Reference: Moore, T.E., C. R. Chappell, M. O. Chandler, S. A. Fields, C. J. Pollock, D. L. Reasoner, D. T. Young, J. L. Burch, N. Eaker, J. H. Waite, Jr., D. J. McComas, J. E. Nordholt, M. F. Thomsen, J. J. Berthelier, and R. Robson, The Thermal Ion Dynamics Experiment and Plasma Source Instrument, Space Sci. Rev., 1995.
Collimator System:
Identical collimator assemblies are installed on each of the seven TIDE polar angle apertures. These collimators consist of two stacks of vanes, one collimating in spacecraft azimuth angle, the other collimating in spacecraft polar angle. The vane stacks are nested with the polar angle stack in the outer position and the azimuth stack in the inner position. The vanes are positioned so as to eliminate all trajectories which would in any case be unacceptable to the mirror/RPA/TOF optics system; that is, all trajectories which lie outside the angular response and effective area of the instrument. This collimator approach is intended to minimize the number of extraneous particles and photons entering the instrument apertures, and particularly to eliminate such particles or photons which would otherwise be incident upon interior surfaces other than the mirror grids. In this way, the susceptibility of the instrument to extraneous particles and photons is minimized.
Collimation of 10 degrees width in spacecraft azimuth is provided by means of parallel vanes which are 0.50 in. deep, 0.005 in. thick, and set on 0.044 in. centers. It should be noted that the M/RPA is designed for a commensurate angular response when its aperture is fully open. When the M/RPA is effectively "stopped down" by choosing a small ratio Vm/VRPA, its angular response is reduced from 10 degrees to as low as approximately 3 degrees.
Collimation of 22.5 degrees in spacecraft polar angle is provided by means of vanes which are set nearly coincident with planes which emanate radially from the instrument symmetry axis. The layout of a single polar angle collimator stack is illustrated in Figure 6. Note that the individual collimator channels each fully illuminate the focal plane (rejection/deflection system entrance aperture), while cutting off rays which would be incident upon the mirror frames or other interior structures. In the figure, "instrument C/L" refers to the symmetry axis of the sensor, around which the individual channels are arranged.
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Figure 6. Geometry for a single TIDE collimator providing 22.5° wide response in polar angle. "Instrument C/L" refers to the symmetry axis of the TIDE sensor, around which the polar angle channels are arranged radially, distributed in polar angle.
The energy resolution of the RPA system is ultimately limited by the angular spread of the ions delivered to it by the mirror system. This spread is inversely related to the breadth of the energy passband defined by the mirror and RPA, which reduces the fraction of the mirror that is active as well as the range of energies that can pass through the system.
The electrical biasing scheme employed in this system is as follows: VRPA is commanded with a 12-bit word through a digital-to-analog converter (DAC) providing a range of 0-300 V with 0.073 V resolution. Vm is commanded through an 8-bit DAC which is referenced to the output of the RPA DAC. In this way, the ratio (Vm/Vrpa) is directly commanded and can be controlled more precisely at the low end of the energy range than with independent commanding for both Vm and VRPA. This directly controls TIDE sensitivity, because the energy pass band (as well as the effective area and solid angle) is directly related to this ratio. The practical range over which TIDE's sensitivity can be varied is approximately two orders of magnitude, as described in the section below on test results.
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