Design of Cooke triplet anastigmats: the effect of non-optimum flint glasses

This study investigates the effects of changing the flint glass of a Cooke triplet from the optimum glass type, in an attempt to determine how many different glass types are necessary. To this end, about 90 triplets were designed, each with a flint glass displaced from its optimum characteristics. Sixteen different combinations of field and aperture, and three different crown glass types were used. Variations of performance with flint changes are presented. An 'acceptable' variation of the flint glass V-value appears to be about plus or minus 2.