1. GRIPS Mask Design Rationale (1) Grid Pitch Parameters Minimum pitch, P min = 1.0 mm (set by detector resolution) Want a finite number, N v, of discrete pitch values (ease of fabrication) Spatial frequencies to be equally spaced (for optimum image quality)  Pitch values = N v * P min / n, with n=1,,,N v Want equal areas for all pitch values ( equal weighting of spatial frequencies) NvNv Max pitch (mm) Max. FWHM (arcsec) Mask Period (mm) Slats per mask period 10 12910055 12 15514466 13 16816991 14 180196105 15 193225120

2. GRIPS Mask Design Rationale (2)  Want to ensure that all spatial frequencies can be observed for any instantaneous combination of roll angle, source location and pointing offset  Mask period must be less than ~16.7 cm (X) or 19.95 cm (Y)  This excludes N v =14 or higher  Larger values of N v  better image quality  To avoid gaps in uv coverable mask phase should be shifted by ½ mask periods in opposite mask halves.  To avoid biasing uv coverage there should be an integral number of ½ mask periods.  To get reasonable (>90%) observational efficiency with +-0.5 degree pointing  active diameter > 40 cm.  Use 2.5 periods of a 13 period mask  42.25 cm active diameter

3. GRIPS Mask (schematic)