Q&A: Chandra Mission

Q:
What makes ACIS better than previous X-ray detectors?

A:
ACIS is better than its predecessors for several reasons: The Chandra mirrors focus X-rays from the cosmic source into a much smaller area - 0.5 arc second, about 50 times smaller than ROSAT, the best previous X-ray mirrors, and thousands of times smaller than ASCA, the best previous telescope that used CCD detectors. Also, the effective area of the X-ray mirror/ACIS combination is larger than previous telescopes.

The greatly improved resolution enables astronomers to detect much finer detail in a source, and to separate closely spaced sources. The use of CCDs makes it possible to measure X-ray spectra at the same time that an image is being made, in essence to make a color picture instead of a black and white one. Both these capabilities Ð separation of sources, and measurement of the spectrum of each source, were critical for discovering the binary black hole in NGC 6240.

For more information, see the following links/publications:

http://cxc.harvard.edu/newsletters/index.html
http://cxc.harvard.edu/proposer/POG/html/ACIS.html

W. Tucker and K. Tucker Revealing the universe: The Making of the Chandra X-ray Observatory (Harvard U. Press: 2001) pp. 184-189.

Here is some background information on CCD detectors: In the late 1970's highly efficient new solid state detectors called charged coupled devices, or CCD's were rapidly becoming the detector of choice for optical astronomers. Now commonly used in digital cameras and camcorders, CCD's were a product of the silicon revolution in which thin silicon wafer or chips were discovered to be capable of many amazing things, all related to their ability to become conductors with the addition of minute electric voltages. The essence of the operation of a CCD is this: an incident photon produces a shower of electrons that fills up little wells, or pixels in a silicon chip. By the clever application of voltages across the wafer, these electrons can be counted, and it can be determined precisely where the photon struck the chip, and what the energy of the photon was. This information can be used to reconstruct an image of the source of the photons, and its spectrum as a function of position. CCD's are, in effect, an extremely efficient type of electronic film. This is perfectly suited for space research, since the electronic signals can be readily converted to radio signals and transmitted to the ground station.