LABEL: A unique label to identify the type and version number of the data base.
DESCRIPTION: Author and brief description of this version of the data base.
PITCH: Measured pitch, based on OGCF data. Values are more
accurate than suggested by the use 5 significant figures, which are more than sufficient for imaging applications. In all cases the difference between front and back grid pitch is sufficiently small that it can be neglected for imaging purposes.
PHASE: As presently defined, this is the distance, orthogonal to the slits, from the origin of the GF (grid-fiducial) coordinate system to a mid-slat location as inferred from OGCF data. The origin of the GF coordinate system is the mid-point between the -X and +X sunny-side fiducials. Phase is positive measured in the -X to +X sense. . The definition and values of this parameter will be changed to incorporate displacements of the grid mounting location relative to the imager axis. Currently tabulated values are nominal.
ORIENTATION: This is the orientation of the slits relative to a spacecraft-based (imaging) axis. The values will correspond to the mid-point between the dark and sunny sides of the grids. Current values are nominal and have not yet been corrected for grid mounting errors.
RMSERROR: The rms deviation of a mid-slit positions from their ideal locations, assuming a best-fit pitch and orientation. Based on OGCF data sampled over the entire active area of the grid.
Measurement errors of ~0.2 microns have been removed.
SLIT: For Tecomet grids, this is the slit width of a single etched layer averaged over all layers and locations. The values are based on a consensus fit among on-axis low-energy x-ray transmission, off-axis low-energy x-ray transmission and far-off-axis high-energy x-ray transmission data and the reconciliation of a mass model to weight data. Measurements of individual layers provide the starting point. For fuzzy-edged (Tecomet) slits, the slit boundary is defined so that the total mass of the grids is independent of the SLITEDGE parameter discussed below.
THICKNESS: Mechanically-measured average thickness. For Tecomet, grids, this is the distance between the outsides of the top and bottom layers. For VBC grids, the values are based on sample blades.
SLITEDGE: Averaging along and over all slits, this is ONE-HALF of the distance measured orthogonal to the slits over which the surface density rises from zero to its full value. The value is based on one of two different models, depending on which gave the best empirical fit to the data set described under SLITS above. In both cases, the models define the profile of the surface density as it rises from zero in a slit to its maximum value in the slat. It is assumed that along any line of sight perpendicular the grid face, the density is uniform. Note that the models do not explicitly reflect the properties of individual layers or stacking. Instead, the models combine the effects of the edge profile of individual layers, stacking errors and variations in slit with both within an individual layer and among the multiple layers
For both models, the surface density rises quadratically from zero at each edge of the slit. In Model 1 (indicated by a positive value for SLITEDGE) the surface density profile also decreases quadratically from its maximum value in the slat so as to behave symmetrically with respect to a point of inflection at the 50% point. In Model 2 (indicated by a negative value for SLITEDGE) the quadratic rise from zero surface density in the slit continues until full surface density is achieved, at which point there is a discontinuity in the slope. Note that for model 1, the average totally clear slit width (which determines the low energy response) is
(SLIT-2*SLITEDGE) and the 'slat width' with full density is
(PITCH-SLIT-2*SLITEDGE). For model 2, the totally clear slit width is (SLIT- 8/3*|SLITEDGE|) and the 'slat width' with full density is (PITCH-SLIT-4/3*|SLITEDGE|).
The SLITEDGE parameter value was found as part of the SLIT consensus fit discussed above
SLAT-LINEAR-DENSITY: Excluding bridges, this is the average linear density of a slat, measured parallel to the slits. (This parameter determines the high-energy modulation of the grid.)
TILT: In the plane orthogonal to the slits and which includes the imaging axis, this is the angle between the direction of maximum grid transmission and the imaging axis. It is positive if the line of maximum response from the grid toward the Sun is directed toward the positive Y axis in the GF system. Tilt is determined by the sum of five components: intrinsic tilt of the slats in the standalone grid; the effects of the grid mounts and their shims; flatness of the grid trays; relative twist of the two grid trays; relative displacement of the two grid trays. The current values include the intrinsic grid values plus the grid mounts. They do not yet include tray-related corrections as measured by the PSI/CMM.
SPIRAL: Spiral is the angle between the average orientation of the sunny-side and dark-side slit orientation, as measured by the OGCF. When combined with grid thickness, it is implies a grid-position-dependent tilt.
SLAT-MATERIAL: Photon-effective material of the slats. The effects of epoxy in the Tecomet grids are not neglected here, but do help determine the grid thickness and slat-linear density. Note that in general, the effective bulk density for a given material will vary from grid to grid.
BRIDGE-MATERIAL: Photon-effective material for the bridges. For the VBC grids, this is a weighted average of the metering structure and spacers, with the Beryllium content neglected. Adopted Fe:Ni:Cu composition ratios by weight for FeNi, FeNiCu and Invar are: 91:09:00, 34:18:48 and 64:36:00 respectively. Small differences among grids 6,7,8 bridge composition have been neglected.
BRIDGE-PERIOD: Effective period of the bridge structures. For Tecomet grids, these values are not exact, but the ratio of bridge-width to bridge-period is believed to be ok. For VBC grids, the effective bridge period is dependent on incident angle and is also different for front and rear grids. The values shown is an effective average over incident angles from 0 to 30 arcminutes.
BRIDGE-WIDTH: The width (parallel to the slits) of the bridge. This is based on a model in which the bridge is a rectangular bar of uniform composition. For Tecomet grids, this is based on the area of a single layer bridge divided by the slit width. The same SLIT-EDGE parameter is also assumed to apply to bridges.
BRIDGE-HEIGHT: The height (parallel to earth-sun line) of the bridge model.
Note that for the VBC grids, the bridge model assumes a single rectangle which incorporates the effects of both top and bottom metering structures and their spacers. The bridge height and width is chosen to reflect the outer envelope of the metering structure and spacers, averaged between slats.
BRIDGE-LINEAR-DENSITY: The linear density of the bridge model along each bridge. The linear density is chosen to reflect the average amount of material between slats with the caveate that this material is also superimposed on the slats. The resulting overestimate of material where the slats are has little, if any effect.