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Our first mirror is modeled closely on the proven 40-cm ASTRO-E design. The focal length has been increased to 4.7 to 8 m, which increases the number of nested shells to 255, corresponding to 2040 foil substrates.
We have already measured the excellent imaging properties of our multilayered replicated foils in an ASTRO-E housing (see attached paper). While this might fall short of the 0.5-arcmin resolution desired for future missions, it will deliver order of magnitude improvements in sensitivity and spatial resolution over current hard X-ray space missions. GSFC and Nagoya are continuously improving the mirror production process with a goal of higher resolution. We have already procured a housing for our first mirror, but the smaller housings for the high energy mirrors have been delayed to incorporate anticipated advances in mirror technology. The current design also has significant advantages as a technology test bed. Individual foils can be replaced or entire quadrants of this mirror can be readily disassembled and reassembled for maximum flexibility in the early phase of this new effort.
Production runs of the foils for the first mirror will begin in May 1999 and we expect the first set to be complete by November. Half of these foils will be multilayered at Nagoya and the other half will be done at GSFC.
Fig. 1. ASTRO-E mirror.
Achievements of the Current Program
In the current SR&T cycle, we have constructed a demonstration telescope. Fig. 2 is the first and only focused hard X-ray image published by any group. It represents first light on a new generation of telescopes. Although this image was made with only 20 foil pairs, it shows all the technology steps necessary for the flight instrument, including successful multilayering and CZT focal plane. Please see the final report and attached paper for more details.
Design of the Flight Telescopes
Multilayering allows a large variety of strategies to optimize the mirror performance. Multilayers can be applied to a variety of substrates and can be made from a variety of materials, and the thickness of the layers can be varied in many ways to broaden the bandpass. We have chosen to use the maximum contrast in Z between the layers to minimize the number of layers required to achieve the desired reflectivity and simplify the multilayering task. Instead of constructing a set of identical mirrors covering the entire bandpass, we chose to cover it using non-overlapping ranges in individual mirrors. This also simplifies multilayer fabrication by allowing us to tune the multilayers in each mirror to a specific band. Additionally, the overall background in each detector is reduced to that within its specific bandpass. The result is a sensitivity nearly equal to broadband multi-telescope systems in an instrument much easier to construct and much less susceptible to thermal and mechanical distortions due to the applied multilayers.While the design of the flight reflectors is based on the demonstration model, the multilayer parameters must be optimized for different radius reflectors in each mirror. Simple scaling laws cannot be used to optimize this design because of the complex structure of the multilayers. For instance at small radii, the reflectance is dominated by the first monolayer. Detailed ray tracing has been performed and the team has agreed on the strategies to be used for the first mirror. These refined designs produce the essentially the same effective area as was proposed in the original proposal.
Fig. 2. Historic first focal plane image of a hard X-ray multilayer focusing telescope.
Description of Foil Fabrication and Multilayer Deposition Techniques
InFOCmS uses the conical mirror design (an approximatin to the Wolter I geometry) invented at GSFC and used on BBXRT, ASCA and ASTRO-E (Serlemitsos 1988; Serlemitsos & Soong 1995). The substrates are conical segments made of ~0.15 thick Al foil, nested to maximize the effective area. The reflecting surface needed for efficient X-ray reflection is applied using epoxy replication off a microscopically smooth mandrel. Multilayers can be applied to the substrates in either of two ways: i.) The 30 or so Pt/C bilayers may be deposited in succession on top of the Au or Pt surface of a reflector which has previously gone through the replication sequence (Serlemitsos et al. 1997, Tamura et al. 1997). ii.) The Pt/C multilayer may be directly deposited instead onto a mandrel in place of the Au or Pt, and then directly replicated onto a substrate. Because multilayer replication facilities exist only at Goddard, we will utilize both these processes as we attempt to share the workload between the two institutions.
Preparation of Flight Mirror
The housing fabrication is complete. Production of the foils will began within a few weeks. The current ASTRO-E production rate is 30 foils/day (flight ready including all yield factors). Nagoya will be able to multilayer 10 foils/day; it will take them 4.5 months to finish their 1020 reflectors. Since we have incorporated the multilayer coating into the production process, the GSFC reflector production rate will be faster.
During this 3 year cycle we will be developing the multilayer capability for the high energy mirrors. In our original proposal, we outlined a strategy for three 30-cm diameter high energy mirrors to cover the range from 65 to 80 keV in 5 keV steps. The innermost 100 reflectors used 400 Ni/C layers for high energy response and the outer reflectors used 300 Pt/C layers to achieve 150-cm2 effective area for each of the 3 mirrors. We delayed the fabrication of the high energy mirror housings because of budget constraints and our expectation that innovations will be introduced into foil mirror construction that lead to higher angular resolution and lower cost. This is the topic of a different proposal. Before constructing the high energy mirrors, in the second year of this effort, we will optimize their design, incorporating structural and fabrication improvements, as well as improvements in multilayer deposition. We are confident that we will be able to deliver at least the level of performance in the original proposal. We will also study alternative mirror configurations that allow us to cover the gap in the current design between 40 and 65 keV. It would be possible to cover this gap with one or two additional mirrors which could be accommodated in an expanded balloon gondola without seriously violating any of the current constraints of the balloon program.
To see the literature references click here.