What is HERO?

Focusing optics have revolutionized X-ray astronomy at lower energies (below 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 order 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.  This approach is being pursued by many groups.

A second approach, adopted here, is to accept the shallow 'graze' angle of conventional optics and to nest many mirrors to build up collecting area.  It should be noted here 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.

This HERO payload is scheduled for flight in Spring 2005, with 6-m-focal-length optics. We have completed the first engineering/test flight in Spring 2001 of a refurbished gondola carrying two mirror modules containing 3-m-focal-length X-ray optics, with 30 arcsec resolution, built specifically to demonstrate the mirror technology and to check out the aspect system in a realistic manner.

Although these optics had only a few cm² of collecting area up to 50 keV, the telescopes had sufficient sensitivity to image bright cosmic X-ray sources. Here is the Spring 2001 balloon flight news and pictures and an accompanying NASA Science News article.

Here is a previous (c. 1999) NASA Science News article with additional information and pictures.

THE OPTICS FABRICATION PROCESS

Click on the tumbnails for a legible gif