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What is HERO?


Focusing optics have revolutionized X-ray astronomy at energies up to about 10 keV. The Chandra X-ray Observatory (CXO) has approximately the same effective collecting area as some of the earliest non-focusing instruments flown on satellites, but has 5 orders of magnitude more sensitivity. This vast increase in sensitivity has so far eluded hard-X-ray astronomy, where large-area non-focusing instruments are still used.

The challenge is to make higher-energy-response mirrors with useful collecting area. As the angle at which X rays scatter drops with increasing energy, the sides of the mirror have to become ever shallower to maintain reflectivity. This means that the projected collecting area becomes ever smaller.

One way to improve collecting areas involves coating the mirror surface to enhance reflectivity through a constructive interference process. These so-called multilayer coatings, involving hundreds of layers of alternating material pairs of slowly varying thickness, provide useful reflectivity at angles several tines the cut-off for the pure gold or iridium coatings on 'conventional' X-ray mirrors.

A second approach, adopted by the X-ray astronomy group at MSFC, is to accept the shallow graze angle of conventional optics and to nest many mirrors to build up collecting area. It should be noted that large collecting areas, though highly desirable, are not necessary for groundbreaking science. The enormous reduction in background afforded by focusing optics means that, roughly, a few cm2 of telescope collecting area provides an equivalent sensitivity to current 1000 cm2 non-focusing systems, in a few hour observation.

We are therefore making use of our development work in light-weight high-resolution replicated optics to construct a hard-X-ray focusing payload for our high-altitude balloon program.& Termed HERO (for High-Energy Replicated Optics), the payload consists of an array of 8 mirror modules, each housing 12 nested shells, to give ~80 cm2 of collecting area at 60 keV, with an angular resolution of ~15 arc seconds. Below these mirror modules is a corresponding array of imaging detectors. tube

This HERO payload has flown several times as a balloon payload to demonstrate the mirror technology and to check out the aspect system in a realistic manner.

The original optics had only a few cm2 of collecting area up to 50 keV. Even these had sufficient sensitivity for imaging bright cosmic X-ray sources for the first time at these energies.


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