The RHESSI instrument

There are two attenuators (thick and thin) designed to cut down on photon flux in order to keep detector dead time low. These have only two possible states, out or in. They are moved in and out through the use of shape memory alloy wires which contract when a large voltage is passed through them. These attenuators are also sometimes referred to as "shutters" or "filters".

Relevant pages: igseact, igseatten.

The SSR is RHESSI's on-board hard drive. It stores photon events detected by the Germanium detectors with a maximum capacity of 4 Gb (or 10^9 photons events). Data are written and read out through two pointers, the read pointer and write pointer. The normal mode of operation corresponds to the read pointer chasing the write pointer (Note that the read pointer only moves during downlinks while the write pointer records continuously). The SSR is a cyclic FIFO (First In First Out) device, meaning that earliest data will be read out first and that the end of SSR is also its beginning.

In principle the read or write pointers for the SSR can be moved, and this is sometimes done in order to delete data from telemetry if downlink capacity does not keep up and if activity is high. Please avoid deleting any day-before or day-after data for major flares, since these are used for gamma-ray background corrections.

Relevant pages: ssrcontrol.

RHESSI holds 9 detectors, each behind grids with a particular spatial resolution. Each detector is a single crystal of pure germanium in the general shape of a cylinder. A gamma ray or X-ray event in the crystal generates electron-hole pairs which, responding to the strong electric field set up by the high-voltage biasing, create a measurable current pulse. The detectors are also separated into front and rear segments (for total of 18 segments). The front segments generally absorb photons up to about 100 keV while the rear segment handles the higher energies more efficiently.

Detectors of this kind have two characteristic problems; pileup and livetime. Pileup is caused by two photons arriving virtually simultaneously and causing the detector to count them together, as one fictitious event with an energy equal to the sum of their individual energies. This is usually only a problem during very large flares or for energies below 6 keV. Pileup problems can be alleviated through the use of clever software. Livetime, on the other hand, cannot. The livetime of a detector measures the probability that a photon will be detected; it is the time which a detector requires to be "reset". During the time which a detector is resetting no photons can be processed. The effect is energy-independent and so only affects the magnitude of the spectrum. You can find more information about the hardware aspects of the detectors here. For the same, plus related data analysis issues, go here.

Relevant pages: monitor_rates, igsepd, ihv, igsehv, igse_dib_pages, igsecom, igsesci.

The Germanium detectors are cooled through a single mechanical Stirling cycle pump. The cryocooler is one of only two essential mechanical subsystems onboard RHESSI. Through its single-state cycle, the cryocooler creates some vibrations which are monitored. Excessive vibration would feed noise into the sensitive detectors, and in fact this routinely happens during shutter motions. The cooler operating voltage can be adjusted in order to control the temperature, and there is also a heater within the cryostat for further control. More information on the cryocooler can be found here.

Relevant pages: igsespc.

The Attitude Control System consists of many separate instruments which all work together to control RHESSI's pointing. The Fine Sun Sensor (FSS) provides the spacecraft with high-bandwidth solar positions. This information is used to direct current to the Torque Rods in order to keep the RHESSI's pointing consistent. The Torque Rods interact with earth's magnetic field in order to create small restoring forces. Very rarely, they are also used to stabilize RHESSI's spin or to do offpointing maneuvers. For more information click here.

Relevant pages: acsmain.

Both of these systems provide data relevant only to RHESSI image reconstruction and so do not affect the spacecrafts attitude. The SAS consists essentially of three lenses and three identical linear pixel arrays (one is redundant), which independently form full-Sun images in one dimension. The solar limb is well defined in these images and provides knowledge of the Sun center to arc-second precision in pitch and yaw. The solution provided by the SAS should coincide with the solution provided by the Fine Sun Sensor (FSS), which provides the spacecraft itself with realtime high-bandwidth pointing information. For more information on the SAS click here; for more information on the RAS click here.

Relevant pages: igseadp, igseimgv, iadpdigital, iadpstatus, iadpbits.

Decimation is a system used to keep the SSR at a resonable fill level. It is set up so that in each segment of the instrument (front and rear) photon counts are deleted. This deletion only applies to a certain fraction of the counts below a specific energy threshold. There are four basic modes that determine these fractions and thresholds: normal-normal, normal-active, active-normal, and active-active. The first word refers to the mode of the front segment and the second refers to the mode of the rear.

Active mode for the front segment can be used to bring down the SSR fill level when the sun is active, and is described by this table. The normal mode of the front segment is described by this table. Active mode for the rear segment can be used to bring down the SSR fill level when there is a lot of high-energy particle precipitation. This mode will delete 3/4 of all counts below an approximate 400 keV threshold. The normal mode for the rear segment will delete 3/4 of all counts below an approximate 200 keV threshold.

Relevant pages: Full explanation of decimation parameters