The instrument is equiped with two identical sensor units pointing the northern and southern ecliptic hemispheres respectively. The field of view (FOV) of each sensor is of 5° x 5° divided on 5 x 5 pixels of 1° square each. The sensor is completed by a mechanical periscope system containing two mirrors at 45° incidence. The moving mirror system allows full sky mapping in Ly-α.
A hydrogen cell is placed in each sensor's optical path, which provides quantitative information on the spectral profile of the Ly-α line, with a resolving power of about 3 x105. The cell contains molecular hydrogen at few hundreds of Pascals pressure, dissociated by two heated tungsten filaments. When the cell is activated, hydrogen atoms, produced by the dissociation process, absorb in resonance the Ly-α line emitted by interplanetary (IP) hydrogen atoms.
| Total Mass | 13.25 kg |
| Mass of Electronic Unit | 2.82 kg |
| Mass of one Sensor Unit | 4.815 kg |
| Average Power | 11 W |
| Telemetry | 200 bps |
| Telecommands | 69 different TC |
| Working Temperature | 0 to +30° C |
| Overall FOV | more than 2π sr for each sensor unit (2-mirror periscope) |
| Instantaneous FOV | 25 pxls of 1° x 1° = 5° x 5° |
| Wavelength range | 115 - 180 nm |
| Photometric Sensitivity | 0.75 counts /sec /Rayleigh /1° pxl |
| Type of detector | Solar blind MgF2 / CsI Cathode - Multianode MCP |
| Expected count rate | from 200/s (IP) to 104 /s (solar corona) per pixel |
| Relative accuracy | better than 1% for 45 sec counting time on IP signal |
| Absolute sensitivity | checked at regular time intervals with stellar calibrations |
| Hydrogen cell | Pyrex vessel (MgF2 lens windows-distance 76mm) Spectral Resolution : better than 0.01Å (Resolving power 105) |
| Last updated on Mon, Feb 7, 2005 | DK |