The Chandra X-ray Observatory

The Chandra X-ray Observatory is the U.S. follow-on to the Einstein Observatory. Chandra was formerly known as AXAF, the Advanced X-ray Astrophysics Facility, but renamed by NASA in December, 1998. The Chandra spacecraft carries a high resolution mirror, two imaging detectors, and two sets of transmission gratings.

Important Chandra features are:

A summary of the telescope mirrors used in various X-ray missions can be found below.

X-ray Telescope Mirrors
Mirror Characteristic Einstein EXOSAT ROSAT BBXRT/ASCA Chandra XMM
Aperture Diameter 58 cm 28 cm 83 cm 40 cm
one module
1.2 m 70 cm
one module
Mirrors 4 nested
one module
2 nested 4 nested 118 nested
one module
4 nested 58 nested
Geometric Area 350 80 1140 1400
two modules
1100 6000
three modules
Grazing Angles
40-70 90-110 83-135 21-45 27-51 18-40
Focal Length (m) 3.45 1.09 2.4 3.8 10 7.5
Mirror Coating Ni Au Au Au Ir Au
Highest Energy Focused (keV) 5 2 2 12 10 10
On-Axis Resolution (arcsec) 4 18 4 75 0.5 20

Chandra is in an elliptical high-earth orbit allowing uninterrupted observing intervals of more than 48 hours in length.

The Chandra telescope was designed to have three times the area of the Einstein mirror at low energies and to have considerable collecting area between 6 and 7 keV, the energy of iron lines emitted by many astrophysical sources. Capability of the observatory goes far beyond that of Einstein. There is an order of magnitude better angular resolution and imaging sensitivity is two orders of magnitude higher. The relative increase in spectroscopy capability is even greater.

The mirror consists of four pairs of nested reflecting surfaces, arranged in the usual Wolter type 1 geometry. The high energy response is achieved by use of relatively small reflection angles and by coating the mirrors with iridium. Improvements in mirror technology since Einstein include significant advances in grinding, polishing, alignment, and testing. Mirrors with a resolution of 0.5 arcseconds have been achieved. The combination of high resolution, large collecting area, and sensitivity to higher energy X-rays makes it possible for Chandra to study extremely faint sources, sometimes strongly absorbed, in crowded fields.

There are two focal plane instruments. One is a High Resolution Camera (HRC). Smaller pore size, larger microchannel plate (MCP) dimensions, lower background, charged particle anticoincidence, and possible energy resolution are all advances over the HRI carried by Einstein and ROSAT. It is used for high resolution imaging, fast timing measurements, and for observations requiring a combination of both.

The second instrument, the Advanced CCD Imaging Spectrometer (ACIS), is an array of charged coupled devices. A two-dimensional array of these small detectors does simultaneous imaging and spectroscopy. Pictures of extended objects can be obtained along with spectral information from each element of the picture. This was done with the Einstein IPC but in a primitive way compared with this Chandra instrument. The new device combines the spatial resolution of the Einstein HRI with the spectral resolution of the Einstein SSS, an order of magnitude improvement over the IPC in both respects. ASCA carries a similar CCD array but the mirror limits the spatial resolution to ~2 arcminutes.

There are two transmission grating spectrometers, formed by sets of gold gratings placed just behind the mirrors. One set is optimized for low energies (LETG) and the other for high energies (HETG). Spectral resolving powers (E/deltaE) in the range 100 to over 1000 can be achieved with good efficiency. These produce spectra dispersed in space at the focal plane. Either the ACIS array or the HRC can be used to record data.


Chandra's capabilities provide unprecedented science and Chandra Users are making important contributions to all areas of astronomy, including the solar system, stars, interacting binaries, compact objects, supernovae, galaxies, and AGN.

How to keep up with it all? Highlights and summaries are provided in several places:

Last modified: 01/08/21