The GEMS satellite is based on Orbital's proven LEOStar-2TM spacecraft bus design. GEMS will be the eighth satellite to be based on the LEOStar-2 platform.
GEMS is made possible by recent breakthroughs in several key technologies. Advances in gas detector technology have enabled exploitation of photoelectric polarimetry, without sacrificing sensitivity. Light-weight mirrors are constructed of especially treated aluminum foils. A deployable boom provides the appropriate separation between the detectors and the mirrors. GEMS has two telescopes on one spacecraft increasing the effective collection area compared to one large telescope. It is the small dimensions and mass of the telescopes and the deployable boom that permits the mounting of two telescopes and vehicle rotation, while retaining a small overall observatory size.
The heart of GEMS is a small chamber filled with gas. When an X-ray is absorbed in the gas, an electron carries off most of the energy, and starts out in a direction related to the polarization direction of the X-ray. This electron loses energy by ionizing the gas; the instrument measures the direction of the ionization track, and thereby the polarization of the the X-ray. The GEMS detector readout employs a time projection chamber to image the track.
The GEMS main science payload, the X-ray Polarimeter Instrument (XPI), consists of two co-aligned telescopes. Each telescope has a high throughput, high heritage, grazing incidence mirror that focuses x-rays onto a high efficiency polarimeter that measures the polarization-depenant direction of the initial photoelectron using a time projection technique. The mirrors are near copies of the mirrors employed on the Suzaku mission. One telescope includes the Student Collaboration experiment, the Bragg Reflection Polarimeter (BRP), which extends GEMS' baseline capabilities.
The two mirrors are mounted on a common Mirror Optical Bench (MOB), which the two polarimeter detectors are mounted on an Instrument Support Structure (ISS). The Telescope Optical Boom (TOB) connects the MOB and ISS. The TOB, which is stowed to a compacted length of only 0.05 m for launch, undergoes a one-time on-orbit deployment to place the mirrors precisely 4.5 m in front of the detector mid-points. The TOB holds the MOB in this position within 0.5 mm throughout the mission, including eclipse periods.
Since no previously existing booms could meet the mission objectives and fit into a Pegasus class launch vehicle, a new four-sided, carbon fiber composite Coilable boom is being designed and manufactured by ATK in Goleta, CA. A flight-like engineering test unit of the new TOB has been built and is being tested to verify the stringent deployment accuracy and repeatability, as well as the ability to withstand the extreme launch environment.
The GEMS primary instrument,the X-ray Polarimeter Instrument, or XPI, uses the photoelectric effect to measure the linear polarization of X-ray flux. It comprises two telescopes which focus source flux into photoelectric polarimeters which employ a Time Projection Chamber readout geometry and have been developed at GSFC for this application. The polarimeters record images of the photoelectron tracks produced after the absorption of X-rays. The initial direction of each track is correlated with the photon electric field direction; the distribution of directions from an ensemble of tracks gives the source polarization.
Sub-milli-second time tagging of each event relates the track direction (measured in detector coordinates) to sky coordinates. The ensemble of track directions in detector coordinates is a sensitive measure of uncalibrated asymmetries in the detector response. Since the experiment uniformly samples all sky angles at all detector angles, such asymmetries are effectively averaged out.
