Reuven Ramaty High Energy Solar Spectroscopic Imager - Imaging

Imaging

Since X-rays are not easily reflected or refracted, imaging in X-rays is difficult. One solution to this problem is to selectively block the X-rays. If the X-rays are blocked in a way that depends on the direction of the incoming photons, then it may be possible to reconstruct an image. The imaging capability of RHESSI is based on a Fourier-transform technique using a set of 9 Rotational Modulation Collimators (RMCs) as opposed to mirrors and lenses. Each RMC consist of two sets of widely-spaced, fine-scale linear grids. As the spacecraft rotates, these grids block and unblock any X-rays which may be coming from the Sun modulating the photon signal in time. The modulation can be measured with a detector having no spatial resolution placed behind the RMC since the spatial information is now stored in the time domain. The modulation pattern over half a rotation for a single RMC provides the amplitude and phase of many spatial Fourier components over a full range of angular orientations but for a small range of spatial source dimensions. Multiple RMCs, each with different slit widths, provide coverage over a full range of flare source sizes. Images are then reconstructed from the set of measured Fourier components in exact mathematical analogy to multi-baseline radio interferometry.

RHESSI provides spatial resolution of 2 arcseconds at X-ray energies from ~4 keV to ~100 keV, 7 arcseconds to ~400 keV, and 36 arcseconds for gamma-ray lines and continuum emission above 1 MeV.

RHESSI can also see gamma rays coming from off-solar directions. The more energetic gamma rays pass through the spacecraft structure, and impact the detectors from any angle. This mode is used to observe gamma ray bursts (GRBs). The incoming gamma rays are not modulated by the grids, so positional and imaging information is not recorded. However, a crude position can still be derived by the fact that the detectors have front and rear pickups. Also, the detectors near the burst shield the ones away from the burst. Comparing signal strengths around the nine crystals, and front-to-back, then gives a coarse, two-dimensional position in space.

When combined with high-resolution time stamps of the detector hits, the RHESSI solution can be cross-referenced on the ground with other spacecraft in the IPN (Interplanetary Network) to provide a fine solution. The large area and high sensitivities of the Ge crystal assembly make RHESSI a formidable IPN component. Even when other spacecraft can provide burst locations, few can provide as high-quality spectra of the burst (in both time and energy) as RHESSI.

Rarely, however, a GRB occurs near the Sun, in the collimated field of view. The grids then provide full information, and RHESSI can provide a fine GRB location even without IPN correlation.