X-ray optics in Lobster Eye arrangement represent promising complementary device to narrow field X-ray optics in common use. We present briefly recent status of design, developments, and tests of X-ray optics including Lobster Eye modules developed and tested within recent space project

Hudec R.

Inneman A.

Maršíková V.

Nentvich O.

Pína L.

Urban M.

English

Digital Library of the Czech Technical University in Prague

Contrib. Astron. Obs. Skalnate´ Pleso 48, 456 – 465, (2018)Lobster Eye X–ray optics for astrophysics:Recent statusR.Hudec1,2, L. Pina3, V.Marsikova1, O.Nentvich1, M.Urban1 andA. Inneman11 Czech Technical University in Prague, Faculty of Electrical EngineeringTechnicka 2, Prague 166 27, Czech Republic(E-mail: hudecren@fel.cvut.cz)2 Engelhardt Astronomical observatory, Kazan Federal University,Kremlyovskaya street 18, 420008 Kazan, Russian Federation3 Czech Technical University in Prague, Faculty of Nuclear Sciences andPhysical Engineering, Brehova 7, Prague 115 19, Czech RepublicReceived: December 16, 2017; Accepted: March 8, 2018Abstract. X-ray optics in Lobster Eye arrangement represent promising com-plementary device to narrow field X–ray optics in common use. We presentbriefly recent status of design, developments, and tests of X–ray optics includ-ing Lobster Eye modules developed and tested within recent space project.Key words: X–ray optics – X–ray telescopes – X–ray astrophysics1. IntroductionX–ray optics in Lobster Eye (LE) arrangement represent promising complemen-tary device to narrow field X–ray optics in common use. The wide field of viewlobster telescope can e.g. detect and provide positions for fast, short lived tran-sients such as gamma–ray bursts and X–rays accompanying gravitational waves,while the narrow field of view Wolter telescope provides high resolution imagesbut can only point to known sources that have been discovered and positionedby other instruments such as the lobster telescope.We present review of the recent status of design, simulations, development,assembly and tests of Lobster Eye X–ray optics including the modules developedand tested within the recent space projects. Various LE test modules were sim-ulated, designed, assembled and tested at several X–ray test facilities. Selectedresults are briefly presented and discussed below.2. Wide–Field Lobster Eye X-Ray OpticsThe Lobster Eye (LE) optics mimic the arrangements of eyes of lobsters and wassuggested in the 1970s for a very wide field X-ray imaging but used in space onlyvery recently mostly due to severe manufacturing problems. Recently, advancedLobster Eye X–ray optics for astrophysics: Recent status 457modules are available for both the Schmidt (Schmidt, 1975) and the alternativeAngel (Angel, 1979) configurations, suitable for space applications.Images in the eye of a lobster are formed through reflections off the internalwalls of a lattice of small square-sided tubes arranged over the surface of asphere. This design can be used in the construction of a grazing incidence systemto focus X-rays.The X-ray objective based on the lobster eye was proposed by (Angel, 1979),and similar design of a wide-field lens by (Schmidt, 1975). Each small channelis aligned along the radius of a sphere. A ray reflected twice off adjacent wallsinside the channel is focused onto a spherical focal plane. Rays reflected onlyonce are focused to a line causing background images to appear as a taperedcross. Some rays go through the lens with no reflections, contributing to diffusebackground. The finite size of the tubes produces specific defocusing in theimage, while the angle subtended by each tube at the focus limits the resolutionof the system.Despite these drawbacks, the great advantage of this design is an almostunlimited field of (Gorenstein, 1979; Gorenstein et al., 1996). This makes itideal for use as an all-sky X–ray monitor. Up to 2017, no X–ray telescopes werelaunched using lobster-eye optics, mainly due to difficulty in manufacturing ofthe reflective tubes. However, improvements in multichannel plate technologyhas