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Glossary | Telescope accessories | General | Type of build | Flattener

The flattener is an additional lens to correct field curvature.

The flattener is an arrangement of lenses that flatten the image field, as this is slightly curved by the primary optical system. This curvature causes stars to appear less sharp toward the edge of the field of view on photographic images. The flattener, also known as a field flattener lens, corrects this error.