flatfield

A flatfield is a region on an optical surface (lens, mirror, or prism) where the curvature has been lost or deliberately removed, creating a flat zone instead of the intended curve. In a lens, this might be a small flat patch on an otherwise spherical surface. In a curved mirror, it appears as a faceted or worn area. The flatfield breaks the optical prescription of the element, scattering or misfocusing light that passes through or reflects from that zone.

Flatfields occur through several mechanisms. Grinding or polishing errors during manufacture can leave a surface under-corrected in one area. Mechanical damage, thermal stress, or impact can flatten a small region of glass or mirror substrate. In precision optics, even minor flatfields (depths of a few wavelengths of visible light) degrade image quality noticeably, introducing coma, astigmatism, or diffuse halos. In large telescope mirrors, flatfields from settling or temperature gradients reduce contrast and resolution.

Detection and consequences

Flatfields are detected using interferometry, optical testing benches, or (for large mirrors) laser collimation. A small flatfield might not render an optical system unusable, but its effect compounds with other aberrations. In a microscope objective or high-aperture lens, even a 1 percent flatfield can produce visible vignetting or field curvature. In astronomical imaging, flatfields in primary mirrors degrade point-spread function uniformity across the focal plane.

The term reflects manufacturing and quality language: a surface that should have figure (precise curvature) instead shows a flat defect. This distinguishes it from ripple, astigmatism, or spherical aberration, which are systematic errors baked into design. A flatfield is always accidental or degradative, something to be avoided or corrected through re-grinding, replacement, or optical compensation downstream.