Optical EngineeringNull testing using the virtual conjugates satisfying the Herschel condition
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Utilizing the Herschel condition, a method is presented for null testing most refractor doublet objectives, including the Baker air-spaced design with equal internal radii, and Maksutov corrector plates using auxiliary spherical mirrors. A special Baker air-spaced objective is offered that utilizes a spherical mirror in null testing that is generated by silvering the front face of the doublet's crown lens or immersing it into mercury. The classical Schmidt corrector is found to deny null testing using the technique.