Chandra High Energy Transmission Grating Spectrometer

Chandra HETG

HETG Principal Investigator
Prof. Claude R. Canizares 

Cambridge, MA 02139

Copyright Permission 

PS version w/Press Releases (2.1 M) 


About Chandra: The Chandra X-Ray Observatory is a major NASA mission of the "great observatory" series, scheduled for launch in July, 1999. The Center for Space Research at MIT has involvement in two of the four scientific instruments on Chandra: the High Energy Transmission Grating, HETG, and the Chandra Advanced Charge-Coupled Device Imaging Spectrometer, ACIS. The Center is also active in the Chandra X-Ray Science Center, CXC which will oversee the operation of Chandra during the mission.
The High Energy Transmission Grating, HETG, consists of 336 gold grating facets mounted on an assembly that can be swung into position behind the Chandra mirrors. The inner two rings are high-energy grating, HEG, facets, and the outer two rings are medium-energy grating, MEG, facets. 

The High Energy Transmission Grating Spectrometer, HETGS, is the combined mirror-HETG-detector system on Chandra . The HETG intercepts the X-rays reflected from the mirrors, changing their direction by amounts that depend sensitively on the X-ray energy. One of the focal plane detectors records the location of the diffracted X-rays, enabling a precise determination of their energies. The HETG gratings are designed to cover an energy range of 0.4 to 10 keV. 

(larger images: 120Kb , 540kb
The HETG grating facets are made of bars which are spaced closer together than a wavelength of visible light; it would take hundreds of bars to equal the thickness of a sheet of paper. The bars are supported by plastic membranes which are as thin as a soap bubble, yet can withstand the trauma of a shuttle launch. The gratings take advantage of the fact that the gold bars are partially transparent to X-rays, so that the diffraction is more efficient, and more X-rays are captured in the high resolution spectrum.  Cross-section of grating

(click for MEG and HEG) 

"Made in Cambridge"

Fabrication: Because the size of the gold grating bars is smaller that a wavelength of visible light, special fabrication techniques are required to make them. Special fabrication techniques were developed at the Space Microstructures Laboratory to fabricate the gratings on the MIT campus in the Center for Space Research. The developement and fabrication of the HETG gratings have led to several industrial spinoffs .
High Resolution Spectroscopy with HETG: The gratings exploit Chandra's sharp mirror focus and matching detector resolution to produce high resolution X-ray spectroscopy. Since the grating spectrometers can measure energy to an accuracy of better than one part in a thousand, they are used in the study of detailed energy spectra, distinguishing individual X-ray lines from specific atoms. Using these spectra, the temperature, ionization and chemical composition of the object can be explored. 

An example of the HETGS high resolution is seen at right as a spectrum from the HETGS first-light observation of the bright star Capella .
(Compare this with our pre-flight simulation of the Capella observation.)

Separation of X-ray energies by HETGS in Capella Spectrum
(click for more details of the observation) 

HETG Science: The HETG spectrometer will be used to observe a variety of astronomical objects which emit X-rays, such as stars, black holes, supernova remnants, galaxies, and clusters of galaxies. 

HETG Press Releases:

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Revised 1/14/00 
(relocated on 8/9/2004.)