For a 2-mirror telescope of Cassegrain form, corrected for spherical aberration, large negative values of bs2 give a CFP behind the secondary, but close to it. As |bs2| reduces to the RC value, the CFP moves backwards almost to the prime focus, to a point normally very close to the exit pupil if the entrance pupil is at the primary. With further reduction of |bs2| to the value for a classical Cassegrain, the CFP is exactly at the prime focus. For lower values, it moves further back, reaching the centre of curvature of the secondary when bs2 = 0 for the DK form. After this, bs2 becomes positive (oblate spheroid) and the CFP moves further away until it is at —ro when the denominator in (3.385) becomes zero. Beyond this, the CFP is in front of the secondary, such cases including the SP form.

The telescope form giving zcfp = ro is defined from (3.385) by

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