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

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 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 degree pointing  active diameter > 40 cm.  Use 2.5 periods of a 13 period mask  cm active diameter

GRIPS Mask (schematic)