- Front Matter: Volume 8861
- Optics for Indirect Gamma-Ray Measurement
- LAUE Lenses I
- LAUE Lenses II
- Telescopes (Flight)
- Telescopes (Development)
- Mirror Technologies
- Alignment and Assembly
- Coatings
- Spectrographs
- Design and Analyses
- Test Methods
- Novel Optics
The next generation of Imaging Atmospheric Cherenkov Telescope will explore the uppermost end of the Very High Energy domain up to about few hundreds of TeV with unprecedented sensitivity, angular resolution and imaging quality.
To this end, the Italian National Institute of Astrophysics (INAF) is currently developing a scientific and technological telescope prototype for the implementation of the Cherenkov Telescope Array (CTA) observatory. The Italian ASTRI program foresees the full design, development, installation and calibration of a Small Size 4-meter class Telescope, adopting an aplanatic, wide-field, double-reflection optical layout in a Schwarzschild-Couder configuration.
In this paper we discuss about the technological solutions adopted for the telescope and for the mirrors. In particular we focus on the structural and electro-mechanical design of the telescope, now under fabrication. The results on the optical performance derived from mirror prototypes are here described, too.
Cosine has developed the technology to bend and directly bond Si mirror plates in order to produce stiff, lightweight Xray optics which are used for large area space based X-ray telescopes. This technology, Silicon Pore Optics (SPO), also allows us to produce other types of high energy optics. Here we present the latest developments in the design and manufacture of a new generation of soft gamma-ray Laue lenses made using SPO technology named Silicon Laue lens Components: SiLC.
The bending and bonding of 300 μm thin Si single crystals allows us to fabricate a single crystal with radially curved crystal planes, which strongly improves the focusing properties of a Laue lens. The size of the focal spot is no longer determined by the size of the individual single crystals, but by the accuracy of the applied curvature, which is as low as a few seconds of arc. Furthermore, a wedge is incorporated in each individual Si crystal to ensure that all crystals are confocal in the radial direction. A secondary curvature in the axial direction can be used to improve the reflectivity of each crystal, and increase the reflected energy bandwidth.
We present the first SiLC crystals which will be manufactured in the fall of 2013. These are technology demonstrators designed for 125 keV radiation, 3.4m focal length and 600mm2 frontal area. The first measurements at synchrotron radiation facilities are planned for November 2013. With these first prototype lenses we want to demonstrate that the SPO stacking technology can be successfully applied to non-ribbed Si wafer plates and subsequently demonstrate the correct focusing in Laue geometry of both the wedges and radial curvature.
ASTRO-H is an astrophysics satellite dedicated for non-dispersive X-ray spectroscopic study on selective celestial X-ray sources. Among the onboard instruments there are four Wolter-I X-ray mirrors of their reflectors’ figure in conical approximation. Two of the four are soft X-ray mirrors1, of which the energy range is from a few hundred eV to 15 keV. The focal point instruments will be a calorimeter (SXS) and a CCD camera (SXI), respectively. The mirrors were in quadrant configuration with photons being reflected consecutively in the primary and secondary stage before landing on the focal plane of 5.6 m away from the interface between the two stages. The reflectors of the mirror are made of heat-formed aluminum substrate of the thickness gauged of 152 μm, 229 μm, and 305 μm of the alloy 5052 H-19, followed by epoxy replication on gold-sputtered smooth Pyrex cylindrical mandrels to acquire the X-ray reflective surface. The epoxy layer is 10 μm nominal and surface gold layer of 0.2 μm. Improvements on angular response over its predecessors, e.g. Astro-E1/Suzaku mirrors, come from error reduction on the figure, the roundness, and the grazing angle/radius mismatching of the reflecting surface, and tighter specs and mechanical strength on supporting structure to reduce the reflector positioning and the assembly errors.
Each soft x-ray telescope (SXT), FM1 or FM2, were integrated from four independent quadrants of mirrors. The stray-light baffles, in quadrant configuration, were mounted onto the integrated mirror. Thermal control units were attached to the perimeter of the integrated mirror to keep the mirror within operating temperature in space. The completed instrument went through a series of optical alignment, thus made the quadrant images confocal and their optical axes in parallel to achieve highest throughput possible. Environmental tests were carried out, and optical quality of the telescopes has been confirmed. The optical and x-ray calibrations also include: angular resolution, effective area in the energy range of ~ 0.4 – 12keV, off-axis response, etc. Some of those are being carried out by our counterpart at JAXA/ISAS, Japan. We report the calibration results of the FM1 and FM2 that were obtained at Goddard Space Flight Center.
