For the Dall-Kirkham (DK), therefore, the CFP lies at the centre of curvature of the spherical secondary, the same as the neutral point for pointing, as must be the case for a spherical secondary. This is not necessarily an advantage since it precludes the possibility of influencing the coma without affecting the pointing (tracking). We shall return to this point later in connection with the active control of decentering coma. Conversely, field scanning without coma for observation in the IR is not possible for the DK form.
For an aplanatic telescope we use (3.364) with (3.375) to give from (3.377)
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