closely related to the use of large X-ray imaging telescopes based mostly on the Wolter 1 X-ray objectives. These systems usually achieve excellent angular resolution as well as very high sensitivity, but are quite limited in the field of view available, which is less than 1 degree in most cases. However, the future of X-ray astronomy and astrophysics requires not only detailed observations of particular triggers, but also precise and highly sensitive X-ray sky surveys, patrol and monitoring. The recently confirmed X-ray counterparts of Gamma Ray Bursters (GRBs) may serve as an excellent example.For recently in detail investigated GRB with precise localization accuracy, in almost all cases variable and/or fading X-ray counterparts/afterglows have been identified. The X-ray identification of GRBs has lead to great improvements in study and understanding of these sources and especially has allowed identifications at other wavelengths due to better localization accuracy provided in X-rays if compared with gamma ray observations. Since most of GRBs seem to be accompanied by X-ray emissions, the future systematic monitoring of these X-ray transients/afterglows is extremely important. However, these counterparts are faint in most cases, hence powerful wide field telescopes are needed. An obvious alternative seems to be the use of wide field X-ray optics allowing the signal/noise ratio to be increased if compared with non-focussing devices. The expected limiting sensitivity of Lobster eye telescopes is roughly 10-12 ergcm-2s-1 for daily observation in soft X-ray range1. This is consistent with the fluxes detected for X-ray afterglows of GRBs.


Fig. 1: The Schmidt lobster eye objective in the double-focusing arrangement. The optical test performance (top right, image area 8.6 x 6.5 mm) as well as the computer-ray tracing (below right, image area 10 x 10 mm) are also shown.

Furthermore, the wide field X-ray telescopes may play an important role in monitoring of faint variable X-ray sources to provide better statistics of such objects (note e.g. the occurrence of two faint fading X-ray sources inside the gamma ray box of GRB9706162) as well as in other fields of X-ray astrophysics. The recent hunting for faint fading X-ray afterglows of GRBs has indicated that there is a large number of faint and/or variable X-ray sources worth of detailed study.

We report on the further development of wide field of view lobster type X-ray telescope prototypes of both Schmidt and Angel geometry, with emphasis on the achievements obtained during the year 1999. Such modules are expected to allow large lobster eye telescopes to be constructed in the near future.

There have been many attempts to increase slightly the available wide field coverage of Wolter and analogous X-ray telescopes. To avoid any confusion, we suggest to restrict the term "wide-field X-ray optics" only for optical systems with field of view >> 1 degree, while to use the term "narrow-field system" for systems with FOV less then 1 degree.


The lobster-eye geometry X-ray optics offer an excellent opportunity to achieve very wide fields of view. One dimensional lobster-eye geometry was originally suggested by Schmidt3, based upon flat reflectors. The device consists of a set of flat reflecting surfaces. The plane reflectors are arranged in an uniform radial pattern around the perimeter of a cylinder of radius R. X-rays from a given direction are focussed to a line on the surface of a cylinder of radius R/2.
The azimuthal angle is determined directly from the centroid of the focused image. At glancing angle of X-rays of wavelength 1 nm and longer, this device can be used for the focusing of a sizable portion of an intercepted beam of X-ray incident in parallel. Focussing is not perfect and the image size is finite. On the other hand, this type of focusing device offers a wide field of view, up to maximum of 2p with the coded aperture. It appears practically possible to achieve an angular resolution of the order of one tenth of a degree or better.

Fig. 2 : The Schmidt objective double-focusing test module, 100 mm x 80 mm plates
Fig. 3: Optical tests of the Lobster eye Schmidt objective from the Figure 2.

Two such systems in sequence, with orthogonal stacks of reflectors, form a double-focusing device. Such device should offer a field of view of up to 1000 square degrees at moderate angular resolution.

It is obvious that this type of X-ray wide field telescopes could play an important role in future X-ray astrophysics. The innovative very wide field X-ray telescopes have been suggested based on these optical elements but have not been flown in space so far. One of the proposals is the All Sky Supernova and Transient Explorer (ASTRE)1. This proposal also includes a cylindrical coded aperture outside of the reflectors which provide angular resolution along the cylinder axis. The coded aperture contains circumferential open slits 1 mm wide in a pseudo-random pattern.

The line image is modulated along its length by the coded aperture. The image is cross-correlated with the coded aperture to determine the polar angle of one or more sources. The field of view of this system can be, in principle, up to 360 deg in the azimuthal direction and nearly 90 % of the solid angle in the polar direction. To create this mirror system, a development of double sided flats is necessary. There is also potential for possible extending the wide field imaging system to higher energy by the use of multilayer or other coatings in analogy to those described for flat reflectors in the Kirkpatrick-Baez geometry.

