MOXE Project

MOXE Instrument Description


The MOXE instrument is a set of six X-ray pinhole cameras that stare continuously at the entire sky, with a bandpass of 2 to 25 keV (note: the onboard software has a provision to change the settings of the lower and upper level discriminators). The detectors are position-sensitive proportional counters that are read out by charge division. Each MOXE module covers 1/6 of the sky (i.e., one face of a cube). For a square detector with side d, the aperture-detector distance should be f = d/2 to cover 2pi/3 steradians. For MOXE, f is chosen slightly smaller so that there is some overlap between the FOVs of the modules to guarantee full sky coverage in case of a slight misalignment. We therefore chose a detector area of 32X32 cm**2 and a 'focal length' f of 15 cm. The pinhole size was chosen such that the diffuse cosmic background and internal detector background were comparable. For modules that each view 1/6 of the sky, the pinhole area should be about 1000 times smaller than the detector area for typical values of diffuse and internal background.

Single instrument layout

The above picture is a cutaway drawing of one of the MOXE cameras. All detector body components are made out of aluminum, except the windows (beryllium), strongback (titanium) and sunshield (titanium, not shown here). The cone and detector sides are plated with tin for particle protection. The six detectors will be placed in holders that are attached to the satellite at various positions to ensure satellite components obstruct a minimal part of any detector's FOV.

In order to optimize the angular resolution, we have chosen an asymmetric pinhole of 2.556 X 0.625 cm2 (Lochner & Priedhorsky 1991). This corresponds to 9.7 X 2.4 sq deg on the sky for the on-axis position. For a given source, the orientation of the long axis of the aperture as projected on the sky will vary from one pointing of SXG to the next. Two sources which are confused in one pointing may not be confused when the projected aperture is turned to a different angle.

Power consumption and weight

The detector modules include proportional counters, cones, apertures, high voltage power supplies, preamplifiers, and a housekeeping/high voltage control box. Each detector module weighs ~13.5 kg, has an envelope of about 45X45X26 cm3, and uses ~2.1 Watts (high voltage on). The central electronics module includes electronics for amplification, A/D conversion, event analysis, commanding, telemetry, memory, interface to the satellite, and two redundant low voltage power supplies. This central electronics module weighs 35 kg and draws 25.1 Watts of bus power (high voltage on). Because Spectrum-X-Gamma flies outside the Earth's magnetosphere, it encounters a high radiation dose. All electronics are designed to withstand at least 20,000 rads.

Internal works of detectors

The active sensors for MOXE are 32X32 cm2 sized, 1 cm deep, Xenon-filled (with 5% CO2 as a quench gas at a total pressure of 1.04 atm at 20'C), permanently sealed, position-sensitive proportional counters with 5-sided anti coincidence. Three co-planar grids (two cathodes and one anode) subdivide the volume in four equally deep layers. The wire grids consist of parallel 0.254cm spaced stainless steel wires, with a diameter of 75 micron for the cathode and 13 micron for the anode. Both ends of each cathode frame are read out separately. Two preamp signals, one from the edge sections and a second from a 1 cm deep guard layer with its own anode, are summed to form an anti-coincidence signal. The wire plane that separates the photon detector layer from the guard layer acts as an over-exposure sensor, its signal feeding a discriminator circuit that triggers should the time-averaged anode current be too large. The detector signals are used to provide anti coincidence, safety against damage, and input to obtain the position and energy in the onboard processor. 5-sided anti coincidence and pulse height discrimination will be used to reject cosmic ray background. The anti-coincidence plane covers the full area of the detector below the main detector volume, while the ends of the x- and y-cathode arrays provide anti-coincidence volumes to guard the sides.

Event energy deposition and localization

Event positions are sensed by charge division in the two resistive cathodes, one for each axis. The ratio of the two signals at the two ends of a given cathode is proportional to the position of the photon interaction and the sum of all four cathode signals is proportional to the photon energy.

Blockages and responses

The low-energy response is limited by the aperture and detector windows, which are made from respectively 75 and 114 micron thick Beryllium. A conical assembly holds the 1.6 cm2 pinhole 15 cm above the detector window. This assembly is filled with helium gas at ~1 atm to relieve the pressure differential across the large detector window. The helium layer absorbs less than 1% of the photons with energies within MOXE's bandpass. A titanium structure on top of the detector entrance window ('strongback') supports any residual pressure differentials, it is designed to withstand up to about 0.3 atm. The strongback has been constructed in such a way, that it causes minimum shadowing of the projected image of the aperture (see Lochner & Priedhorksy 1991).

Intense flux from the Sun, the brightest source in the X-ray sky, raises a particular problem. Since MOXE covers 4pi steradians, the Sun will always be in view of at least one detector and sometimes three. The Sun can appear anywhere in a 20X80 sq deg region, relative to spacecraft coordinates, as Spectrum-X-Gamma moves from one pointing to the next. To avoid overloading the detectors or saturating the telemetry stream, parts of three detectors corresponding to this 20X80 sq deg region (4% of the whole sky) are blocked by a heavy titanium shield. This system is backed up by software that exclude Sun counts and, for the highest rates, turns off the detector high voltage should the Sun stray outside the shielded area. The Sun is recognized by its extremely high count rate compared to other sources.

Digitization and onboard software

Since the total expected counting rate from the six modules is about 260 cts/s (with about half from point sources), each detected photon can be individually encoded. We use 24 bits per event: 7 bits for each position axis, 4 bits of pulse height (photon energy), 3 bits of differential time information, and 3 bits to identify the detector. The total expected telemetry rate is 7.4 kbits/s, which is stored in a mass memory of 827 net Megabits, sufficient for 30 hours at average countrate, and downloaded every 24 hrs at ground contact. The flight software cannot be modified from the ground but is controlled by about 1000 variables that can be commanded from the ground.

Hardware status

The six flight detectors are, at the time of this writing, almost all completely built, and the process of testing has recently been started. It is expected that all of the flight hardware will be delivered for integration by December 1994.