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The blurring is due to a phenomenon known as "field curvature" which stems from the inability of a telescope to get all the light in focus at exactly the same distance from the objective (main lens or mirror). Differing designs have differing degrees of field curvature. In ancient days astronomers would have curved photographic plates to reduce field curvature's impact on photographs.

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Posted by waqqas1

Lens designed to correct field curvature of field.

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Posted by sagitraz

its a lens designed to correct the curvature of field

for description visit the following links
http://www.answers.com/topic/field-flattener
http://www.idigitalphoto.com/dictionary/field_flattener

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Posted by Hash007

Your guess is correct. The blurring is due to a phenomenon known as "field curvature" which stems from the inability of a telescope to get all the light in focus at exactly the same distance from the objective (main lens or mirror). Differing designs have differing degrees of field curvature. In ancient days astronomers would have curved photographic plates to reduce field curvature's impact on photographs. When looking through a telescope you can see it by focusing at the center of the field of view and then noticing that stars at the edge are out of focus--often not by much. Eyepieces used in telescopes also contribute their own field curvature to the view. The field flattener is a corrective lens which reduces the distortion and helps project a better image on a flat ccd chip for electronic imaging

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Posted by ferna