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The preferred modern type of large telescope today is the Ritchey-Chretien form. The field size is aberrationally limited by undercorrected astigmatism and field curvature. A pair of equally and oppositely oriented flat transmittng plates can neutralize either or both of these aberrations locally in the field to restore the image. These plates, relatively inexpensive and positionally noncritical, may be used to extend the available image area by an order of magnitude.
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Rosin et al. (1972) studied this question.
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