led to a proposal for a lobster-eye X-ray telescope could be placed on theInternational Space Station, and to an application on BeppiColombo (Fraseret al., 2010), and the alternative glass foil technology developed in the CzechRepublic has lead to the construction of several first LE telescopes (Innemanet al., 1999, 2000). These efforts in the Czech Republic have started soon afterannouncing the idea of lobster optics (Hudec et al., 1995). The first LE modulein space is the 1D LE optics on the VZLUSAT–1 minisatellite (Urban et al.,2017), see the dedicated section below.The wide-field mirror modules offer advantageous application in astrophysics.The major scientific achievements of the X–ray astronomy in the past are closelyrelated to the use of large X-ray imaging telescopes based mostly on the Wolter IX–ray objectives. These systems usually achieve excellent angular resolution asFigure 1.: LE 2D optics – composed of two 1D sub-modules, 2 reflections, energyrange optical to 10 keV.458 R.Hudec, L. Pina, V.Marsikova, O.Nentvich, M.Urban and A. InnemanFigure 2.: LE 1D optics – 1 reflection, energy range optical to 30 keV.well as very high sensitivity, but are quite limited in the field of view avail-able, which is less than 1 degree in most cases. However, the future of X–rayastronomy and astrophysics requires not only detailed observations of particu-lar triggers, but also precise and highly sensitive X–ray sky surveys, patrol andmonitoring. The wide field lobster eye telescopes are hence expected to play animportant role in future X–ray astrophysics missions and analyses.The 2D LE optical system is illustrated on Fig. 1. For some applicationswhere wider energy coverage (extension to harder X–rays) is necessary, the 1DLE system can be considered (Fig. 2). The imaging properties of these systemsin optical light are documented in Fig. 3.Figure 3.: LE system – test in visible light, left 1D, right 2D.3. SimulationsFor every Lobster Eye test module developed and assembled, ray–tracing sim-ulations were performed in order to simulate the ideal imaging performance(Fig. 4). Few selected results are given below.Lobster Eye X–ray optics for astrophysics: Recent status 4590 2 4 6 8 10 12 14x (mm)02468101214y (mm)0102030Counts (-)0 2 4 6 8 10 12 14x (mm)02468101214y (mm)0510152025Counts (-)0 2 4 6 8 10 12 14x (mm)02468101214y (mm)02004006008001000Counts (-)Figure 4.: REX LE optics simulation for source-detector distance 10 m at 8 keV,from left to right two 1D images and combined image.4. Lobster Eye X–ray optics on VZLUSAT–1The first LE X–ray optics in space is small Lobster Eye telescope onboardVZLUSAT–1. This cubesatellite represents example of 2U cubesatellite with ad-vanced astrophysical payload onboard (Da´niel et al., 2016). One from payloadsis represented by one dimensional (P´ına et al., 2016) miniature X–ray telescope(Pina et al., 2015) with Timepix detector in its focal plane (Baca et al., 2016).The main mission goal is technological verification of the system but scientificoutcome is also expected for bright celestial X–ray sources (Urban et al., 2017;Da´niel et al., 2016; Blazek et al., 2017). The satellite represents the 5th satellitewith Czech X–ray optics onboard.The 1D Lobster Eye module onboard VZLUSAT–1 has focal length of 250 mmand is composed of 116 wedges and 56 reflective double-sided gold-plated foils(thickness 145 microns) Input aperture is 29× 19 mm2, outer dimensions are60× 28× 31 mm3. Active part of the foils is 19 mm in width and 60 mm in lengthand the energy range is 3 to 20 keV. The detailed parameters are documentedin Tab. 1 and the pictures of the optics are in Fig. 5.Figure 5.: The 1D LE module for the VZLUSAT–1 minisatellite (left), the 1DLE optics in frame (right).460 R.Hudec, L. Pina, V.Marsikova, O.Nentvich, M.Urban and A. InnemanAt the time of writing, the VZLUSAT–1 X-ray payload was still in testingphase. The Timepix detector collects incident X–ray radiation on the satelliteand all the electronics and communications of this experiment work well. Inconsequence of a temporary