HEXTE Cluster Source/Background Beamswitching

Each HEXTE cluster has a simple ``rocking'' mechanism so that it can alternate between observing the source and observing ``blank'' sky on either side. This process is known to ground-based astronomers as ``beamswitching''. A typical beamswitch cycle consists of observing the source, switching (i.e. moving rapidly) to one side off-source, returning to the source position, then switching to the other side. This cycle is then repeated continuously throughout any given observation. The on-source dwell time can be selected to be 16, 32, 64, or 128 s.

[animated GIF] [Timing pattern GIF]
GIF Animation by Art Lecompte
The animation shows a typical HEXTE observing setup: synchronized two-cluster source/background switching (speeded 4 times up for clarity), with the timing pattern shown at right (red=cluster A, blue=cluster B). The example shows ±1.5 ° offsets and a 16s on-source dwell time; the circles represent the HEXTE clusters' 1 ° FWHM fields of view.

Note that each cluster only moves to a background position after its partner has moved back on-source, thus ensuring continuous on-source coverage. The cluster's motion takes 2s, which reduces the amount of time spent observing the background.

The rocking mechanisms may also be commanded to dwell indefinitely (i.e. stare) at any of the on- or off-source positions in order to satisfy scientific objectives such as fast timing on bright sources where background subtraction is not important. Since the rocking axes of the two clusters are orthogonal to each other, 4 background regions are usually sampled around a given source position. The off-source positions can be selected to be either 1.5° or 3.0° from the source, with the option of one- or two-sided beamswitching.

Since the rocking axes of the HEXTE clusters are fixed relative to the RXTE spacecraft, the background positions on the sky depend on the spacecraft's attitude, which is defined by the source coordinates (RA, Dec) and the ``roll offset'' from the sun (RXTE's sensitive instruments and star trackers must be shielded from the sun's glare). Of course the sun's position changes relative to the stars with time, so to help astronomers determine the HEXTE background fields of view, the HEXTE team has provided software with a WWW interface (HEXTErock), which calculates and displays HEXTE background positions on the sky for a given observation. Try it out for an observation near the Galactic Center (RA=266°, Dec=-28°) and you will see how careful one must be to avoid contaminating sources in the HEXTE background!

HEXTE virtual tour