Metering Structures and Grid Trays
Closeup of Grid TrayIntegrated into HESSI imager

The trays are the supporting platform for the grids. Made of a special aluminum alloy which was chosen with its easiness to machine and weld in mind. The material also needed to show no deviation from the shape with time and temperature. Trays are hollow, with beams welded from parts to provide the required stiffness at an acceptable mass. To release residual stresses after the production, trays were tempered in a moderate heat treatment. The final grid-bearing surfaces of the trays were flat within ~10 micrometers. The trays were manufactured by Paul Scherrer Institute.

Front Grid Tray The critical alignment requirement for the metering structure is to maintain the relative twist of the finest grid pair to within one arcminute. The proposed metering structure is based on the Telescope Demonstration Unit (TDU) built and tested at GSFC. The TDU's on-orbit alignment performance was simulated using SINDA thermal models and NASTRAN structural models. The results of this analysis and the TDU thermal and vibration tests demonstrate that the proposed design satisfies all the mission requirements.
Rear Grid TrayTo maintain alignment following thermal and vibrational stresses, flexural elements are used to mount the grids to the grid trays and the telescope tube to the spacecraft. This approach was demonstrated with the TDU and used successfully by Co-I van Beek in the Hard X-ray Imaging Spectrometer (HXIS) on SMM, where the alignment requirements were significantly more stringent. The fine grids will be precision aligned in twist by elastic deflection of these flexure elements.  These will be manufactured by Thermo Electron Tecomet.