The Chandra X-ray Observatory is one of NASA's Great Observatories, together with the Hubble Space Telescope, the Spitzer Space Telescope and the now deorbited Compton Gamma Ray Observatory. The Chandra Observatory carries a telescope with the spectacular angular resolution of the order of one arcsecond, two imaging cameras and two different types of transmission gratings with dedicated read-out detectors.
Chandra was launched by Space Shuttle Columbia in 1999 and since then contributes importantly to the study of the structure and evolution of the Universe.
The imaging capability of the telescope allows detailed spectral mapping of extended sources such as nearby supernova remnants and distant clusters of galaxies. The grating spectrometers provide detailed spectra from many different classes of objects, ranging from coronae of nearby stars to active galactic nuclei in the distant Universe.
SRON in collaboration with the Max Planck Institut für Extraterrestrische Physik (MPE) in Garching (near München) has provided the Low Energy Transmission Grating (LETG). The actual gratings were manufactured and tested by MPE, and SRON produced the high-precision ring-like structure in which the gratings are placed. SRON is the Principal Investigator institution for the LETG, ultimately responsible for the entire low energy grating experiment.
Chandra has been put in an elliptical orbit by the Space Shuttle in July 1999 and operates since then successfully.
The Chandra mission addresses a large number of astrophysical subjects, ranging from deep sky surveys to detailed spectroscopy of plasmas. The spectral coverage of Chandra ranges from 0.1 to 10 keV. The Low Energy Transmission Grating addresses high-resolution spectroscopy at the longer wavelengths in this range, up to about 140 Angstrom. Studies of astrophysical plasmas in point-like sources form the core of its scientific objectives.
Prime candidates for study are stellar coronae, white dwarf atmospheres, X-ray binaries, cataclysmic variables and active galactic nuclei. Measurements of plasma emission lines allow the determination of physical parameters such as gas temperature and density, ionization state, elemental abundances, velocities, and red shifts.
The Chandra Observatory contains a spectrometer for soft X-rays of between 0.2 and 17.5 nanometers in wavelength, which is an energy of between 0.07 and 10 keV. The spectrometer operates using a transmission grating, so-called Low Energy Transmission Grating (LETG).
The grating can be placed behind the mirrors and intercepts the converging bundle there. The grating consists of no less than 540 elements. These elements have been made by the Max Planck Institute MPE. Each element has a gossamer-thin structure of a thousand parallel independent gold threads per millimeter. The grating elements are located in rings with a diameter of 1.5 centimeters. Two supportive structures have been applied to support the thin threads: a linear grating with a distance of 25.4 micrometers between the wires and a triangular pattern of wires with a distance of 2 millimeters between the wires. SRON produced the ring-shaped structure in which all of the grating elements are located. As the Principal Investigator SRON was also responsible for the entire grating.
With the grating, a spectral resolution of 1000 is possible. This means that the energy of the photons captured can be measured to an accuracy of 0.1%. The grating has an average efficiency of 10%, with a peak of 20% for radiation of 2 keV.
The radiation diffracted by the grating is focused per 'color' on the focal plane at a distance of almost 10 meters. There, the detectors that observe the X-rays are located. The user of the satellite can opt for CCDs (Charge Coupled Devices) or MCPs (Multi Channel Plates), dependent on the wavelength that he/she wants to observe. The lower noise CCDs are only sensitive for radiation up to 6 nanometers whereas the MCPs can measure across the entire wavelength range. MCPs make use of the fact that an X-ray photon causes an electrical discharge in a gas-filled tube.
The working principle of a grating is based on the wavelength characteristic of electromagnetic radiation. A grating bends radiation at very specific angles, which satisfy the requirement that the difference in wavelength between two adjacent slits constitutes exactly one or more wavelengths. The wavelength can be derived for the angle at which the radiation is bent and the distance between the wires of the grating. In Chandra, the grating elements are located in the convergent bundle. The shape of the structure in which the elements are located has been chosen such that the 'light' of each wavelength is focused at a separate point.