Reinhold Henneck

High-Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission for the next (2000) solar maximum

The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum.

RHESSI imager and aspect systems

RHESSI uses nine Rotating Modulation Collimators (RMCs) for imaging, each consisting of a pair of grids mounted on the rotating spacecraft. The angular resolutions range from 2.3 arcsec to 3arcmin. The relative twist between the two grids of each pair is the most critical parameter. It must be less than 20 arcsec for the finest grid. After precision alignment, it is monitored by the Twist Monitoring System (TMS) to a few arcsec. The Sun-pointing must be known better than 0.4 arcsec for the image reconstruction. This is achieved by the Solar Aspect System (SAS), which consists of a set of three Sun sensors. Each sensor is focusing the filtered Sun light onto a linear CCD. The onboard Aspect Data Processor (ADP) selects the 6 limb positions, which over-define the pointing offset of the Sun center in respect to the imaging axis of the imager. The Roll Angle System (RAS) continuously measures the roll angle of RHESSI within arcmin accuracy. The RAS is a continuously operating CCD star scanner. The time of the passage of a star image over the CCD is recorded and defines the roll angle, comparing its pixel position and amplitude with a star map.

A prestorage method to measure neutron transmission of ultracold neutron guides

There are worldwide efforts to search for physics beyond the Standard Model of particle physics. Precision experiments using ultracold neutrons (UCN) require very high intensities of UCN. Efficient transport of UCN from the production volume to the experiment is therefore of great importance. We have developed a method using prestored UCN in order to quantify UCN transmission in tubular guides. This method simulates the final installation at the Paul Scherrer Institute’s UCN source where neutrons are stored in an intermediate storage vessel serving three experimental ports. This method allowed us to qualify UCN guides for their intended use and compare their properties.

Solar Aspect System (SAS) for the High-Energy Solar Spectroscopic Imager (HESSI)

The HESSI SAS is a set of three Sun sensors, which shall provide high bandwidth information on the solar pointing of the rotating spacecraft. The precision of <EQ 0.4 arcsec relative is necessary in order to obtain the HESSI imaging resolution of 2 arcsec; the absolute accuracy of 1 arcsec is required for comparison with other measurements. Each SAS is based on focusing the Sun through a narrow bandwidth filter on to a 2048-element x (13(mu) )2 linear CCD. A digital threshold algorithm is used to select N pixels that span each solar limb for inclusion in the telemetry. Determination of the 6 limb crossing locations provided by the 3 subsystems defines the position offset of the Sun in the rotating frame. In this paper we describe the mechanical and electronic configuration of the SAS FM and the results of the first test measurements.

RHESSI Aspect System and In-flight Calibration

Precise knowledge of the pointing and the roll angle of the rotating spacecraft is needed in order to reconstruct images with 2 arcsec resolution using the modulation patterns seen on each of the detectors of the bi-grid rotating collimators. Therefore, the aspect system consists of two subsystems of sensors, the Solar Aspect System (SAS) and Roll Angle System (RAS). The transmitted data consists of Solar limb data from the SAS (CCD pixels around the intersection of a Solar image with three linear CCDs) and Star event data from the RAS (CCD pixels induced by passages of Star images over a linear CCD). In order to meet the RHESSI requirements, the reconstructed pointing needs to be ≤ 0.4 arcsec (rms) relatively (≤ 1 arcsec absolutely) and the determination of the roll angle needs to be better than 1 arcmin (rms). Beside of understanding and calibrating each sensor, the error budget on the aspect system requires an alignment of the relevant features of the 1.55 m extended telescope on a micron level. This could be achieved by a combination of on-ground and in-flight calibration.

EUVID — a compact, high-resolution imaging detector for the wavelength range 20 ≤ λ ≤ 1200 Å

Abstract The E xtreme UV I maging D etector EUVID consists of a compact “micro-channel-plate-wedge-and-strip anode” imaging system and corresponding data read-out electronics with 32 MB of memory. EUVID is suited for imaging in the wavelength range 20 ≤ λ ≤ 1200 A . The performance of this space-qualified, 2.5 kg detector system was measured in the FUV. We obtained a spatial resolution of about 70 μm over the full detector area of ∼ 7 cm2. Image distortions are small and can easily be corrected for. Detector efficiency depends on the choice of photocathode and ranges between 50% in the FUV up to 90% in the EUV.

EUVID: A compact, high resolution imaging detector for the wavelength range 20-A <= lambda <= 1200-A

Abstract The E xtreme UV I maging D etector EUVID consists of a compact “micro-channel-plate-wedge-and-strip anode” imaging system and corresponding data read-out electronics with 32 MB of memory. EUVID is suited for imaging in the wavelength range 20 ≤ λ ≤ 1200 A . The performance of this space-qualified, 2.5 kg detector system was measured in the FUV. We obtained a spatial resolution of about 70 μm over the full detector area of ∼ 7 cm2. Image distortions are small and can easily be corrected for. Detector efficiency depends on the choice of photocathode and ranges between 50% in the FUV up to 90% in the EUV.

Low-reflectivity materials for the vacuum UV: Mg and Ag surfaces

We have developed absorptive Mg coatings for the vacuum UV (VUV) wavelength range. The total hemispherical reflectivity at normal incidence was measured at 121.6 nm. The reflectivity of the Mg coating produced by magnetron sputtering (presumably from the vapor phase) is close to 1%, comparable to the best known coatings. In contrast to the latter they are mechanically robust. After storage in air for 2 years the reflectivity increased by a factor 2. The reflectivity of Mg and Ag coatings produced by thermal evaporation was observed to be about 2 - 3%. The reflectivity of the Ag coatings proved to be stable over a period of 58 months in air.

Transmission of thin indium filters in the EUV and lifetime tests

The transmission of thin indium filters was measured in the EUV wavelength range between 500 and 1200 angstroms. Our results for the shape of the transmission peak are consistent with previous measurements, indicating a FWHM bandpass between 750 and 900 angstrom with the maximum at 770 angstrom. The absolute transmission values however differ significantly from former measurements. The results are compared to currently used predictions. The filter transmission was remeasured after 20 months storage in N2. On average, the transmission was observed to be reduced to about 60% of the original value.

121.6-nm stray light reduction methods for solar wind characterization instruments

In satellite-borne particle characterization instruments, for example, in the solar wind charge-energy-mass spectrometers and ion traps, there is a need to suppress the effect of the 121.6 nm Lyman-alpha line of the hydrogen spectrum, the most intensive line of the solar UV radiation resulting in high level of the detector’s noise. To reduce this effect, the electrodes of instruments are usually covered with electroconductive light-absorptive coatings having low reflectivity at the wavelength of 121.6 nm. In this paper the physical mechanisms are considered applicable to reduce the reflectivity of the A1-based coatings to be applied on electrodes of the particle analyzing instruments. Particular emphasis has been given to the role of three phenomena: (i) multiple light scattering light traps of the rough surface, (ii) diffraction of the incident light at the rough surface of the coating, and (iii) electron scattering in a skin layer. It is presented the behavior of reflectivity of the A1-based coating in the course of mechanical and environmental tests simulating standard shipping, storage, launching, flight, and operating conditions of the space equipment. The noise measurements of the Faraday cups used on board the INTERBALL-1 mission are also given. As the hydrogen is the most prevalent substance in the Universe, perhaps, the 121.6 nm stray light problem is the common one not only for the solar wind missions and solar astrophysics telescopes, but also for the Far UV astronomy and future UV space astrophysics missions.