First Lobster-eye X-ray Schmidt telescope prototype consists of two perpendicular arrays of flats (36 and 42 double-sided flats 100 mm 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 is about 6.5 degrees.

Fig. 4: X-ray focal images of the Schmidt telescope prototype from the Fig. 2 at 1.5 nm taken in the test facility of the X-ray astronomy group, University of Leicester, UK.
Fig. 5: Intensity distribution in the focal plane (40x40 mm detector) according to mathematical ray-tracing for the identical test module and test arrangement as given above; the microroughness of reflecting surfaces is assumed to be 1 nm.

The results of optical and X-ray tests have indicated the performance close to those provided by mathematical modeling (ray-tracing). The X-ray test have been carried out in the test facility of the X-ray astronomy group, University of Leicester, UK. In 1999, another test modules of Schmidt geometry have been designed and developed, based on 0.1 mm thick gold coated glass plates 23 x 23 mm at 0.3 mm spacing. The aperture/length ratio is 80. One module is represented by 60 such plates. Two analogous modules represent the 2D test system for laboratory tests, providing focus to focus imaging with focal distances of 85 and 95 mm.

The innovative gold coating technique has improved the final surface microroughness rms to 0.2-0.5 nm. Various modifications of this arrangement have been designed and developed both for imaging sources at final distances (for laboratory tests) as well as of distant sources (the corresponding double-focusing array has f = 250 mm and FOV = 2.5 deg).

fig6 fig7
Fig. 6: Tapping AFM images of the surface of the double-sided flats taken in the test facility of the Astronomical Observatory in Brera, Italy - the resulting microroughness RMS is 0.3 nm. Fig. 7: Intensity distribution in the focal plane (15 x 15 mm detector) for the 23 x 23 mm Schmidt objective, mathematical ray-tracing for l = 1 nm and s = 1 nm.
fig8 fig9
Fig. 8, 9: The Schmidt objective test modules based on two sets of 60 23 x 23 mm flats, 0.1 mm thick (front views).
fig10 fig11
Fig. 10: The X-ray reflexivity measurements of 0.1 mm float glass plate (used in the 23 x 23 mm Schmidt objective). The reflecting surfaces were coated by Au/NiCr layers. Fig. 11: The X-ray reflexivity measurements for BOROFLOAT 1.1 mm thick flat sample (with no additional layers).
fig12 fig13
Fig. 12, 13: The Microsource X-ray test facility at Reflex, Prague (left) and the X-ray image of a 10 micron source at 8 keV detected by X-ray CCD camera (right) in the focal plane of the 23 x 23 mm Schmidt objective, the image area 2.5 x 2.5 mm.

Besides the Schmidt objectives described above, there is also an alternative based on slightly different arrangement, sometimes referred as two-dimensional lobster eye optics. The idea of two dimensional lobster-eye type wide-field X-ray optics was first mentioned by Angel5. The full lobster-eye optical grazing incidence X-ray objective consists of numerous tiny square cells located on the sphere and is similar to the reflective eyes of macruran crustaceans such as lobsters. The field of view can be made as large as desired, and good efficiency can be obtained for photon energies up to 10 keV. Spatial resolution of a few seconds of arc over the full field is possible, in principle, if very small reflecting cells can be fabricated.

The arrangement described above was however never been further developed because of difficulties with production of numerous polished square cells of very small size(about 1 x 1 mm or smaller at lengths of order of tens of mm).

The early feasibility studies have shown that this demand can be also solved by electroformed replication and first test cells as well as objective prototypes have been already successfully developed this way. The recent approach is based on electroforming and composite material technology to produce identical triangular segments with square cells while these segments will be aligned in quadrants onto a sphere.


Fig. 14: The schematic arrangement of the Angel lobster-eye objective (bottom) and the proposed triangular segments of this arrangement (top).

The 1st Angel telescope prototype developed consists of linear arrangement of 47 square cells of 2.5 x 2.5 mm, 120 mm long (i.e. length/size ratio of almost 50), with focal length of 1.3 m. The 2nd Angel telescope prototype is represented by an array of 6 x 6 i.e. 36 square cells, 2.5 x 2.5 mm each, 120 mm long, focus and length/size ratio as above. Both of these prototype modules have been produced already and are tested recently. The microroughness of the inner reflecting surfaces is better than 1 nm. The 3rd prototype has been finished in 1999, and consists of 2 x 18 perpendicular arranged cells 2.5 x 2.5 mm, 120 mm long. The 4rd prototype module is in development, with 96 x 96 = 9216 square cells with length and focus length as above.

