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Design and testing of a low-resolution NIR spectrograph for the EXoplanet Climate Infrared TElescope
The mirror segments are mounted to the meta-shell using a novel four point semi-kinematic mount. The four point mount deterministically locates the segment in its most performance sensitive degrees of freedom. Extensive analysis has been performed to demonstrate the feasibility of the four point mount and meta-shell approach. A mathematical model of a meta-shell constructed with mirror segments bonded at four points and subject to launch loads has been developed to determine the optimal design parameters, namely bond size, mirror segment span, and number of layers per meta-shell. The parameters of an example 1.3 m diameter mirror assembly are given including the predicted effective area. To verify the mathematical model and support opto-mechanical analysis, a detailed finite element model of a meta-shell was created. Finite element analysis predicts low gravity distortion and low sensitivity to thermal gradients.
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.
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