malfunction the attitude determination and controlsystem (ADCS) and thus caused fast swinging of satellite, it is necessary tocapture images with short exposure time. Obtaining images of the source inspace is thus distinctively more difficult and statistically improbable. The firstimage was taken by Timepix on July 13, 2017 and were made more than 20 000orbital measurements since launch. The results from the satellite operationalphase will be presented in consecutive publications.Table 1.: Parameters of the 1D LE optics for cubesat experiment VZLUSAT–1Properties ValueAperture 29× 19 mm2FOV 3.3 × 2 degreesFocal length 250 mmRadius of convergence 500 mmDimensions of foils 60× 25× 0.145 mm3Number of foils 56Spacing 0.3 mmReflective surface AuEffective area @ 8 keV and 80 % reflectivity ∼ 0.5 cm2Effective area @ 10 keV and 50 % reflectivity ∼ 0.3 cm2Angular resolution 2 arcminDetector TimepixDetection material 300µm Si detectorDetector resolution 256× 256 pixelsPixel size 55µm5. Lobster Eye X–ray optics for rocket experimentAnother LE flight module was designed and manufactured for REX rocketexperiment. The goal of this X-ray telescope is the Vela nebula observationduring a sounding rocket flight. The Water Recovery X–ray Rocket (WRX–R) experiment is organized by the Pennsylvania State University (PSU), USAwith a primary payload of a soft X-ray spectroscope. The Czech team developeda hard X-ray Lobster-eye telescope REX (see Fig. 6; left) as a secondary pay-load. The Czech experiments astrophysical object of study is the Vela pulsarin the centre of the Vela nebula.The discussed optics for REX rocket experiment is two dimensional (2D)type. The LE module consists of two sub-modules addressed as Horizontal andLobster Eye X–ray optics for astrophysics: Recent status 461Table 2.: Parameters of the LE optics for rocket experiment REXProperties ValueAperture 54× 54 mm2FOV 1.3×1.6 degreesFocal length 1 065 mmRadius of convergence 2 130 mmDimensions of foils 150× 75× 0.35 mm3Number of foils in one sub–module 47Spacing 0.75 mmReflective surface AuEffective area @ 8 keV and 80 % reflectivity ∼ 1.4 cm2Effective area @ 10 keV and 50 % reflectivity ∼ 0.5 cm2Angular resolution 1.1× 1.4 arcminDetector TimepixDetection material 300µm Si detectorDetector resolution 256× 256 pixelsPixel size 55µmVertical in our case. The Horizontal sub-module is the one with longer focallength, closer to the observed object, which has mirrors oriented horizontally.The Vertical sub-module is placed closer to the camera, with mirrors placedvertically. The final optical system has focal length of 1065 mm and is com-posed of 114 wedges and 47 reflective single-sided gold-plated foils (thickness350 microns) in each 1D module. Input aperture is 54× 54 mm2, outer dimen-sions of one 1D module are 80× 80× 170 mm3 and the active part of the foilsis 54 mm in width and 150 mm in length. Total energy range is 1 to 15 keV overthis energy range it work as soller slit. The main parameters of LE sub-modulesare listed in Table 2 and a few selected measurement results are listed in nextsections. For more information about the rocket experiment see (Da´niel et al.,2017; Stehlikova et al., 2017).6. X–ray tests at Boulder, Pennstate, Iowa and PragueSeveral Lobster Eye modules were assembled and prepared for X–ray tests atthe US and Czech test facilities. These LE modules were tested in vacuumchamber at CASA (University of Colorado at Boulder), taking into account theexperimental arrangement of the test facility (focus to focus), hence these LEtest modules were of focus to focus design, with source to optics distance: 10 m,optics to detector distance: 8 m, module position adjustment in visible light(Xe lamp), and a MCP detector as X–ray focal plane detector, with diameterof 1 inch.462 R.Hudec, L. Pina, V.Marsikova, O.Nentvich, M.Urban and A. InnemanFurther X-ray tests were performed at X-ray test facilities in Iowa, Pennstate,and Prague, few selected results are illustrated below. For Pennstate tests,Timepix detector (Jakubek et al., 2014) was used to record the