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Greater Collecting Area

Astronomers are in a constant search for better ways to collect light. The more light an astronomer collects, the easier it is to determine the properties of an object and to discover new phenomena. The light-collecting area is a measurement of how much light a telescope may collect at one time.

Chandra X-ray Observatory
Chandra X-ray Observatory

Rossi  X-ray Timing Explorer
Rossi X-ray Timing Explorer

In high-energy astronomy, astronomers collect photons using either X-ray or gamma-ray telescopes (as in the case of the Chandra X-ray Observatory or the European Space Agency's XMM-Newton Observatory) or large area detectors (as in the case of the Rossi X-ray Timing Explorer (RXTE)).

More photons means better images and better spectra. Better images will be obtained because greater detail can be picked up, as more photons create a sharper image, and better spectra are possible because weaker emission lines become more evident as we detect more photons in them. But better images and better spectra also need higher resolution in the detector, in addition to greater collecting area.

Binary Star System
An artist's conception of an X-ray binary system. Collecting enough photons from this kind of source can tell us whether it contains a black hole.

A larger collecting area also means better light curves. For bright sources, large area means collecting more photons in a shorter amount of time. Hence, we can detect phenomena that occur within a very short time. With its large collecting area of 6,250 cm2, RXTE detected quasiperiodicities (that is, a recurrent behavior pattern within a larger, more unpredictable pattern) in X-ray binaries down to milliseconds, and provided evidence of the signature of material just before if falls into a black hole.


Fermi gamma-ray telescope
Fermi Gamma-Ray Space Telescope

Reasearchers are constantly taking steps toward launching missions with larger collecting areas. Launched in December 1999, the XMM-Newton mission uses compact X-ray optics to produce an effective collecting area of 2,500 cm2 at low X-ray energies. Among the many types of objects it studies, XMM-Newton uses this collecting area to study to the nature of the diffuse X-ray background, which ROSAT had shown to be discrete sources, such as quasars and galaxies.

Another mission using larger collecting area is the Fermi Gamma Ray Space Telescope (Fermi), which studies objects that emit gamma rays with energies ranging from 10 MeV (megaelectron volts) to 100 GeV (gigaelectron volts). Fermi has an effective collecting area of at least 8,000 cm2, compared to the EGRET instrument on the Compton Gamma Ray Observatory (CGRO), which has an area of 1,500 cm2. In addition, Fermi is able to view an area four times larger at any one time than EGRET did. With its large area and sensitivity, Fermi is addressing the evolution of supermassive black holes in the centers of some galaxies, the nature of particle jets emanating from these black holes, and the search for radiation from weakly interacting massive particles (WIMPS), which may make up part of the invisible substance called dark matter.

Last Modified: May 2011

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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