GIF Animation by Art Lecompte |
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!