A field flattener is an optical accessory used in astrophotography to correct for field curvature and other optical aberrations that can occur when imaging through a telescope.

Telescopes, particularly those with fast focal ratios, can produce curved or distorted fields of view. This means that objects at the center of the image may be in focus, but those towards the edges may appear blurry or distorted. A field flattener is designed to correct for these distortions by flattening the field of view and providing a sharp, well-corrected image across the entire field.

Field flatteners come in various designs and configurations, but generally consist of a lens or lens system that is placed in the light path between the telescope and the camera. They are typically used in conjunction with a camera and other accessories, such as a filter wheel or focuser.

Field flatteners can be particularly useful when imaging wide-field views of the sky, such as with a camera lens or a small refractor telescope. By correcting for field curvature, they allow for sharp, distortion-free images of extended objects such as nebulae or galaxies.

It’s important to note that not all telescopes require a field flattener, and the specific type of field flattener needed depends on the telescope’s design and the camera being used. It’s important to carefully match the field flattener to the telescope and camera system for optimal results.