Future high energy astrophysics missions will require high performance novel X-ray optics to explore the Universe beyond the limits of the currently operating Chandra and Newton observatories. Innovative optics technologies are therefore being developed and matured by the European Space Agency (ESA) in collaboration with research institutions and industry, enabling leading-edge future science missions.
Silicon Pore Optics (SPO) [1 to 21] and Slumped Glass Optics (SGO) [22 to 29] are lightweight high performance X-ray optics technologies being developed in Europe, driven by applications in observatory class high energy astrophysics missions, aiming at angular resolutions of 5” and providing effective areas of one or more square meters at a few keV.
This paper reports on the development activities led by ESA, and the status of the SPO and SGO technologies, including progress on high performance multilayer reflective coatings [30 to 35]. In addition, the progress with the X-ray test facilities and associated beam-lines is discussed [36].
Silicon Pore Optics is an enabling technology for future L- and M-class astrophysics X-ray missions, which require high angular resolution (~5 arc seconds) and large effective area (1 to 2 m2 at a few keV). The technology exploits the high-quality of super-polished 300 mm silicon wafers and the associated industrial mass production processes, which are readily available in the semiconductor industry. The plan-parallel wafers have a surface roughness better than 0.1 nm rms and are diced, structured, wedged, coated, bent and stacked to form modular Silicon Pore Optics, which can be grouped into a larger optic. The modules are assembled from silicon alone, with all the mechanical advantages, and form an intrinsically stiff pore structure.
The optics design was initially based on long (25 to 50 m) focal length X-ray telescopes, which could achieve several arc second angular resolution by curving the silicon mirror in only one direction (conical approximation).
Recently shorter focal length missions (10 to 20 m) have been discussed, for which we started to develop Silicon Pore Optics having a secondary curvature in the mirror, allowing the production of Wolter-I type optics, which are on axis aberration-free.
In this paper we will present the new manufacturing process, the results achieved and the lessons learned.
X-ray optics is an essential component of every conceivable future x-ray observatory. Its astronomical utility is measured with two quantities: angular resolution and photon collecting area. The angular resolution determines the quality of its images and the photon collecting area determines the faintest sources it is capable of detecting and studying. Since it must be space-borne, the resources necessary to realize an x-ray mirror assembly, such as mass and volume, are at a premium. In this paper we report on a technology development program designed to advance four metrics that measure the capability of an x-ray mirror technology: (1) angular resolution, (2) mass per unit photon collecting area, (3) volume per unit photon collecting area, and (4) production cost per unit photon collecting area.
We have adopted two approaches. The first approach uses the thermal slumping of thin glass sheets. It has advantages in mass, volume, and cost. The objective for this approach is improving its angular resolution. As of August 2013, we have been able to consistently build and test with x-ray beams modules that contain three co-aligned Wolter-I parabolichyperbolic mirror pairs, achieving a point spread function (PSF) of 11 arc-second half-power diameter (HPD), to be compared with the 17 arc-seconds we reported last year. If gravity distortion during x-ray tests is removed, these images would have a resolution of 9 arc-seconds, meeting requirements for a 10 arc-second flight mirror assembly. These modules have been subjected to a series of vibration, acoustic, and thermal vacuum tests.
The second approach is polishing and light-weighting single crystal silicon, a material that is commercially available, inexpensive, and without internal stress. This approach has advantages in angular resolution, mass, and volume, and objective is reducing fabrication cost to make it financially feasible to fabricate the ~103 m2 mirror area that would be required for a future major x-ray observatory.
The overall objective of this technology program is to enable missions in the upcoming years with a 10 arc-second angular resolution, and missions with ~1 arc-second angular resolution in the 2020s.
Lightweight and high resolution optics are needed for future space-based X-ray telescopes to achieve advances in highenergy astrophysics. The Next Generation X-ray Optics (NGXO) team at NASA GSFC is nearing mission readiness for a 10 arc-second Half Power Diameter (HPD) slumped glass mirror technology while laying the groundwork for a future 1-2 arc-second technology based on polished silicon mirrors. Technology Development Modules (TDMs) have been designed, fabricated, integrated with mirrors segments, and extensively tested to demonstrate technology readiness. Tests include X-ray performance, thermal vacuum, acoustic load, and random vibration. The thermal vacuum and acoustic load environments have proven relatively benign, while the random vibration environment has proven challenging due to large input amplification at frequencies above 500 Hz. Epoxy selection, surface preparation, and larger bond area have increased bond strength while vibration isolation has decreased vibration amplification allowing for space launch requirements to be met in the near term.
