The trouble with gamma-rays is that they just go right through mirrors, so we use another technique called coded aperture imaging. Coded aperture is a new technology. In the 1990s and 2000s, virtually every gamma-ray instrument that’s flown uses this coded aperture technique.

Schematic of a coded aperture telescope.
Credit: B.J. Mattson, L3/NASA/Goddard Space Flight Center

In a coded-aperture-based telescope, there’s a “mask” mounted above the detectors at a distance of about 1m that has small lead tiles glued to it in a known pattern. There are also open areas on the mask. Gamma-rays hitting the lead tiles are stopped, but those going through the open areas continue on. We measure the pattern of gamma-rays on the detector plane, and then use a computer to deconvolve, or isolate, the patterns. This can tell us the location of the gamma-ray source in the sky.

The advantage of the coded aperture telescope is that it can observe a broad patch of sky. So if we want to view a lot of sky at once, a coded-aperture-based system is best. But it’s more complicated and not as sensitive as a focusing system would be. Until recently there was no technology for focusing gamma-rays, but that’s changing now, at least for the "low energy" part of the gamma-ray range with energies just above the X-ray band, so-called "hard X-rays."

An artist's concept of NuSTAR.
Credit: NASA

I promised to tell you about focusing optics. Fiona Harrison at Caltech has developed the technology that can focus hard x-rays and soft gamma-rays (20-100 keV) onto imaging detectors. The technology increases the sensitivity for hard x-rays by coating the mirrors with multiple layers of platinum/siliconcarbite and tungsten/silicon that increase the reflectivity of the mirror so that it can reflect higher energy levels onto the detector. A team at Goddard is building the optics for this telescope. The technology was first tested on a balloon but there are plans to fly it on a satellite called NuSTAR in a few years. That will be the first time that we’ve used the focusing optics technique above the atmosphere.