images.The experimental setup at Pennstate test facility is in Fig. 6, examples ofobtained results in Fig. 7. The comparison between simulation and experimentalresults for Prague measurement of VZLUSAT–1 1D module with used setupschema in Fig. 8 is in Fig. 9.7. X–ray tests of LE module at PANTERThe full illumination X–ray tests of the LE module designed and assembledfor the REX rocket experiment were performed at the Max Planck Institutefor Extraterrestrial Physics PANTER facility, Germany in 2017. A few selectedpreliminary results are shown in Fig. 10. More details on these tests (includingother types of X–ray optics) are given in another paper in this Volume (Pinaet al., 2018).8. ConclusionsVarious LE X–ray optics modules were assembled and tested, based either onglass foils or high–quality silicon wafers, all of them in multi–foil arrangement,i.e. as stacked modules with flat X–ray reflecting substrates. These modules weretested at several X–ray test facilities, with very promising results, with around1–2 arcmin FWHM in the best cases.The LE X–ray optics represent promising alternative to widely used Woltermirrors, mainly because of much larger FOV albeit at the cost of reduced angularresolution. The LE optics is suitable for use in wide–field X–ray monitors withvery wide coverage of various astrophysical sources.The availability of high-quality novel materials such as superior silicon wafersand/or glass foils enables the design and construction of LE X–ray optical sys-tems with improved angular resolution for various applications both in space,Figure 6.: X–ray tests at the Pennstate X-ray test facility. Left the REX LEmodule, right the Widepix detector used to record the focal images.Lobster Eye X–ray optics for astrophysics: Recent status 4630 2 4 6 8 10 12 14x (mm)050100150200Counts (-)0 2 4 6 8 10 12 14y (mm)050100150200Counts (-)0 2 4 6 8 101214x (mm)02468101214y (mm)050100150Counts (-)Figure 7.: X-ray tests of the REX LE module at the Pennstate X–ray test facility.The measured FWHM was 1.7 arcmin horizontal and 1.8 arcmin vertical.Figure 8.: Hard X–ray tests at RITE Prague, experimental setup. Cu Kα X-raytube was used as source and Timepix as focal detector. LE optics was placed inthe middle between source and detector.0 2 4 6 8 10 12 14x (mm)02468101214y (mm)05101520Counts (-)0 2 4 6 8 10 12 14x (mm)02468101214y (mm)0200040006000800010000Counts (-)Figure 9.: Experimental tests of 1D LE optics, hard X-rays 8 keV at X–raylaboratory RITE Pregue, left simulated image, right measured image. Source –detector distance was 600 mm, exposure time 0.4 s and maximum signal = 104,after summation 18× 105.astronomy, as well as in the laboratory. In the near future, we plan to continuewith assembling and testing LE modules with longer focal length and improved464 R.Hudec, L. Pina, V.Marsikova, O.Nentvich, M.Urban and A. Inneman0 2 4 6 8 10 12 14x (mm)02468101214y (mm)010203040Counts (-)Figure 10.: LE REX PANTER X–ray test results. 2D LE X–ray optics (REX)– image of the 2D focus at 8.04 keV for different focal distance. Best focusin vertical direction 1.21 mm FWHM (1.5 arcmin) and in horizontal direction0.94 mm (1.3 arcmin).performance, based on superior quality substrates.Acknowledgements. We thanks the teams at the X–ray test facilities at Boulder,Pennstate, Iowa and MPE PANTER for their assistance and help with measurementsof LE modules there.We acknowledge GA CR grants 13-33324S, 18-10088Y and AHEAD EU Horizon2020 project (Integrated Activities in the High Energy Astrophysics Domain), grantagreement n. 654215. This work was partly supported by the Grant Agency of theCzech Technical University in Prague, grant No. SGS18/186/OHK3/3T/13 and MEYSRVO 68407700 as well.ReferencesAngel, J. R. P., Lobster eyes as X-ray telescopes. 1979, Astrophys. 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Lobster Eye X-ray optics for astrophysics: Recent status

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Lobster Eye X-ray optics for astrophysics: Recent status

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