Fokalgrid

Fokalgrid, or focal grid, is a calibration feature used in optical instrumentation to project or imprint a regular grid pattern at the focal plane of a camera, spectrograph, or telescope imager. It provides a known reference geometry that enables precise assessment of the instrument’s optical performance, including alignment, scale, and distortion.

Construction and forms vary. A focal grid can be a physical mask with an etched grid inserted into a calibration unit, or a projected grid produced by a light source shining through a patterned element. Patterns are commonly square or rectangular, though radial or crosshair layouts also appear. Grid spacing is chosen to match the detector’s pixel size and focal length, typically spanning tens of micrometers to millimeters, with multiple grids sometimes included to cover different scales.

Applications encompass astrometric calibration, distortion mapping, and plate-scale determination. By recording images of the illuminated grid, engineers extract precise grid line positions and fit distortion models (polynomial or radial) to compute correction coefficients. The grid also aids in optical alignment checks, detector nonuniformity assessment, and wavelength calibration in spectrographs when grid lines appear across the spectrum. These calibrations are performed during commissioning, after instrument maintenance, or as part of regular monitoring to ensure stable performance.

Operation notes. The focal grid is usually placed at or near the focal plane and activated during dedicated calibration sequences. Accuracy depends on grid fabrication quality, alignment precision, illumination uniformity, and detector sampling. In some designs, the grid is removable or switchable, allowing different spacings or patterns for various calibration tasks. The term encompasses both fixed calibration masks and dynamically projected spatial references used to calibrate and validate imaging systems.