What is a "Flat Field"?

[yagon]

Could someone please explain what a "Flat Field" is when refering to scope optics?

[rogerg (Roger)]

I could help you out with Flat Frame with respect to astro photography, but Flat Field.

I know of Field Flatenners and could guess at what they do, making the field appear more flat (less like you're looking at the sky moving across a sphere) - flatenning the field by reducing edge distortion. But I'd be guessing at that.

Roger.

[Blue Skies (Jacquie)]

No, I think you're about right there, Roger. A flat field means there is no distortion or coma at the edge of the field. The view is even or 'flat' all the way across. I can recall seeing a really good flat field in a Nagler eyepiece once.

[74tuc]

In an optical system with a simple lens or a mirror off axis light is focused on a curved surface this is called field curvature. One gets field curvature and other distortions "for free"- this is the nature of singlet lenses and mirrors.

When viewing an image on this curved surface the eye will compensate by quickly focusing on the off axis area one is looking at. If we place a CCD at the on axis focal point then the image will be distorted because the focal plane is curved. To enable the CCD to provide an undistorted pic the focal plane has to be made flat - the field is now known as a flat field.

Distortion is the property of an optical system to image straight lines as curved. Wide field eyepieces provide an increase AFoV (above about 57 degrees) by purposely distorting the image - I think type 4 Naglers have about 25% distortion.

Jerry,