Field flattener lens

Field flattener lens is a type of lens used in modern binocular designsTemplate:Efn and in astronomic telescopes to improve edge sharpness. Field flattener lenses counteract the Petzval field curvature of an optical system, mitigating the field-angle dependence of the focal length of a system.

Details

The object in designing a field flattening lens is to create a lens that shifts the focal points of the Petzval surface to lie in the same plane. Consider inserting a pane of glass in a focusing beam. Due to refraction, the focal point of the beam is shifted by ${\delta }_x$ dependent on the thickness of the glass. Thus we have a thickness as a function of focal shift:

$t({\delta }_x)=\left(\frac{n}{n-1}\right){\delta }_x$.

${\delta }_x(y)$ is given by the radius of curvature of the Petzval surface, $R_p$. It can be shown, then, that the radius of curvature for the lens that would flatten out the field is given by

$R_f=\left(\frac{n-1}{n}\right)R_p.$

Examples of use

In the 21st century, the New Horizons spacecraft, which was an unmanned space probe sent past Pluto and the Kuiper belt, had a telescope instrument called the Long Range Reconnaissance Imager. LORRI was a reflecting telescope but incorporated a field-flattening lens, with three elements.

See also

• Petzval field curvature
• Coma corrector

References

Template:Optics-stub