Greater Collecting Area

In high-energy astronomy, astronomers gather light by counting photons, which are particles of light. The limit to our knowledge about the size, energy, or mass of an object depends on how certain we are of the number of photons collected. The relative uncertainty in the number of photons collected decreases as the number of photons increases. Hence the the more light we collect, the better we can know.

Chandra X-ray Observatory

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 X-ray Multiple-Mirror Mission) or large area detectors (as in the case of the Rossi X-ray Timing Explorer).

Rossi X-ray Timing Explorer

More photons means better images and better spectra. Better images because greater detail can be picked up as more photons create a sharper image. Better spectra 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.

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

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 6250 cm2, RXTE detected quasi-periodicities in X-ray binaries down to milliseconds, and provided evidence of the signature of material just before if falls into a black hole.

Steps toward flying missions with larger collecting area are constantly being taken. Launched in December 1999, the X-ray Multiple-Mirror Mission uses compact X-ray optics to produce an effective collecting area of 2500 cm2 at low X-ray energies. Among the many types of objects it studies, XMM uses this collecting area to study to the nature of the diffuse X-ray background, which ROSAT has shown to be discrete sources, but whose nature is still largely a mystery.

XMM

Another mission using larger collecting area is the Fermi Gamma Ray Space Telescope (Fermi), which will study objects emitting gamma-rays having energies ranging from 10 MeV to 100 Gev. Fermi has an effective collecting area of at least 8000 cm2, compared to the EGRET instrument on the Gamma Ray Observatory, which has an area of 1500 cm2. In addition, Fermi is able to view 4 times more of the sky 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 search for radiation from weakly interacting particles, which may make up the dark matter in the universe.

Fermi Gamma-Ray Space Telescope

Updated: September 2008