The next generation of TDMs, which demonstrates a lightweight structure supporting more mirror segments, is currently being fabricated. Analysis predicts superior performance characteristics due to the use of E-60 Beryllium-Oxide Metal Matrix Composite material, with only a modest cost increase. These TDMs will be larger, lighter, stiffer, and stronger than the current generation.
Preliminary steps are being taken to enable mounting and testing of 1-2 arc-second mirror segments expected to be available in the future. A Vertical X-ray Test Facility (VXTF) will minimize mirror gravity distortion and allow for less constrained mirror mounts, such as fully kinematic mounts. Permanent kinematic mounting into a modified TDM has been demonstrated to achieve 2 arc-second level distortion free alignment.
Silicon Pore Optics (SPO) provide a high angular resolution with a low areal density as required for future X-ray telescopes for high energy astrophysics. We present progress in two areas of ESA’s SPO development activities: Stray light baffling and environmental qualification.
Residual stray light originating from off-axis sources or the sky background can be blocked by placing suitable baffles in front of the mirror modules. We developed two different mechanical implementations. The first uses longer, tapered mirror plates which improve the stray light rejection without the need of mounting additional parts to the modules or the telescope. The second method is based on placing a sieve plate in front of the optics. We compare both methods in terms of baffling performance using ray-tracing simulations and present test results of prototype mirror modules.
Any optics for space telescopes needs to be compliant with the harsh conditions of the launch and in-orbit operation. We present new work in improving the mechanical and thermal ruggedness of SPO mirror modules and show recent results of qualification level tests, including tests of modules with externally mounted sieve plate baffles.
Hartmann testing of x-ray telescopes is a simple test method to retrieve and analyze alignment errors and low-order circumferential errors of x-ray telescopes and their components. A narrow slit is scanned along the circumference of the telescope in front of the mirror and the centroids of the images are calculated. From the centroid data, alignment errors, radius variation errors, and cone-angle variation errors can be calculated. Mean cone-angle, mean radial height (average radius), and the focal length of the telescope can also be estimated if the centroid data is measured at multiple focal plane locations. This test is the only viable way of verifying the alignment of tightly nested x-ray telescopes.
In this paper we present the basic equations that are used in the analysis process. These equations can be applied to full circumference or segmented x-ray telescopes. We use the Optical Surface Analysis Code (OSAC) to model a segmented x-ray telescope and show that the derived equations and accompanying analysis retrieves the alignment errors and low order circumferential errors accurately.
The development and calibration of eROSITA mirror modules is supported by continuous measurements at the X-ray test facility PANTER. To obtain comparable measurement results after each new integration robust alignment procedures are needed to place the mirror module on the optical axis.
Here we present the different methods that we use to align eROSITA like mirror modules. One method uses the symmetry of single reflection images, another one is based on a symmetry of the intensity distribution, and the last one on the symmetry of the half energy width (HEW).
The characterization of large aperture (> 2 meters), long focal length (> 10 meters) X-ray mirrors for X-ray astronomy with synchrotron radiation poses signi cant problems related to the available space at synchrotron radiation facilities. Intrafocal pencil beam characterization of part of the optics is advantageous if its results can be shown to have predictive capabilities with respect to the full system.
In this paper we present the routine characterization of silicon pore optics at the X-ray Pencil Beam Facility of the Physikalisch-Technische Bundesanstalt, located at the synchrotron radiation facility BESSY II (Berlin, Germany). In particular we show how measurements taken in the standard beamline con guration (detector at ve meters from the optics) can e ectively be used to predict the optical performance of the optics at their design focal length by comparing data taken on 20-meter focal length Silicon Pore Optics unit in the 20-meter beamline con guration (available only for a few weeks every year) with extrapolated 5-meter measurements.
An open question in the measurement of X-ray optics for telescopes in space is what the point spread function (PSF) looks like in orbit and what is the focal length for an infinite source distance. In order to measure such a PSF, a parallel X-ray beam with a diameter of several centimeters to meters is needed.
For this purpose it is studied of how to collimate the X-rays using a zone plate. Furthermore, a configuration study is presented to characterize X-ray optics with such a collimated beam at the PANTER X-ray test facility. In particular, estimations for segmented optics for future X-ray missions such as ATHENA+ with a focal length of 10m to 20m are presented.