An innovative technique of production of 120 x 120 mm sized modules consisting of large number of 3 x 3 mm cells, 120 mm long, is also under development.

fig15 fig16
Fig. 15, 16: The Angel lobster eye linear module with 47 cells 2.5 x 2.5 mm, 120 mm long
fig17 fig18
Fig. 17, 18: The Angel lobster eye test module, in L-shaped array of 2x18 cells, 2.5 x 2.5 mm, 120 mm long
fig19 fig20
Fig. 19 ,20: The frame with 23 x 23 mm flats for the Angel objective prototype after gold coating (left) and the assembled modules (right)

It is obvious that the first lobster eye prototypes confirm the feasibility to design these telescopes with currently available innovative technologies. We propose the following steps to be undertaken for a real wide field X-ray telescope: (1) To further reduce the microroughness as well as the slope errors of the reflecting surfaces in order to improve the angular resolution and the system reflectivity/efficiency. The 1999 development has already lead to significant microroughness improvement (to 0.2-0.5 nm) (2) To design and to construct larger or multiple modules in order to achieve larger fields of view (of order of 1000 square degrees and/or more) and enhanced collecting area (3) To further reduce the aperture of the cells (for the Angel arrangement) and/or spacing and plate thickness (Schmidt arrangement) and to enhance the length/aperture ratio (recently nearly 50-80), and (4) To study the multilayer application on reflecting surfaces and/or other approaches in order to improve the energy coverage for higher energies.


The application of very wide field X-ray imaging systems could be without doubts very valuable in many areas of X-ray and gamma-ray astrophysics. Results of analyses and simulations of lobster-eye X-ray telescopes have indicated that they will be able to monitor the X-ray sky at an unprecedented level of sensitivity, an order of magnitude better than any previous X-ray all-sky monitor. Limits as faint as 10-12 erg cm-2 s-1 for daily observation in soft X-ray range are expected to be achieved, allowing monitoring of all classes of X-ray sources, not only X-ray binaries, but also fainter classes such as AGNs, coronal sources, cataclysmic variables, as well as fast X-ray transients including gamma-ray bursts and the nearby Type II supernovae. For pointed observations, limits better than 10-14 erg sec-1cm-2 (0.5 to 3 keV) could be obtained, sufficient enough to detect X-ray afterglows to GRBs. As indicated by our preliminary results, the production of corresponding optical elements can be reasonably achieved by methods of electroforming and composite replication as an alternative to other methods6. For the Schmidt objectives, the results obtained with the development of technology for production of large area and high quality double-sided X-ray foils are very promising and together with composite material technologies represent an important input for the further development of this type of X-ray optics. The production of Angel lobster eye cells is much more complicated, nevertheless the first prototypes of the lobster eye Angel cells have been also successfully designed and developed.


The design and development of innovative X-ray telescopes is supported by the grant 105/99/1546 provided by the Grant Agency of the Czech Republic. We further acknowledge the support provided by the X-ray astronomy group of the University of Leicester, UK, in tests of the Schmidt telescope prototype and by the Astronomical Observatory Brera, Italy, in AFM surface measurements. We also acknowledge the support in the X-ray measurements provided by the ROTAN Laboratory, Physical Institute of the Academy of Sciences of the Czech Republic, and by the Reflex, Prague. The LE plates of Schmidt arrangement were gold-coated by the TTS, Prague. The Optical Development Workshop in Turnov has participated in the LE construction.


  • 1. P. Gorenstein: "All sky supernova and transient explorer (ASTRE)", in Variability of Galactic and Extragalactic X-ray Sources, A. Treves Ed. Associazione per L'Avanzamento dell'Astronomia, Milano-Bologna, 1987.
  • 2. J. Greiner, IAUC No. 6722, 1997.
  • 3. W. H. K. Schmidt, Nucl. Instr. And Methods 127, 285, 1975.
  • 4. K. D. Joensen, P. Gorenstein, J. Wood, F. E. Christensen and P. Hoghoj, SPIE Vol. 2279, 1994.
  • 5. J. R. P. Angel, Astroph. J. 233, 364, 1979.
  • 6. W. C. Priedhorsky, A. G. Peele. and G. A. Nugent, Mon. Not. R. Astron. Soc. 279, 733, 1996.


    Page generated: