X-ray Grating Spectrometer (XGS)
The specific implementation of the X-ray Grating Spectrometer (XGS) has not yet been chosen, but the basic technology will be the same no matter which design is ultimately selected. A spectrum is simply an plot of the intensity of light in different energies (or wavelengths). A rainbow is a spectrum of visible light, but spectra can be made from any wavelengths of light and not all of the wavelengths need to be represented in a spectrum. In fact, the relative amount of light from each wavelength tells scientists about the composition of the material which made or reflected the light, and can reveal some details of the processes which made the light.
Detail of the Chandra Low-Energy Transmission Grating (LETG). Chandra uses transmission gratings, which are also under consideration for Constellation-X; however, the specific implementation of the grating spectrometer (transmission or reflection) has yet to be selected.
A spectrum grating makes a spectrum by physically separating the different wavelengths of light. There are two kinds of gratings – a reflection grating, which creates a spectrum when light reflects off a surface and a transmission grating, which creates a spectrum when light passes through the grating. A prism is a transmission grating, since light has to pass through it to be separated into a rainbow. A common reflection grating is a CD or DVD. The rainbow of colors that appears on the back of a CD or DVD appears because the tiny grooves on the disk separate light into its constituent colors.
Both types of gratings are under consideration for the Constellation-X XGS. Regardless of which is chosen, some of the X-rays entering the Soft X-ray Telescope will encounter the XGS to be separated into different energies. This spectrum will be focused onto CCD (charge-coupled device) detectors. CCDs are becoming a familiar technology, since they are used in hand-held digital cameras. Unlike the CCD in your camera, the XGS CCDs will be sensitive to X-ray light rather than visible light. Scientists will then process the CCD data into spectra.
