The High Energy Focusing Telescope (HEFT)
HEFT Instrument Overview
instruments operating in the hard X-rays today (20–100 keV) fall into two
categories: coded aperture telescopes and collimated experiments. Detector
background, resulting from particle interactions
in the detector is the dominant factor in determining the minimum
detectable X-ray flux. For non-focusing experiments,
the detector area is approximately equal to the collecting area.
For focusing experiments, however,
a mirror concentrates the incoming flux onto a small spot on
the detector, greatly reducing background—making much fainter astronomical
How to focus at high energies: Present day soft X-ray focusing telescopes
rely on total external reflection to focus the X-rays.
X-rays are reflected from surfaces at very small incidence angles. The
critical graze angle (the maximum incidence angle at which reflection can
occur), is inversely proportional to the photon energy, so that reflecting
high-energy X-rays requires very small incidence angles. This
makes designing practical mirrors very difficult, unless some method is
used to increase the typical reflection angles.
To reflect high energy photons at angles greater than the critical graze
angle, HEFT uses multilayer coatings. A multilayer coating is a thin film
coating consisting of alternating layers of high and low density materials.
Constructive interference among small reflections at adjacent interfaces
produces high reflectivity at angles above the critical graze angle.
HEFT requires position-sensitive hard X-ray detectors with
< 1 mm spatial resolution, and good energy resolution. We are developing
room-temperature solid-state cadmium zinc telluride (CdZnTe) pixel detectors
with a custom low-noise ASIC readout. In these
detectors, the CdZnTe sensor has one monolithic contact, with the other
(anode) segmented into pixels. Each pixel contact is bump
bonded to a custom readout chip, with one complete pulse processing chain
for each pixel. The photo shows the HEFT 8 × 8 pixel prototype
HEFT is employing the conical-approximation to the Wolter-I geometry
used on the
experiments. We have developed new thermally-formed glass substrates for
the HEFT mirrors.
The glass was developed for flat panel displays, is available in thin sheets,
and is smooth on the relevent length scales. The mirrors shells are
made in quadrant section. The photo shows HEFT glass mirror
substrates before and after coating with depth-graded multilayers.
The HEFT optics are coated with depth-graded multilayers. Multilayers
coatings consist of alternating layers of high and low index of refraction
materials, and operate on the principle of Bragg reflection. Constructive
interference between reflections occurring at the interfaces between materials
provides reflectance at angles larger than can be achieved with single
metal coatings. The HEFT coatings have typically 500 layer
pairs, which vary in thickness between 20 Å for a pair,
and 200 Å per pair. The range in layer pair thickness in the coatings
provides reflectivity over a wide range of X-ray energies.
HEFT uses two different sets of material pairs to achieve response over
the broadest possible energy band. The multilayers are
coated using magnetron sputtering, in coating chambers specifically designed
for the purpose.